dynamic vs static compression

[grumpyvette]

its obvious some of the guys on this site need to understand the difference between static and dynamic compression ratios, and that's understandable as its a difficult concept to grasp at first.
your engines tendency to get into detonation is the result of higher effective compression that the fuel octane your using can permit, but your piston is only compressing the fuel air mix, trapped between the rising piston and combustion chamber, AFTER both valves seat.and that normally happeneds well after the piston passes BDC and starts back up the bore.
detonation is a condition that occurs as the result of both cylinder temperature compressions effect on your fuel/air ratio and several other factors that cause fuel to self ignite under some conditions, lowering the EFFECTIVE compression , lowering the heat in the combustion chamber , or designing the engines quench,squish,turbulence and other factors to greatly reduce the engines tendency to get into detonation will obviously help prevent it.
but you need to understand it before selecting a combos components ,
keep in mind a huge percentage of the details you need to understand is contained in the sub links so don,t just skim thru, take the effort to read the links also as youll save hundreds of dollars and weeks or nu-necessary work doing so in avoiding potential problems
http://www.wallaceracing.com/dynamic-cr.php

http://victorylibrary.com/mopar/cam-tech-c.htm

http://www.empirenet.com/pkelley2/DynamicCR.html

http://www.not2fast.com/turbo/compression/cranking_pressure.shtml

http://garage.grumpysperformance.com/index.php?threads/port-speeds-and-area.333/

https://www.uempistons.com/index.ph...e=comp&zenid=1e826335bfac0f356463eabed4958558

https://www.uempistons.com/index.ph...=comp2&zenid=1e826335bfac0f356463eabed4958558

https://www.rbracing-rsr.com/compstaticcalc.html

http://www.diamondracing.net/tools/

http://www.hotrod.com/events/coverage/0311em-power-squeeze/
, if you have any engine and want too find the true compression you need to deal in verified fact, not assume what the manufacturers suggest is always correct.
you just cc the heads, combustion chamber volume, place the piston 1" down the bore and seal the rings gap around the pistons,
above the rings with moly grease or Vaseline and measure that volume and calculate what a bore diameter cylinder 1" tall would
contain, minus the volume you,ve measured, the piston dome took up.then calculate the true compression.

measure the piston deck height after the machined block has the rotating assembly test fitted

read related linked info

http://garage.grumpysperformance.co...n-chamber-or-piston-dome-or-port-volume.2077/

http://garage.grumpysperformance.com/index.php?threads/ccing-my-heads.14187/#post-71989

http://garage.grumpysperformance.co...ng-combustion-chambers.2630/page-3#post-77963
what the chart below shows is the theoretical gain in torque with an increase in compresion
example
if you had 8:1 compression, and boosted compression to 12:1 you could theoretically expect a 11.5% boost in torque as a result

Ive built enough engines that I think I can post a bit of info regarding the effect of the need to consider dynamic compression, in the engine planing

fact
usable torque the engine produces tends to go up as cylinder pressure increases
fact
your fuels OCTANE or tolerance, to heat before you tend to get into potential detonation, issues limits the effective compression.
fact
uncontrolled detonation will rather quickly damage pistons and rings
fact
cylinder pressure is greatly influenced by the cylinders,displacement, and valve timing as well as the static compression ratio
fact
on a N/A engine you'll rarely if ever exceed the static compression, with the effective dynamic compression, but you can markedly reduce it with cam timing changes, that delay the valves closing as the piston compresses the cylinder volume.

you might want to read thru these link's carefully
http://victorylibrary.com/mopar/cam-tech-c.htm

http://members.uia.net/pkelley2/DynamicCR.html

http://www.wallaceracing.com/dynamic-cr.php

http://www.projectpontiac.com/ppsite15/compression-ratio-calculator

http://www.pcengines.com.au/calculators/Calculate dynamic Comp Ratio.htm

OK, first fact! the piston can,t compress anything being trapped in the cylinder by the piston compressing it as it raises,until both valves seat & seal

the chart above can be used as a rough guide to match cam duration at .050 lift and static compression in engines obviously other factors come into play so its only a rough guide

an engines static compression is calculated from the pistons sweep volume from bottom dead center to top dead center , compressing that volume into the remaining combustion chamber volume., if the valves don,t set until the piston reaches lets say 280 degrees past bottom dead center, that greatly reduces the volume of fuel air mix trapped in the cylinder at lower rpms

READ THESE RELATED LINKS
viewtopic.php?f=55&t=2718&p=12119&hilit=calculate+octane#p12119

viewtopic.php?f=52&t=4081&p=10861#p10861

http://victorylibrary.com/mopar/cam-tech-c.htm

http://www.engineeringtoolbox.com/free- ... _1190.html

http://www.wallaceracing.com/dynamic-cr.php

http://victorylibrary.com/mopar/otto-c.htm

http://www.pcengines.com.au/calculators ... 0Ratio.htm

viewtopic.php?f=99&t=4458&p=12155#p12155

http://www.projectpontiac.com/ppsite15/ ... calculator

http://www.enginebasics.com/Advanced%20 ... namic.html

Ive found the KB calculator to give good results
https://www.uempistons.com/index.php?ma ... bed4958558

try this also
http://www.projectpontiac.com/ppsite15/ ... calculator

http://www.wallaceracing.com/dynamic-cr.php

http://cochise.uia.net/pkelley2/DynamicCR.html

http://www.steigerperformance.com/products/sp90005.html

I generally look to see about 145-160 psi on a street gas compatible engine compression test, but even if the reading are down near 120-130 psi I look more at cylinder consistency that the number

viewtopic.php?f=50&t=41&p=49&hilit=leak+down#p49

static compression is theoretical , in that its based on the piston stroke compressing the cylinder volume trapped above the piston from bottom dead center to top dead center being compressed into the combustion chamber and head gasket volume and you also need the piston deck to block deck heights and piston dome or dish volume, you need to know that static compression volume to start your calculations, its theoretical because all modern cams use timing figures that don,t close both the valves at BDC, allowing the full cylinder volume to be compressed.

when you build an engine you generally have some idea as to its intended operational rpm range and the octane level of the fuel you want to use.
normal quality blended gasoline tends to produce its best power at about a 12.5:1 fuel/air ratio, and lowest exhaust emissions at about a 14.7:1 f/a ratio
the higher the effective compression ratio (up to about 13:1) the more torque you can expect to produce from an engine,if you use, high octane gas or race fuels, but the octane of the fuel currently available from pump gas drops the max effective dynamic compression ratio to about 8:1
obviously the components selected must work in the desired rpm and intended power range.
you need to select a cam duration and LSA , and static compression that BOTH matches the static compression ratio and your cars gearing so the effective dynamic compression ratio falls in that range, and intended rpm/power band

viewtopic.php?f=55&t=2718&p=12119&hilit=octane#p12119

viewtopic.php?f=52&t=1070

http://www.jeepstrokers.com/calculator/

viewtopic.php?f=52&t=82

viewtopic.php?f=71&t=741

http://www.projectpontiac.com/ppsite15/ ... calculator

http://www.technovelocity.com/chevyhack ... cation.htm

http://www.pcengines.com.au/calculators/Calculate dynamic Comp Ratio.htm

viewtopic.php?f=55&t=109
read thru this thread
viewtopic.php?f=52&t=1477

https://www.uempistons.com/index.php?ma ... bed4958558

viewtopic.php?f=52&t=5078&p=14433#p14433


dynamic compression uses that that static compression volume as a base calculation but factors in the fact that the pistons not compressing a darn thing until both valves seat, and thats always AFTER BOTTOM DEAD CENTER, as the piston moves upward. so you compress a good deal less cylinder volume as the cams duration and LSA change the valve seat timing

http://garage.grumpysperformance.com/index.php?threads/crowers-valve-timing-charts.4299/


heres an extremely useful chart giving cam timing info

viewtopic.php?f=52&t=2782&p=7214#p7214

viewtopic.php?f=52&t=2077&p=9049#p9049

viewtopic.php?f=53&t=726&p=8809&hilit=quench+squish#p8809

you really might want to read thru these threads and links also, as theres a ton of good info included in the links

http://www.empirenet.com/pkelley2/DynamicCR.html

http://www.empirenet.com/pkelley2/DCR_FAQ.html

http://www.jeepstrokers.com/calculator/

viewtopic.php?f=52&t=4299

http://www.projectpontiac.com/ppsite15/ ... calculator

viewtopic.php?f=53&t=726&p=2302#p2302

http://victorylibrary.com/mopar/rod-tech-c.htm

http://www.circletrack.com/techarticles ... index.html

http://www.not2fast.com/turbo/compressi ... sure.shtml

viewtopic.php?f=55&t=2718

https://www.uempistons.com/index.php?ma ... bed4958558

http://www.cswnet.com/~carother/compression_ratio.htm

http://www.skipwhiteperformance.com/det ... 3831P-1230

http://victorylibrary.com/mopar/piston_position-c.htm

http://www.empirenet.com/pkelley2/DynamicCR.html

http://www.cpgnation.com/category/produ ... comp-cams/

viewtopic.php?f=52&t=1477

http://victorylibrary.com/mopar/cam-tech-c.htm

the difference between STATIC COMPRESSION RATIO AND DYNAMIC COMPRESSION RATIO is where the piston is in the cylinder when the valves close and the piston can actually start compressing the REMAINING VOLUME IN THE CYLINDER VS the STATIC COMPRESSION THAT ASSUMES THE PISTON STARTS COMPRESSING THE INSTANT IT LEAVES BOTTOM DEAD CENTER AND STARTS UPWARD ON THE COMPRESSION STROKE!

let me try and explain, the short version is that the PISTON COMPRESSES NOTHING untill BOTH VALVES ARE CLOSED, ....higher compression INCREASE the engines mechanical efficiency up to a point...that's the only compression ratio that matters,.... since its the only compression ratio the engine ever sees.

btw your max effective STATIC compression ratio naturally vary with the cylinder heat and fuel octane,ETC. but after about 13.7:1 the percentage of gains in hp from compression increases tend to drop off fairly rapidly, with most comon (GAS)

static compression is simply the difference between the cylinder volume at BOTTOM DEAD CENTER(BDC) and its compressed volume at TOP DEAD CENTER (TDC), into the combustion chambers,... dynamic compression takes into account that on the pistons upward compression stroke the valves have not yet closed and nothing gets compressed by the piston until they do, that of course depends on the cam and rockers, pistons and connecting rods, the cylinder volume, the rod/stroke ratio, ETC.,used, in the combo, and the rpm levels to some extent BTW, ALUMINUM HEADS can usually operate at a higher dynamic compression simply because ALUMINUM releases heat to the coolant much faster than iron, its the lower heat levels that remain in the cylinder that help prevent detonation..when you increase the dynamic compression the heat levels in the heads combustion chamber rise , the difference in the RATE heat leaves the cylinder allows a slightly higher dynamic compression level from aluminum before the same HEAT levels are REACHED & MAINTAINED in the combustion chambers , KEEP IN MIND ENGINE TEMP AND YOUR FUELS OCTANE , YOUR TRUE AIR/FUEL RATIO,YOUR IGNITION TIMING ADVANCE CURVE,HOW FAST YOUR KNOCK SENSOR REACTS, and THE ENGINES GENERAL CONDITION, EFFECT THE RESULTS AND WHEN DETONATION BECOMES A PROBLEM
!aluminum cylinder heads tend to allow you to run about 1/4-to-1/2 point more effective compression, IE, if iron heads get into detonation at 10:1 ALUMINUM might ALLOW YOU TO RUN 10.3-10.4:1 BEFORE GETTING INTO DETONATION, BUT ON THE PLUS SIDE AT LEAST IN THEORY IRON HEADS AT ANY GIVEN CPR WILL HAVE A SLIGHT ADVANTAGE IN HP
but in my real world testing the difference is much closer almost non-existent
the main advantage I see in aluminum heads is lighter weight and their much easier to repair when damaged
an aluminum cylinder head allows heat transfer to the engine coolant at a significantly higher rate than an iron head, and generally about .25-.50 higher compression can be tolerated but theres No absolute real definite answer, depends on quench, cam LSA, cylinder head design, advance curve, fuel/air mixture, intake temperature, coolant temp, spark plug design, piston design, heat barrier coatings, combustion chamber surface texture, and a bunch of other parameters.


here is a calculator for static cpr, which you need to figure first

http://www.rbracing-rsr.com/compstaticcalc.html

then lets assume your 350 sbc engine has a static compression ratio of 11:1 but youve installed this cam

http://www.cranecams.com/?show=brows...81&lvl=2&prt=5

first look at this chart
http://www.iskycams.com/ART/techinfo/ncrank1.pdf

and this one
viewtopic.php?f=52&t=4299


looking at the cam specs we see that the effective stroke is not the 3.48" that the static compression ratio is measured from ,at BDC, BUT from about 2.6 inches from tdc where the valves close as the piston moves upward, so your true working compression is closer to 8.1:1 NOT 11:1

heres some different calculators

viewtopic.php?f=52&t=4299

heres an extremely useful chart giving cam timing info

http://www.projectpontiac.com/ppsite/co ... iew/16/30/

https://www.uempistons.com/index.php?ma ... bed4958558

http://www.wallaceracing.com/dynamic-cr.php

http://members.uia.net/pkelley2/DynamicCR.html

average the results..of all four calculators.

READ thru THESE LINKS CAREFULLY

https://www.uempistons.com/index.php?ma ... bed4958558

http://cochise.uia.net/pkelley2/DynamicCR.html

viewtopic.php?f=55&t=2718&p=7057#p7057

http://www.kennedysdynotune.com/Dynamic Compression Tech.htm

viewtopic.php?f=55&t=613&hilit=boosters

http://www.popularhotrodding.com/tech/0 ... index.html

http://victorylibrary.com/mopar/cam-tech-c.htm
http://www.daytona-sensors.com/tech_tuning.html

read these
viewtopic.php?f=52&t=2077

viewtopic.php?f=52&t=2630&hilit=+valves

viewtopic.php?f=52&t=1159&hilit=lapping

I think back on the many dozens of times Ive pocket ported,heads un-shrouded combustion chambers, port matched, intakes and checked deck to piston heights

and I can,t help but grin, if you guys are discussing the potential for mis-reading the burret level when CCing your cylinder heads ,
http://74.125.93.132/search?q=cache:I5W ... clnk&gl=us
a potential mistake that might result in a 1/2cc difference, now in an ideal world we all strive to get things exactly correct, but,

lets assume you do find a full cc difference ,between combustion chamber volumes,the real question is, what do you do to correct that, mis-match and if you don,t how much does it matter, to your engines performance?

http://www.csgnetwork.com/compcalc.html

input your average 70cc sbc heads on a 9:1 cpr sbc then change the chamber volume 1 cc and see how it changes the true results, in most production engines youll be very lucky to be within that range

just keep in mind the piston can,t compress a darn thing UNTIL both valves are closed, so your cam timing effects the true volume compressed in the cylinders
ok lets start from the basics

EACH COMPONENT YOU SELECT IS A COMPROMISE IN SOME AREA
If you've selected a cam that's got too much duration , for your application and rear gear ratio,
if you say the lopey, idle and lack of low rpm torque its a problem,its frequently the result of the cam, you've selected as TORQUE is the result of the EFFECTIVE USE of both DISPLACEMENT and cylinder pressure, so your compression ratio maters here in relation to cam timing, and the longer the cams duration the further up the cylinder the pistons traveled BEFORE the valves fully close, the less trapped volume or displacement you compress, hurting the low rpm torque. to much cam duration is effectively wasting a good percentage of your EFFECTIVE DISPLACEMENT
reducing duration and tightening the LSA tends to increase low rpm torque because for a GIVEN DURATION the valves close sooner in the crank rotation trapping more displacement and resulting in a higher percentage of the fuel/air mix being trapped, burnt and resulting in cylinder pressure turning the crank, but it also results in the valves being open less time so it tends to restrict high rpm breathing or cylinder fill efficiency
IF your running out of high end power it can be the result of a restrictive intake, carb, restrictive exhaust or lack of valve control, but its also common for a cam with too little duration to effectively restrict the cylinders from filling due to there being less time to fill and empty the cylinders

look heres a chart displaying piston location in crank degrees

http://www.iskycams.com/ART/techinfo/ncrank1.pdf


heres a chart showing valve timing in relation to duration
viewtopic.php?f=52&t=4299

look at the chart,

heres free cam selection software to narrow your choices

http://www.compcams.com/Pages/409/camquest-6.aspx

AS your displacement per cylinder increases the effective valve size per cubic inch decreases so you need a slightly tighter LSA and these charts should help.

he following are different static compression ratios (SCR) with a 3.48" stroke and different camshaft intake closing points @ 0.050" tappet lift on Keith Black's dynamic compression ratio calculator. The calculator, starting at 8.25:1 SCR. The dynamic compression ratio (DCR) has been kept at just over 8.0:1, a figure that is acceptable for use with today's pump gas with a little cushion:

• Static CR....Intake closing point @ 0.050"....Dynamic CR.
• 8.25........10* ABDC......8.010........................................................................................................
• 8.50........20............8.012........................................................................................................
• 8.75........27............8.022........................................................................................................
• 9.00........33............8.018........................................................................................................
• 9.25........37............8.061........................................................................................................
• 9.50........42............8.029........................................................................................................
• 9.75........46............8.016........................................................................................................
• 10.00.......49............8.038........................................................................................................
• 10.25.......52............8.043........................................................................................................
• 10.50.......55............8.029........................................................................................................
• 10.75.......57............8.069........................................................................................................
• 11.00.......60............8.022........................................................................................................
• 11.25.......62............8.038........................................................................................................
• 11.50.......64............8.042........................................................................................................
• 11.75.......66............8.035........................................................................................................
• 12.00.......68............8.017........................................................................................................

heres a chart I found that I don,t fully agree with, I think its a bit conservative, by about 3%-5% on the required cam duration ,required to avoid detonation with todays crappy octane fuel, but it at least gives you a base to work from, but Id suggest selecting a bit more duration

CC'ing Heads — How To Measure Combustion Chamber Volume

Measuring your cylinder head's combustion chamber volume is a critical step to ensure that you have your desired compression ratio.

www.enginelabs.com

How do I cc my cylinder head? · Help Center

ccing cylinder heads - Google Search

 

[grumpyvette]

A FEW USEFUL CALCULATIONS AND RELATED LINKED INFO
http://www.csgnetwork.com/compcalc.html

https://www.rbracing-rsr.com/compstaticcalc.html

https://www.uempistons.com/index.php?main_page=calculators&type=comp

http://www.wallaceracing.com/cr_test2.php

http://victorylibrary.com/mopar/cam-tech-c.htm

http://www.diamondracing.net/tools/

http://crt-performance.com/compression.htm

http://www.hotrod.com/articles/ctrp-0505-calculating-engine-compression/

viewtopic.php?f=52&t=399

http://www.bgsoflex.com/crchange.html

http://garage.grumpysperformance.com/index.php?threads/cooler-denser-air.8961/#post-54528

http://garage.grumpysperformance.co...-descriptions-dont-tell-you.12357/#post-61139

http://en.wikipedia.org/wiki/Compression_ratio

http://www.chevytalk.org/fusionbb/showt ... tid/92966/

http://www.rbracing-rsr.com/compstaticcalc.html

https://www.uempistons.com/index.php?main_page=index

viewtopic.php?f=86&t=1330


http://www.race-cars.net/calculators/compression_calculator.html

http://www.projectpontiac.com/ppsite15/compression-ratio-calculator

http://www.wallaceracing.com/dynamic-cr.php

http://www.rbracing-rsr.com/comprAdvHD.htm

http://performancetrends.com/Compression_Ratio_Calculator_V2.3.htm

http://www.wallaceracing.com/cr_test2.php

http://www.pcengines.com.au/calculators/Calculate dynamic Comp Ratio.htm

http://www.csgnetwork.com/compcalc.html

http://www.diamondracing.net/tools/

https://www.uempistons.com/index.php?main_page=calculators&type=comp

https://www.rbracing-rsr.com/compstaticcalc.html

more info


this charts based on a 350-383 chevy or similar size engine, but its a good rough guide on most engines under 400cid displacement on matching the duration to the intended operational rpm band

IDEALLY you would select the horsepower goal, and the displacement,youll work with too reach that goal. then the cylinder heads, intake and exhaust are selected that supply the necessary flow rates,in that rpm band, you pick the cam too match the intended rpm band,and flow rates and power range for the application, you then match the compression ratio, to the cam timing too maintain the correct dynamic compression ratio, and you select the matching drive train and gearing to keep the engine IN the matched rpm band the vast majority of the time.
naturally if your limited to a set displacement or compression ratio the other factors must be selected with those limits in mind

the only true way to find the exact dynamic compression ratio,is to locate both the piston, in its bore and the exact point the intake valve seats firmly, and do the calculations with that info, all the other calculators get you close, but not exact numbers

example
heres a cam card



it says the intake valve closed at 41 degrees abdc at the .050 lift off the seat point,the silvolite calculator says add 15 degrees making it 56 degs abdc, but lets assume your using a dial indicator and find the true seat is at 59 degrees abdc , how much of a difference does that make on your average 350 with 5.7" rods? and how close are the charts

heres the crower chart and your cam in this example is at 222 degrees at .050 lift,off the seat, on a 114 lsa, the chart says 41 degrees, just like the cam card,remember thats at at .050 lift,off the seat,

http://www.crower.com/valve-timing-chart


heres the isky chart and with some math, we find
http://www.iskycams.com/ART/techinfo/ncrank1.pdf

55 ABDC Position: 2.919
56 ABDC Position: 2.897
57 ABDC Position: 2.876
58 ABDC Position: 2.855
59 ABDC Position: 2.834
60 ABDC Position: 2.813

so the difference in effective stroke , is 0.063"
between the calculator and the dial indicator data.
on a 350 chevy 4.00" bore the difference is 291.239 cubic inches being compressed at 56 degrees and 284.90 cubic inches at 59 degrees
changing the true dynamic cpr by about 2%
or not enough to really worry about, keep in mind the lobe ramps differ between manufacturers and slight tolerances, during manufacturing

 

[grumpyvette]

The problem with working "BACKWARDS" in your parts selection process, IE matching the cam you select,too the existing static compression ratio, and displacement,etc. too get the intended dynamic compression ratio, is that youll seldom get the ideal cam timing for the engines most effective volumetric efficiency and torque production in the intended rpm band, if your working too match an existing compression ratio, vs building the combo with the idea of matching ALL the components too maximize efficiency in a certain rpm range and flow rate, and horse power range.

read thru this thread

viewtopic.php?f=52&t=322

you DON,T WANT TO GET INTO DETONATION

http://www.sdsefi.com/meltdown.htm

http://www.popularhotrodding.com/tech/0 ... index.html

http://www.misterfixit.com/deton.htm

http://www.kennedysdynotune.com/Dynamic Compression Tech.htm

http://www.misterfixit.com/deton.htm

https://www.uempistons.com/index.php?main_page=index

HEY GRUMPYVETTE!
"I recently disassembled my engine because of rapidly dropping oil pressure, bearing clearances were larger than installed and bearing shells were loose in the main and rod caps and the crank was mildly scored,One very peculiar sight is the combustion chambers and exhaust ports which were polished when assembled,are rough and it feels , gritty almost like pool sand., and the Tops of pistons same way
No major damage to any of this, except the FROSTED appearance ,on the piston tops ,and combustion chamber surfaces, some it just scrapes off with fingernail. "


what your describing quite well is damage done or at least contributed too by DETONATION

if the edges of the piston lands look rounded off slightly and FROSTED that's the results of detonation, the rough stuff it micro sized bits of molten piston aluminum , that got transferred to the combustion chamber and exhaust port surfaces

btw heres typical detonation damage, and in this case, resulting from a bit of nitrous, that boosted the pressure, but the results would be similar on a high compression engine subjected to crappy fuel and high loads at high rpms without nitrous, notice the sugary/frosted appearance and rounded edges of the melted areas

Alright you guys got me good and confused the stock zz4 uses 10 to 1 cr and has very poor quench it use a .0501 head gasket and the pistons are down in the block .025.
so why is there no detonation danger here

its not that theres no detonation danger, its that its fairly LOW, because of the cam timing, wide 112 lsa and aluminum heads, combustion chamber design, etc that your running, theres no absolute compression level that will induce detonation, where if your at %5 lower your totally immune
fuel octane, air coolant and oil temps, spark advance and plug heat range all effect the results, when you see a chart like these below, they provide a good guide to keep you out of potential trouble, but each combos unique and has a different potential to cause detonation, simply slowing the ignition advance , and changing to a different plug heat range,and enriching the fuel/air ratio, is sometimes all thats required to run lets say a 8.8:1 dynamic compression in one combo, but a similar one may only tollerat a 8.4 dynamic compression with the same mods

 

[grumpyvette]

OK FACTS time, every guy wants to stick a killer turbo or supercharger on his current N/A engine combo at some time! forgetting that theres a differance in the way the engines make hp

an 11:1 cpr engine with 8 lbs of boost will effectively be running at about 14:1 compression ratio minimum and require about 115 octane fuel, a 12.5:1 cpr combo would be at 17:1 cpr with that same 8 lbs of boost, DETONATION tends to start at about 9:1 effective compression ratio with regular pump gas octane fuel

compressing air builds heat, higher heat increases detonation, even intercoolers won,t allow a 11:1 compression ratio to work at full efficiency without retarding the ignition timing and result in less potential power than a similar displacement engine combo could accheive with a lower static compression ratio

http://www.kennedysdynotune.com/Dynamic Compression Tech.htm

http://www.empirenet.com/pkelley2/DynamicCR.html

http://www.turbofast.com.au/TFcompB.html

http://forum.grumpysperformance.com/viewtopic.php?f=52&t=1343

http://forum.grumpysperformance.com/viewtopic.php?f=86&t=1106&p=2444&hilit=detonation#p2444

http://forum.grumpysperformance.com/viewtopic.php?f=44&t=937&p=1922&hilit=+detonation#p1922

http://forum.grumpysperformance.com/viewtopic.php?f=52&t=727&p=1024&hilit=+detonation+dynamic#p1024

viewtopic.php?f=86&t=1330

viewtopic.php?f=86&t=1330&p=2905#p2905

http://www.musclemustangfastfords.com/tech/mmfs_070022_ford_compression_ratio/index.html


yes the urge to slap a supercharger on that LT1-LT4 or similar high performance engine sure looks attractive, the adds say youll see a 40% instant boost in hp,
(don,t forget youll need a major software tuning upgrade and probably larger injectors, improved ignition,fuel system upgrades )on a basically stock motor , with thousands of miles of wear, already on the short block, by adding a supercharger and boosting the effectinve compression,and you DON,T THINK youll eventually damage the basically stock engine , with its very high effective dynamic compression, stock ring gaps and clearances ,and significantly increase the chance of detonation damage? while IM sure you could bolt it together, and get it running, your not looking a a long term dependable combo and anyone suggesting it will be durable long term is blowing smoke under your skirt, the 12 month warrantys on the supercharger NOT the engine!

LOOK IM not trying to kill your dreams IM trying to prevent you finding out the hard expensive way about what works and what won,t! and a superchargers a very GOOD IDEA if ...IF the engine its mounted on is DESIGNED from the bottom up to BE A BOOSTED ENGINE, but starting with a worn, high compression stock clearance engine is LOOKING for failure!

have a reputable shop like

http://www.ohiocrank.com/chev_sb_shortb.html

http://www.lewisracingengines.com/

build you an 8:1 or 9:1 cpr 383-396 with forged components and put the correctly designed SUPERCHARGER COMPATABLE roller cam in that short block with that supercharger,add a low restriction exhaust, and a large aluminum radiator, and its a totally differant deal, youll be all smiles

 

[guysmontess]

Although I didnt build my big block with any thought to dynamic cr,after reading these articles,I believe I am getting a grasp on the differences.It would appear that between 7.5 & 8.5 is the ideal dynamic # to look for.Using the supplied calculations,mine is right around 7.9.If I were to do it a bit different knowing what I know now,I would have a tad more static cr and a bit less quench.A static cr of 10.5 would give me a dynamic # of 8.22,and a gasket with a compressed thickness 0.003 less than what I have now would give me a quench height of .041.Oh well,maybe next time around...
Guy

 

[grumpyvette]

even if you had no idea about the concepts IN this thread before you built your engine combo, from what you stated you did much better than most people do, CONGRATULATIONS!

 

[guysmontess]

Thanks Grumpy,coming from you I consider that a compliment !! Just out of curiosity,if I had built my bbc with these figures,do you think there would have been much difference in HP ?? If so,maybe I would pull the motor next year.Could the heads be milled enough to produce an extra 1/2 point of compression ?? Also,even though I have heard the term for many years,I dont really understand the difference betewwn regular milling and angle milling.Would you mind explaining the difference ??
Guy

 

[grumpyvette]

I doubt youll see a significant change in power levels unless you change several components,
a change in quench and a slight increase in static compression of 1/2 a point alone using the same components should result in about a 2% gain, on a 400hp engine thats 7-8hp hardly worth the effort

http://en.wikipedia.org/wiki/Octane_rating

first youll want to read this carefully, as the octane number displayed on the gas pump varies a good deal, with the effective detonation resistance, of the fuel, between locations
example 100 octane rated fuel in western Europe is close to the same detonation resistance as 93 octane rated fuel in the USA, due to the difference in the methods used to rate the octane

read thru these links

lets assume your building a 454 chevy with edelbrock heads with 100cc combustion chambers and your looking to find the STATIC COMPRESSION RATIO

http://www.carcraft.com/techarticles/ca ... index.html

https://www.uempistons.com/index.php?ma ... alculators

http://www.csgnetwork.com/compcalc.html

http://www.circletrack.com/techarticles ... mples.html

viewtopic.php?f=55&t=2718&p=7057&hilit=octane+compression#p7057

viewtopic.php?f=52&t=727

ok, lets assume your cylinder head volume is 100cc
and your looking at use of a .032 thick head gasket with a 4.310 diam.
and your piston is .010 below the block deck when measured,
and a flat top piston is selected as a start point,
and your bore diam. is 4.281(.030 over)
and your using a stock 4" stroke crank
and stock rods
lets use the calculator above from KB
you see you have a 9.58:1 cpr

now lets assume you select three different pistons and compare results, with the only change being the piston dome or dish

the first has a 5cc valve clearance notch
which would result in a 9.2:1 cpr

the second piston choice has a 18cc dome
which would result in a 11.26:1 cpr


the third piston choice has a 20cc dish
which would result in a 8.26:1 cpr

 

[grumpyvette]

Based on your recommendation I bought a Crane #119661 cam for my HSR 383. However before I got to install it I got a killer deal on some LE ported TFS 195 heads. Because the new heads have 58cc chambers they are going to push my CR out to 12:1.
Will the above cam give me an acceptable DCR ; or should I be looking bigger like a Crane #119841 ( GM 847 cam ) same lift but 234 / 242 ?
Have 95 and 98 octane pump gas here.

Combo is all forged 383 HSR ,TFS 195's flow 274/190 @.500 .282/204 @.600.All the good gear;Pro Mag rollers, 30lb,etc.
A4 3K stall , 3.73
With the current out of the box TFS (11.2:1 CR) 195's was happily spinning to limiter set at 6500 with ZZ409 cam ,no problems
Your comments most appreciated.
Regards
Rod
Sydney
Australia


ok example time to make this concept easier to understand

first the GOAL on a street engine running common pump available octane levels, is a dynamic cpr of between 7.7:1 and about 8.5:1, but the lower range is preferable on a street car to provide some extra safety with bad gas and detonation

remember this chart

heres a link to a very useful chart for calculating your dynamic compression

http://www.crower.com/valve-timing-chart

lets look at your example using those two cams

the crane 119661 has a 230/238 dur. on a 112 LSA (LOBE SEPARATION ANGLE)

the crane 119841 has 234/242 dur. on a 112 LSA

use the chart above and we find that the intake valve on the 119661 closes at about 43 deg at .050 lift

use the chart above and we find that the intake valve on the 119841 closes at 45 deg at .050 lift


http://garage.grumpysperformance.co...-descriptions-dont-tell-you.12357/#post-61139

use that info and this calculator
add the 15 degrees it says to add to the .050 timing figures, before entering the data into the KB calculator, follow instructions on the others
http://www.kb-silvolite.com/calc.php?action=comp2

heres some different calculators if you want a more precise answer

http://www.projectpontiac.com/ppsite/co ... iew/16/30/

https://www.uempistons.com/index.php?ma ... alculators

http://www.wallaceracing.com/dynamic-cr.php

http://members.uia.net/pkelley2/DynamicCR.html

average the results..of all



119661 =10:1 DCR
119841=9.9:1 DCR

so obviously if you keep the same heads and pistons youll need more cam duration or a wider LSA to get to the intended DCR GOAL, a quick calculation puts the required valve close point at .050 lift at about 65-68 degs


or a cam like this




WITH THAT CURRENT OCTANE RANGE IN PUMP FUEL

OBVIOUSLY youll want to EITHER run octane booster or race gas with the current crane cams OR use different pistonsas that is generally the least expensive route that allows you to keep the expensive heads and current roller cam, to reduce the CPR (PROBABLY WHY THE HEADS WERE FOR SALE AT A GOOD PRICE,yes it a P.I.T.A. to get the rotating assembly re-balanced but its the least expensive option, because the more radical cam also requires a new rear gear ratio and higher stall converter.
so you either resell the heads, get new pistons or run a octane booster, or knock sensor that will retard timing significantly and an octane booster

viewtopic.php?f=55&t=613&hilit=+booster

and IF YOUR NOT AWARE IF IT, you can generally retard the cam timing 4-5 degrees and that drops the dynamic compression ratio results, and tends to move the whole torque curve about 200rpm higher.
aluminum heads and a good cooling system and a large baffled oil pan, oil cooler, trans cooler etc. will generally allow you to get away with DCR of up to about 8.9:1
ONE of the reasons I generally prefer to install cams (STRAIT UP) or split overlap, vs DOT-TO-DOT which is usually 4 degrees ADVANCED

viewtopic.php?f=52&t=90


IF you want to index the cam drive or a chain drive,theres a fast simple answer that takes a bit less effort and work.

http://www.romac.com.au/Std_&_Offset_Crank_Info.pdf

http://store.summitracing.com/partdetai ... toview=sku


you simply drill out the hole in the CAM timing gear that the cam pin inserts thru, then following the instructions carefully you insert the OFFSET bushing anfd then the bolts that hold the cam to the timing gear, it indexes the cam the desired degrees advance or retarded , depending on the bushing used and the location of the thick section on the bushing, then you install the cam button and retainer plate locking it solidly together.
NATURALLY YOULL CHECK THE RESULTS ON THE DEGREE WHEEL

http://store.summitracing.com/partdetai ... toview=sku


just be sure you verify all clearances
(piston to valve,
rocker to rocker stud,
rocker to retainers, etc. )
I think you made a good choice from what I see posted so far
Id install it strait up, IE no advance split overlap_

degree it in correctly and get it to last,cam install info,

IM frequently asked (why bother degreeing in a cam, the timing set comes with index marks to install it?) it basically comes down to, a question of do you accept a random install where the timing can be almost any place randomly or do you insist on maximizing the engines potential and know...

garage.grumpysperformance.com

checking piston to valve clearances

this is an old post I got asked to repost, naturally the first step is to install and correctly degree in your cam, https://garage.grumpysperformance.com/index.php?threads/calculate-required-octane-for-compression-ratio.2718/#post-50341...

garage.grumpysperformance.com

valve train clearances and problems

you might want to read thru these links and sub links it will help you avoid costly mistakes http://www.summitracing.com/parts/pro-66830/overview/ https://www.harborfreight.com/multipositional-magnetic-base-with-fine-adjustment-5645.html...

garage.grumpysperformance.com

sellecting valve springs, and setting up the valve train

How do you determine the spring pressure needed to keep the valves under control for a given lift, duration, and max rpm. It might take you several hours to read thru all the links and sub links but its time very well spent as it could save your engine from destruction and save you thousands of...

garage.grumpysperformance.com

rocker/push rod wear issues

Put vortec heads on my L98 and ordered 1417 1.52 Roller Rockers and 7808 Pushrods from CompCams. Installed screw in rocker studs with guide tool before assembly of the engine. Checked valve geometry and the roller left a mark right on the center of the valve like its supposed to. Oil pressure...

garage.grumpysperformance.com

http://garage.grumpysperformance.com/index.php?threads/how-to-read-a-cam-spec-card.1477/


yeah lots of info/reading but it potentially prevents several issues


http://garage.grumpysperformance.co...ng-cam-and-shifting-the-lca.10553/#post-44949

http://garage.grumpysperformance.com/index.php?threads/cam-degreeing.9010/#post-35474

http://garage.grumpysperformance.com/index.php?threads/precision-measuring-tools.1390/#post-12997

http://garage.grumpysperformance.co...hanics-of-adv-ret-a-camshaft.4532/#post-12050

http://garage.grumpysperformance.com/index.php?threads/degreeing-in-a-cam-correctly.3097/#post-8240

http://garage.grumpysperformance.com/index.php?threads/cam-degree-equipment-tools.1759/#post-4440

http://garage.grumpysperformance.co...rect-custom-length-pushrods.14241/#post-72355

 

[grumpyvette]

retarding the cam to the split overlap, location VS the factory location that is in most cases ADVANCED 4 degrees moves the whole torque curve up about 150rpm-170rpm, higher in the rpm band and reduces the effective dynamic compression ratio just a bit, now each combo will differ but thats a constant, that can be fairly well relied on.
remember the piston only starts compressing the cylinder volume once BOTH VALVES SEAT, so the full potential stroke and displacement is not used, to calculate the effective true working compression,so a change of a few degrees that delays the valve seating reduces the working compression at low engine speed, but as the rpms increase the inertia of the mass of air moving in the ports and exhaust scavenging tends to help fill the cylinders, more effectively, up to the point where the time becomes to limited to allow efficient flow past the valves.
remember that at 700rpm, your valve opens and closes, only about 6 times a second but by 6500rpm, its up to 54 times a second and that limits the flow



lets look at an example, heres a crane 114132
that I use frequently in mild builds

http://www.cranecams.com/?show=browsePa ... vl=2&prt=5

heres its factory dot-to-dot timing at 4 degrees advanced
TAPPET @.050
Lift: Opens Closes Max Lift Duration
Intake (4) ATDC 34 ABDC 109 210 °
Exhaust 47 BBDC (11) BTDC 119 216 °


heres an ISKY chart that shows piston location in the bore in degrees of crank rotation


http://www.iskycams.com/ART/techinfo/ncrank1.pdf

now lets look at the cam timing as it relates to piston location, as I stated earlier, the valve actually seats at roughly 13 degrees more duration than the .050 timing, so looking at the crane .050 timing of 34 degrees (47 degrees for valve seat timing ,(ABDC) we see the piston in a typical 350sbc with 5.7" rods is near 3.045" down the bore, but retarding it 4 degrees moves that to about 2.945" down the bore, and while thats hardly a huge change, that difference of about 0.100 inch will effect the dynamic compression and allow the engine to continue to fill its cylinders just a bit longer at high rpms.

now the rather obvious question, is... why do many of the cam company's set up their cams to be 4 degrees advanced to begin with?
well they know from long years of experience that the vast majority of guys purchasing cams will select cams with a bit more duration than they need, thinking "MORE IS BETTER" and advancing the cam timing tends to compensate,for that tendency and that the more knowledgeable guys will know how to degree in the cam to get the power curve that they want

 

[grumpyvette]

What else influences your car's octane requirements?

* Temperature: Hotter air and engine coolant increases your engine's octane requirements
* Altitude: Higher altitudes decrease your engine's octane requirements
* Humidity: Drier air increases your engine's octane requirements
* Engine spark timing: If your engine's spark timing is increased, the octane requirement increases
* Driving method: Rapid acceleration and heavy loading increase your octane requirement.

If your thinking of running pump fuel, the simple answer..
try to keep your dynamic compression ratio at 8:1,or lower, your intake air temp as low as possible,your oil temperature below about 220f and your coolant temp below about 190f and use 92-or higher octane fuel, and use an ignition system with a knock sensor if possible
READ THRU THE LINKS ITS WELL WORTH THE EFFORT

viewtopic.php?f=52&t=727

http://www.motorsportsracingfuels.com/O ... lator.html

http://chemistry.about.com/cs/howthings ... 70401a.htm

For a typical carbureted engine, without engine management [27,38]:-
Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %

Modern engines have improved significantly on this, and the changing fuel specifications and engine design should see more improvements, but significant gains may have to await improved engine materials and fuels.

Based on this information I extrapolated the following expansion of the octane chart

DCR Octane #
7.1 87.5
7.2 88.0
7.3 88.5
7.4 89.0
7.5 89.5
7.6 90.0
7.7 90.5
7.8 91.0
7.9 91.5
8.0 92.0
8.1 92.4
8.2 92.8
8.3 93.2

http://www.faqs.org/faqs/autos/gasoline ... ion-1.html

http://www.csgnetwork.com/octaneratecalc.html

http://www.anycalculator.com/octane.htm

http://www.torquecars.com/articles/fuel ... atings.php

viewtopic.php?f=53&t=726&p=5640&hilit=quench#p5640

http://www.chevyhiperformance.com/tech/ ... index.html

viewtopic.php?f=44&t=937&p=6449&hilit=+detonation#p6449

http://www.mr2.com/TEXT/gasoline_faq.txt

Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %


Read more: http://www.faqs.org/faqs/autos/gasoline ... z0cEhLg8Gn

viewtopic.php?f=52&t=727

http://en.wikipedia.org/wiki/Compression_ratio

http://en.wikipedia.org/wiki/Octane_rating

http://www.empirenet.com/pkelley2/DynamicCR.html

http://dnr.louisiana.gov/sec/EXECDIV/TE ... ns/b/b.htm

http://www.acl.co.nz/automotive-2/

http://www.corvettefever.com/techarticl ... atios.html

http://www.sacoriver.net/~red/uccr.html

http://victorylibrary.com/mopar/cam-tech-c.htm

viewtopic.php?f=52&t=1343&p=2942&hilit=+booster#p2942

http://www.hotrod.com/pitstop/hrdp_0706 ... index.html

http://www.serioussolutions.com/evo/octcalc.htm

 

[grumpyvette]

http://www.popularhotrodding.com/tech/0 ... index.html

http://en.wikipedia.org/wiki/Compression_ratio

Chevy V8 bore & stroke chart
I saw this online and figured I would post it.

from an old Hot Rod Tech article with this comment from Joe Sherman:

“Fully assembled, the engine's static compression ratio came in at 11.02:1. Yikes! Compression is good for making power if the engine doesn't get into detonation, but at first glance, this high a ratio seems excessive for an iron-headed engine on 91-octane. However, with the big-overlap cam, cranking compression was only 182-185 psi, well within Sherman's comfort zone. In his experience, anything less than 200 psi is permissible for running a small-block Chevy successfully on pump gas. And in fact, running 91-octane, the engine would make its best numbers with 36 degrees of total advance with no evidence of detonation using NGK UR6 plugs gapped at 0.039 inch.”

Two common, non-factory smallblock combinations:

377 = 4.155" x 3.48" (5.7" or 6.00" rod)
400 block and a 350 crank with "spacer" main bearings
383 = 4.030" x 3.75" (5.565" or 5.7" or 6.0" rod)
350 block and a 400 crank, main bearing crank journals
cut to 350 size

ALL production big blocks used a 6.135" length rod.
CHEVY BIG BLOCK V-8 BORE AND STROKE


366T = 3.935" x 3.76"
396 = 4.096" x 3.76"
402 = 4.125" x 3.76"
427 = 4.250" x 3.76"
427T = 4.250" x 3.76"
454 = 4.250" x 4.00"
477= 4.5" bore x 3.76" stroke
496 = 4.250" x 4.37" (2001 Vortec 8100, 8.1 liter)
502 = 4.466" x 4.00"
557T= 4.5 bore 4.375" stroke
572T = 4.560" x 4.375" (2003 "ZZ572" crate motors)

T = Tall Deck

ALL production big blocks used a 6.135" length rod.
its important to keep in mind that the piston can,t compress a darn thing UNTIL BOTH valves seat, so your cam timing effects the dynamic compression,ratio which is ALWAYS lower than the theoretical STATIC COMPRESSION ratio

[

Volumetric Efficiency: Is calculated by dividing the mass of air inducted into the cylinder between IVO and IVC divided by the mass of air that would fill the cylinder at atmospheric pressure (with the piston at BDC). Typical values range from 0.6 to 1.2, or 60% to 120%. Peak torque always occurs at the engine speed that produced the highest volumetric efficiency.
keep in mind as rpms increase so do port speeds and volumetric efficiency UP TO A POINT, WHERE THE TIME LIMITATION TO FILL AND SCAVENGE the cylinder limits power

COMPRESSION RATIO -vs- COMPRESSION PRESSURE


Compression Ratio as a term sounds very descriptive. Compression ratio by itself, however, is like torque without RPM or tire diameter without a gear ratio. Compression ratio is only useful when other factors accompany it. Compression pressure is what the engine actually sees. High compression pressure increases the tendency toward detonation, while low compression pressure reduces performance and economy. Compression pressure varies in an engine every time the throttle is moved. Valve size, engine RPM, cylinder head, manifold and cam design, carburetor size, altitude, fuel engine/air temperature and compression ratio all combine to determine compression pressure. Supercharging and turbocharging can drastically alter compression pressures.

The goal of most performance engine designs is to utilize the highest possible compression pressure without causing detonation or a detonation-related failure. A full understanding of the interrelationship between compression ratio, compression pressure, and detonation is essential if engine performance is to be optimized. Understanding compression pressure is especially important to the engine builder that builds to a rule book that specifies a fixed compression ratio. The rule book engine may be restricted to a 9:1 ratio, but is usually not restricted to a specific compression pressure. Optimized air flow and cam timing can make a 9:1 engine act like a 10:1 engine. Restrictor plate or limited size carburetor engines can often run compression ratios impractical for unlimited engines. A 15:1 engine breathing through a restrictor plate may see less compression pressure than an 11:l unrestricted engine. The restrictor plate reduces the air to the cylinder and limits the compression pressure and lowers the octane requirements of the engine at higher RPM.

At one time compression pressure above a true 8:1 was considered impractical. It still is in many cases with today's gas. The heat of compression, plus residual cylinder head and piston heat, initiated detonation when 8:1 was exceeded. Some of the 60's 11:1 factory compression ratio engines were 11:1 in ratio, but only 8:1 in compression pressure. The pressure was reduced by closing the intake valve late. The late closing, long duration intake caused the engine to back pump the air/fuel mix into the intake manifold at speeds below 4500 RPM. The long intake duration prevented excess compresson pressure up to 4500 RPM and improved high RPM operation. Above 4500 RPM detonation was not a serious problem because the air/fuel mix entering the cylinder was in a high state of activity and the higher RPM limited cylinder pressure due to the short time available for cylinder filling.

The following compression guide should be considered realistic for sea level operation. Cam timing and special applications can move the recommendations around some, but in most cases the following recommendations work.

PUMP GAS (regular)
8.5:1-Non-quench 2 valve head road use standard sedan, without knock sensor.

8.5:1- Quench head engine for tow service, motor home and truck with torque cam.

9.0:1- Street engine with proper .040" quench, 200° @ .050" lift cam, iron head, sea level operation.

9.5:1- Same as 9:1 except aluminum head used. Light vehicle and no towing.

10:1- Used and built as the 9.5:1 engine with more than 220° @ .050" lift cam.

10:1- 4" and smaller bore, high RPM cam, cold plugs, good fuel distribution, full power limited to 30 seconds W.O.T.

RACE GAS
13:1- is the highest compression ratio suggested with unrestricted gasoline engines.

13:1- Plus o'kay with restricted intake engines at high RPM.

ALCOHOL
15.5:1- is the highest compression ratio suggested for unrestricted alcohol fuel engines. Probably will lose power at 15.5:1 in most applications.

Satisfactory use of 14:l - 17:1 compression engines can be made when restrictor plate or small carburetor use are mandated by race sanctioning. High altitude reduces cylinder pressure, so if you only drive at high (above 4500') altitude, a 10:1 engine can often be substituted for a 9:1 compression engine. General compression rules can be violated, but it is usually a very special case, such as a 600 Hp normally aspirated engine in a 1500 lb. street car with a 12:1 compression ratio. The radical cam timing necessary for this level of performance keeps low and medium RPM cylinder pressure fairly low. At high RPM, detonation is less of a problem due to chamber turbulence, reduced cylinder fill time, and the fact that you just can't leave the above conbination turned on very long without serious non-engine related consequences.

Piston temperature and Hp are interrelated. High Hp per cubic inch engines not only make more Hp, but they make more heat. How the excess heat is handled has a significant effect on total engine power and longevity. Some engines have ceramic exhaust port insulation lines that allow cooler cylinder head operation, while keeping exhaust temperatures elevated for efficient catalytic converter operation. The same ceramic-type insulation on a piston top has not been quite so successful. Ceramic insulation on pistons can insulate the piston too well. The piston stays cool while the very top surface gets so hot that the intake air is immediately heated on contact with the piston. The heated intake charge expands and reduces the air flow into the cylinder.

On the compression cycle, the now over heated intake charge offers more resistance to being compressed and, because of the higher compression pressure and temperature, is more likely to detonate during combustion. Ideal piston temperatures in an operating engine would suggest refrigeration during the intake and compression stroke, and incandescence during the combustion and exhaust stroke. The advantage of a hot piston on the power stroke is that less combustion energy is going to be absorbed by the piston. So far, it is not practical to heat and refrigerate a piston 6000 times a minute. However, if the incoming air is not heated by the piston and the piston reflects the heat of combustion, you start to approach ideal conditions. A polished piston runs during the power stroke. A smooth polished piston runs cooler than a non-polished piston even after combustion deposits have turned both pistons black. A cool, smooth piston will transmit a minimum of heat to the incoming fuel air mix.

The KB Performance Piston line gives the racer a real "out of the box" advantage with smooth, diamond-turned piston heads. A polish is relatively easy to achieve and does improve the already excellent reflectivity of the KB Piston. If a buffing wheel is used on hypereutectic pistons, you will note a gray cast to the finished piston. The gray results from the exposure of the silicon particles that are dispersed throughtout the piston. Our forged pistons will polish like chrome.

Experimental work to reduce piston heating of the incoming fuel mix has been very limited, but in theory a thin coating may prove to be beneficial. A thin, smooth coating over a polished piston should still reflect combustion heat while reducing caron buildup and protecting the piston polish. It is easier for a thin film to change temperature with each engine cycle than it is for the whole piston to do the same. A thin film can be cooled by the first small percentage of inlet fuel mix, allowing the main quantity of fuel mix to remain relatively cool. Tests have shown that polishing the combustion chamber, valves and piston top can increase Hp and fuel economy by 6%. So far it has proven difficult to keep a coating on a polished piston.

All this polishing is counter to the practice of dimpling the combustion chamber. Dimpling has been shown to put wet flow back into the air flow and improve combustion. We do not recommend dimpling, but do suggest cutting a small discontinuity close to the valve seat to turbulate wet flow. Some bench flowed cylinder heads encourage fuel separaton at the inlet port. If a small step is added at the valve seat to force the wet flow over the resulting sharp edge, fuel will re-enter the air stream and give you the same affect as dimpling, only without losing the benefit of a completely polished chamber.

As you re-atomize wet flow you will improve combustion and most likely need to install leaner carburetor jets. Leaner jets compensate for the excess fuel that is available when we flow is put back into the air/fuel mix. Significant additional Hp gains can be had with careful attention to cylinder-to-cyliner fuel distribution by allowing all cylinders to be set "just right." More on this in the "Combustion Science and Theory" article.

Combustion chamber design work has increased steadily in the last ten years. Some of the work is mandated by the EPA and some is the result of race engine development. Some of the smog work has actually enhanced race engine development. Combustion chamber science is now focused on the effects of swirl, tumbling, shrouding of the valve, quench, flame travel, wet flow and spark location. A combustion chamber shaped dish piston can improve the flame travel in the combustion chamber, but may increase piston temperature. A new step dish allows the flame to travel further and expand more before it is stopped by a metal surface. This rapid flame travel makes it unnecessary to run big spark advance numbers. Ideally, we would like to be able to initiate ignition at top dead center since this would reduce negative torque in the engine that now results from spark advance. We are some time away from a practical spark ignition system that will make optimum power with a TDC setting. Don't go out and buy dished pistons for your non-supercharged open chamber 454. The advantage in flame travel is more than offset by the low compression ratio this combination yields. Small combustion chambers respond well to dished pistons, expecially reversed dome "D" cups and step dish. A 400 small block Chevy can use a 22cc D-cup piston and still have 10.4:1 compression. A trend in modern engine design to smaller combustion chambers with slightly recessed piston tops is resulting in more Hp per cubic inch with less fuel.

Ignition timing on most KB installations should be 30-34 degrees total with full mechanical advance dialed in. More advance may feel better off the line, but the engine lays down as the combustion chamber components come up to temperature. At the drag strip set timing for maximum MPH, not best ET. Even at max MPH (max Hp) you likely have 4 cylinders wanting more timing and 4 cylinders wanting less. We worry about the cylinders that want less. Too much spark advance will shorten the life of any engine, sometimes drastically.

John Erb
Chief Engineer
KB Pistons

 

[grumpyvette]

COMPRESSION RATIO -vs- COMPRESSION PRESSURE


Compression Ratio as a term sounds very descriptive. Compression ratio by itself, however, is like torque without RPM or tire diameter without a gear ratio. Compression ratio is only useful when other factors accompany it. Compression pressure is what the engine actually sees. High compression pressure increases the tendency toward detonation, while low compression pressure reduces performance and economy. Compression pressure varies in an engine every time the throttle is moved. Valve size, engine RPM, cylinder head, manifold and cam design, carburetor size, altitude, fuel engine/air temperature and compression ratio all combine to determine compression pressure. Supercharging and turbocharging can drastically alter compression pressures.

The goal of most performance engine designs is to utilize the highest possible compression pressure without causing detonation or a detonation-related failure. A full understanding of the interrelationship between compression ratio, compression pressure, and detonation is essential if engine performance is to be optimized. Understanding compression pressure is especially important to the engine builder that builds to a rule book that specifies a fixed compression ratio. The rule book engine may be restricted to a 9:1 ratio, but is usually not restricted to a specific compression pressure. Optimized air flow and cam timing can make a 9:1 engine act like a 10:1 engine. Restrictor plate or limited size carburetor engines can often run compression ratios impractical for unlimited engines. A 15:1 engine breathing through a restrictor plate may see less compression pressure than an 11:l unrestricted engine. The restrictor plate reduces the air to the cylinder and limits the compression pressure and lowers the octane requirements of the engine at higher RPM.

At one time compression pressure above a true 8:1 was considered impractical. It still is in many cases with today's gas. The heat of compression, plus residual cylinder head and piston heat, initiated detonation when 8:1 was exceeded. Some of the 60's 11:1 factory compression ratio engines were 11:1 in ratio, but only 8:1 in compression pressure. The pressure was reduced by closing the intake valve late. The late closing, long duration intake caused the engine to back pump the air/fuel mix into the intake manifold at speeds below 4500 RPM. The long intake duration prevented excess compresson pressure up to 4500 RPM and improved high RPM operation. Above 4500 RPM detonation was not a serious problem because the air/fuel mix entering the cylinder was in a high state of activity and the higher RPM limited cylinder pressure due to the short time available for cylinder filling.

The following compression guide should be considered realistic for sea level operation. Cam timing and special applications can move the recommendations around some, but in most cases the following recommendations work.

PUMP GAS (regular)
8.5:1-Non-quench 2 valve head road use standard sedan, without knock sensor.

8.5:1- Quench head engine for tow service, motor home and truck with torque cam.

9.0:1- Street engine with proper .040" quench, 200° @ .050" lift cam, iron head, sea level operation.

9.5:1- Same as 9:1 except aluminum head used. Light vehicle and no towing.

10:1- Used and built as the 9.5:1 engine with more than 220° @ .050" lift cam.

10:1- 4" and smaller bore, high RPM cam, cold plugs, good fuel distribution, full power limited to 30 seconds W.O.T.

RACE GAS
13:1- is the highest compression ratio suggested with unrestricted gasoline engines.

13:1- Plus o'kay with restricted intake engines at high RPM.

ALCOHOL
15.5:1- is the highest compression ratio suggested for unrestricted alcohol fuel engines. Probably will lose power at 15.5:1 in most applications.

Satisfactory use of 14:l - 17:1 compression engines can be made when restrictor plate or small carburetor use are mandated by race sanctioning. High altitude reduces cylinder pressure, so if you only drive at high (above 4500') altitude, a 10:1 engine can often be substituted for a 9:1 compression engine. General compression rules can be violated, but it is usually a very special case, such as a 600 Hp normally aspirated engine in a 1500 lb. street car with a 12:1 compression ratio. The radical cam timing necessary for this level of performance keeps low and medium RPM cylinder pressure fairly low. At high RPM, detonation is less of a problem due to chamber turbulence, reduced cylinder fill time, and the fact that you just can't leave the above conbination turned on very long without serious non-engine related consequences.

Piston temperature and Hp are interrelated. High Hp per cubic inch engines not only make more Hp, but they make more heat. How the excess heat is handled has a significant effect on total engine power and longevity. Some engines have ceramic exhaust port insulation lines that allow cooler cylinder head operation, while keeping exhaust temperatures elevated for efficient catalytic converter operation. The same ceramic-type insulation on a piston top has not been quite so successful. Ceramic insulation on pistons can insulate the piston too well. The piston stays cool while the very top surface gets so hot that the intake air is immediately heated on contact with the piston. The heated intake charge expands and reduces the air flow into the cylinder.

On the compression cycle, the now over heated intake charge offers more resistance to being compressed and, because of the higher compression pressure and temperature, is more likely to detonate during combustion. Ideal piston temperatures in an operating engine would suggest refrigeration during the intake and compression stroke, and incandescence during the combustion and exhaust stroke. The advantage of a hot piston on the power stroke is that less combustion energy is going to be absorbed by the piston. So far, it is not practical to heat and refrigerate a piston 6000 times a minute. However, if the incoming air is not heated by the piston and the piston reflects the heat of combustion, you start to approach ideal conditions. A polished piston runs during the power stroke. A smooth polished piston runs cooler than a non-polished piston even after combustion deposits have turned both pistons black. A cool, smooth piston will transmit a minimum of heat to the incoming fuel air mix.

The KB Performance Piston line gives the racer a real "out of the box" advantage with smooth, diamond-turned piston heads. A polish is relatively easy to achieve and does improve the already excellent reflectivity of the KB Piston. If a buffing wheel is used on hypereutectic pistons, you will note a gray cast to the finished piston. The gray results from the exposure of the silicon particles that are dispersed throughtout the piston. Our forged pistons will polish like chrome.

Experimental work to reduce piston heating of the incoming fuel mix has been very limited, but in theory a thin coating may prove to be beneficial. A thin, smooth coating over a polished piston should still reflect combustion heat while reducing caron buildup and protecting the piston polish. It is easier for a thin film to change temperature with each engine cycle than it is for the whole piston to do the same. A thin film can be cooled by the first small percentage of inlet fuel mix, allowing the main quantity of fuel mix to remain relatively cool. Tests have shown that polishing the combustion chamber, valves and piston top can increase Hp and fuel economy by 6%. So far it has proven difficult to keep a coating on a polished piston.

All this polishing is counter to the practice of dimpling the combustion chamber. Dimpling has been shown to put wet flow back into the air flow and improve combustion. We do not recommend dimpling, but do suggest cutting a small discontinuity close to the valve seat to turbulate wet flow. Some bench flowed cylinder heads encourage fuel separaton at the inlet port. If a small step is added at the valve seat to force the wet flow over the resulting sharp edge, fuel will re-enter the air stream and give you the same affect as dimpling, only without losing the benefit of a completely polished chamber.

As you re-atomize wet flow you will improve combustion and most likely need to install leaner carburetor jets. Leaner jets compensate for the excess fuel that is available when we flow is put back into the air/fuel mix. Significant additional Hp gains can be had with careful attention to cylinder-to-cyliner fuel distribution by allowing all cylinders to be set "just right." More on this in the "Combustion Science and Theory" article.

Combustion chamber design work has increased steadily in the last ten years. Some of the work is mandated by the EPA and some is the result of race engine development. Some of the smog work has actually enhanced race engine development. Combustion chamber science is now focused on the effects of swirl, tumbling, shrouding of the valve, quench, flame travel, wet flow and spark location. A combustion chamber shaped dish piston can improve the flame travel in the combustion chamber, but may increase piston temperature. A new step dish allows the flame to travel further and expand more before it is stopped by a metal surface. This rapid flame travel makes it unnecessary to run big spark advance numbers. Ideally, we would like to be able to initiate ignition at top dead center since this would reduce negative torque in the engine that now results from spark advance. We are some time away from a practical spark ignition system that will make optimum power with a TDC setting. Don't go out and buy dished pistons for your non-supercharged open chamber 454. The advantage in flame travel is more than offset by the low compression ratio this conbination yields. Small combustion chambers respond well to dished pistons, expecially reversed dome "D" cups and step dish. A 400 small block Chevy can use a 22cc D-cup piston and still have 10.4:1 compression. A trend in modern engine design to smaller combustion chambers with slightly recessed piston tops is resulting in more Hp per cubic inch with less fuel.

Ignition timing on most KB installations should be 30-34 degrees total with full mechanical advance dialed in. More advance may feel better off the line, but the engine lays down as the combustion chamber components come up to temerature. At the drag strip set timing for maximum MPH, not best ET. Even at max MPH (max Hp) you likely have 4 cylinders wanting more timing and 4 cylinders wanting less. We worry about the cylinders that want less. Too much spark advance will shorten the life of any engine, sometimes drastically.

John Erb
Chief Engineer
KB Pistons

 

[grumpyvette]

"Dynamic Compression Ratio Explained"



In our attempt to help our customers understand performance and what makes an engine produce power we are going to explain the concept of dynamic compression ratio (DCR). While seemingly esoteric, this is an essential concept in designing an engine for performance use.



The first thing to understand is that "compression ratio" (CR) as it is usually talked about is best termed "static compression ratio". This is a simple concept and represents the ratio of the swept volume of the cylinder (displacement) to the volume above the piston at top dead center (TDC). For example, if a hypothetical cylinder had a displacement of 450cc and a 50cc combustion chamber (plus volume over the piston crown to the head) the CR would be 500/50, or 10:1. If we were to mill the head so that the volume above the piston crown was decreased to 40cc, the CR would now be 490/40, or 12.25:1. Conversely, if we hogged the chamber out to 60cc, the CR would now be 510/60, or 8.5:1.



Everyone knows that high performance engines typically have higher compression ratios. Simply put, higher compression makes more hp. Higher CR also improves fuel efficiency and throttle response. So why not bump up the CR even further? Once CR exceeds a certain point, detonation will occur. Detonation kills power and it kills engine. The amount of compression a given engine can handle is determined by many factors. These include combustion chamber design, head material, use of combustion chamber coatings, etc. Once these mechanical aspects of the engine have been fixed, the main variable is fuel octane. Higher octane = more resistance to detonation and the ability to tolerate more compression.



The above brings up the question that is often on the mind of performance enthusiasts and engine builders: how high should my CR be? Even if you know all about your engine and have decided what fuel you are going to use, the question cannot be answered as phrased. Why? Because without reference to the camshaft specs, talking about (static) CR is next to meaningless!



How is this so? Well, think about the Otto cycle and how a four stroke engine works. The power stroke has been completed and the piston is heading up in the bore. The intake valve is closed and the exhaust valve is open. As the piston rises it is helping to push the spent combustion gasses out the exhaust port. The piston reaches TDC and starts back down. The exhaust valve closes and the intake valve opens. Fresh fuel and air are drawn into the cylinder. The piston reaches bottom dead enter (BDC) and starts back up. This is the critical point as far as understanding DCR. At BDC. the intake valve is still open. Consequently, even though the piston is rising up the bore, there is no compression actually occurring because of the open intake valve. Compression does not begin until the intake valve closes (IVC). Once IVC is reached, the air fuel mixture starts to compress. The ratio of the cylinder volume at IVC over the volume above the piston at TDC represents the dynamic compression ratio. The DCR is what the air fuel mixture actually "sees" and is what "counts", not the static CR. Because DCR is dependent upon IVC, cam specs have as much effect on DCR as does the mechanical specifications of the motor.



DCR is much lower than static CR. Most performance street and street/track motors have DCR in the range of 8-8.5:1. With typical cams, this translates into static CR in the 10.0-12.0:1 range. Higher than this, there may be detonation problems with pump gas. Engines with "small" cams will need a lower static CR to avoid detonation. Engines with "big" cams have a later IVC point and can tolerate a higher static CR. When race fuel is used, much higher DCR (and static CR) may be used because of the detonation resistance of the fuel. Of course, race motors also have much larger camshafts which is another reason they can get away with such high static CR, often in the 13-15:1 range.



Note: there is some confusion about use of the term "Dynamic Compression Ratio". Some people use it to refer to the characteristics of an engine combo running at high speed. In that case, the engines volumetric efficiency will have a major effect on cylinder pressure. In this case, a larger cam will increase cylinder pressure when within its' rev range. Thus, more power and more cylinder pressure will be created. We prefer to think of this concept as "cylinder pressure" to avoid confusion.

 

[grumpyvette]

I got asked if altitude effects your engines effective compression, well if youve ever been out in the mountains up 7000-10,000 plus feet you know how different it is when you go to do much that requires physical effort, the first time I went elk hunting it was a huge shock to find out that even minor physical effort ran you short of breath very quickly and your engines no different whe you gain altitude your air density drops and you have far less actual volume being compressed, the links below will help you get a better grasp on the situation ,so read thru them
thinner air density at altitude is also why engines need turbo chargers to operate at near full potential at upper altitudes, this is one area where EFI is vastly superior, as the oxygen sensor feed back keeps the fuel/air ratio consistent while carb jets, not changing flow rates, won,t do as well with changes in altitude
example
in WWII both the P38 lightning and P51 mustang first proto type engines either lacked turbos, or had turbos added when it became mandatory to operate at high altitudes ,once the turbos were added the power increase made a huge difference, in the case of the p51 they also changed from allison to merlin engines also



RELATED LINKS
http://www.wallaceracing.com/dynamic-cr.php

http://victorylibrary.com/mopar/cam-tech-c.htm

 

[grumpyvette]

Its important to get the cam timing, compression, and gearing to match the intended power range and fuel octane limitations, its a balance, higher compression generally produces a more efficient burn and more torque but also a greater tendency to get into detonation, and result in engine damage, longer cam lobe durations allow higher rpm levels, and better upper rpm power, but can cause low rpm torque and drive-ability to suffer.
Don,t be in a huge rush to buy parts, do the required calculations, do the research, first.
so you know what you need for the application your intending to build.
Its ridiculous, to rush out and buy a great set of heads, a new cam and a short block or rotating assembly ,and and the best intake you can find etc. only to find one or more of the components won,t work in the combo your planing to build, and that youll need to replace parts before you start or if youre really stubborn, of course you can, just slap parts together and find it runs like crap...EXPENSIVE CRAP!
all the factors can be matched but it takes some research and throwing parts together randomly is almost guaranteed to result in a less than effective combo.
btw Im going to point out that you can change the EFFECTIVE compression ratio your engine has a bit by changing the amount of air trapped in the cylinders when both valves seat, obviously changing heads to a design with a larger combustion chamber or flat top or dished pistons can be used to get a lower effective compression but both those options cost a good deal of money, smaller changes might be all thats required, this can be done by changing the valve timing or the combustion chamber volume, thus changes like selecting a thicker head gasket, that increases the effective combustion chamber volume or retarding the cam timing so the valves seat a few degrees later in relation to piston rotation, or selecting a cam with a wider LSA, I.E. moving from a 108 LSA to a 112 LSA in a similar duration cam, can all reduce the effective amount of trapped fuel air mix trapped when the valves seat.
I recently had a guy stop over with a 400 Pontiac engine that had the annoying habit of getting into detonation that you could hear rather easily if the owner used anything but premium fuel,and at times even with premium fuel, naturally this was a problem.
we checked his ignition advance and it was set at factory specs, I suggested he back off the timing 1-2 degrees as a test to see if that helped.
I ask what engine temps the car usually ran at and he said at about 190f-200f which is generally fine, if he goes with a 180f t-stat it might also help but generally its not as effective as smoothing the combustion chamber and retarding the cam.
I suggested he pull the heads, polish the combustion chambers surface by hand with #80 then #160 sand paper , and retard the cam 4 degrees, the combo of a smoother combustion chamber with less potential hot spots and having all the sharp edges contoured and smoother and the slightly larger size removing a small amount of material to do it and retarding the cams valve close point will generally reduce an engines tendency to get into detonation.
I looked at his spark plugs and measured his cylinder heads exhaust temps with my IR temp gun and the fuel air ratio and burn pattern looked reasonably good, I suggested modifying the ignition advance so it came in slower and did not reach full advance till about 800 rpm higher in the rpm range.
obviously he could richen the fuel/air mix just a bit also as this tends to reduce an engines tendency to get into detonation by reducing the burn temps slightly.
I suggested verifying the damper and timing tab TDC marks were really at TDC.
I suggested new spark plugs , be gaped at .045 and one step colder in heat range.

how to go about getting the desired compression ratio?

if youve read a bunch of the threads on this web site your sure to eventually have considered theres,
more than one route to build any engine to allow it to get to the desired combustion chamber, or true compression ratio.
http://garage.grumpysperformance.com/index.php?threads/dynamic-vs-static-compression.727/

https://www.summitracing.com/newsandevents/calcsandtools/compression-calculator

http://www.diamondracing.net/tools/

https://www.rbracing-rsr.com/compstaticcalc.html

most people will start out thinking about a flat top piston that at TDC is about at the block deck height.
but theres hundreds of different piston designes , some domed, some dished and theres a huge variation in combustion chamber and intake runner and valve sizes, and port flow can vary from the restrictive peanut port that just barely out flows the vortec sbc heads in stock form to bbc heads that easily flow well over 400 cfm.

seems simple enough! your piston is at BDC and its rotated to TDC, the bore diameter times the stroke is the cylinder volume,
that volume is compressed into the cylinder heads combustion chamber .
lets say were dealing with a 454 BBC your 454 displacement divided by 8 cylinder is approximately, 56.75 cubic inches per cylinder volume
combustion chambers are generally measure in cc's theres 16.3871 cc's in a cubic inch
so 56.75 x 16.3871=roughly 930 cc's being compressed into a combustion chamber volume. which will generally fall in the 96cc-123cc combustion chamber volume
remember open and closed chamber heads on THE BBC.

and yes the piston may or may not have calve notches that generally add 5cc-7cc more combustion chamber effective volume, ahh,
but theres a head gasket, between the block and heads, that can be anywhere from about .021-.060,
and the deck height and piston too deck height will be different and must be accurately measured to do the calculations.
and of course theres DOMED pistons in dozens of configurations.

TYPICAL FLAT TOP PISTON WITH ONE VALVE RELIEF NOTCH

TYPICAL DISHED PISTON USED TO LOWER EFFECTIVE COMPRESSION


http://garage.grumpysperformance.co...in-height-compression-height.5064/#post-66240

http://garage.grumpysperformance.co...in-height-compression-height.5064/#post-66240





THESE PICTURES BELOW ARE NOT PONTIAC HEADS BUT GIVE YOU SOME IDEA OF WHAT IM SUGGESTING,
polishing the combustion chambers and smoothing contours tends to reduce detonation and improve power

BEFORE
AFTER

MAKE SURE YOU POLISH AND CC MATCH THE CHAMBERS

RELATED INFO/THREADS

http://www.iskycams.com/techinfo_index.html

http://www.steigerperformance.com/products/sp90005.html

http://www.iskycams.com/techinfo_index.html

viewtopic.php?f=52&t=4299

http://zhome.com/ZCMnL/PICS/detonation/detonation.html

viewtopic.php?f=52&t=727&p=1024&hilit=burret#p1024

viewtopic.php?f=70&t=1809

viewtopic.php?f=55&t=109

viewtopic.php?f=70&t=202

viewtopic.php?f=52&t=8460&p=29682&hilit=angle+curtain+seat#p29682

viewtopic.php?f=69&t=8540&p=30192&hilit=angle+curtain+seat#p30192

viewtopic.php?f=52&t=2630&p=13142&hilit=angle+curtain+seat#p13142

viewtopic.php?f=70&t=4683

viewtopic.php?f=52&t=2077&p=5565&hilit=burret#p5565

http://garage.grumpysperformance.co...ber-of-people-that-don-t-use-resources.12125/

 

[87vette81big]

Grumpy what is The Algorithm mathetical Long Hand Formula Keith Black UMP & Otherss using for Dynamic Compression ratio calculators ?
What VE Are they assuming ?
How do they acount for different combustion chamber designs ?
What ambient air temps assumed & humidity levels ?
What engine working loads in percentage ?
10-30% ?
80-100% ?
Low Rpms ? 900-3,000 ?
High rpms 5-8k ?
Aluminum heads assumed used ?

 

[grumpyvette]

I don,t have a clue what they use, but heres a fairly good detailed explanation

http://cochise.uia.net/pkelley2/DynamicCR.html

HUGES ENGINES
#15---What affects my cylinder pressure?




What affects compression ratio and cranking cylinder pressure.

Compression ratio has a great effect on engine performance, but cylinder pressure has an even greater influence on power output. Cylinder pressure is what you get when you install a pressure gauge in the spark plug holes and spin the crankshaft with the starter. This cylinder cranking pressure number is the result of several important engine components.

1.Compression ratio: a higher ratio = higher pressure, and a lower ratio lowers the cylinder pressure.

2.Altitude: As the altitude gets higher, the air pressure drops. That is why airplanes pressurize the cabins (to keep yourpressure up). The altitude effect on an engine is a gradual condition. It can have a small effect around 1000’ – 1400’. And from 2500’ and up you must find some way to offset the pressure loss. The easiest way is to raise the compression ratio. We have customers who run 14:1 with 91 octane gas at higher altitudes.

3.Cylinder heads: Cylinder heads made of aluminum dissipate (for those who think big government is a good thing, dissipate means to make disappear or vanish—like taxes and liberties) more heat than cast iron heads. Less heat means less pressure in the combustion chamber. The effect on power is to reduce it approximately the same as lowering the compression ratio about 1 ½ points. Some “experts” will tell you that you “can” raise your compression ratio to make up for the heat/pressure loss when using aluminum heads. I’m saying you “better” raise it or you are giving HP away.

4.Camshaft: Actually camshaft size, which is based on duration, when the valves open and close, not lift. The compression stroke begins when the intake valve closes. The bigger the cam, the later, past bottom dead center, the valve closes and the compression stroke starts. Later closing means a shorter effective compression stroke. The shorter the compression stroke, the less volume is compressed and the lower the cylinder pressure will be. This is why big cams need high compression ratios to make cylinder pressure and power.

A discussion of cylinder pressure is not complete without addressing octane ratings. The subject of compression ratio and fuel octane comes up more when dealing with customers who want to run pump gas rather than with customers running race gas.

The octane number tells you how much cylinder pressure the gas can handle without detonating. That detonation is not a bad thing. It is a very bad thing. Can you say “the motor blowed up”? You not only can say it, you can do it with detonation. Cast iron heads can tolerate somewhere around 165# of cylinder pressure with 93 octane gas. Switch to aluminum heads and the cylinder pressure can go to 195# with 93 octane, both at sea level. There are ways to build your engine to tolerate more cylinder pressure, such as very tight quench heights, with the same 93 octane gas, but generally the above numbers are safe.

Now, based on the preceding information, it looks like it is a juggling act when putting an engine together to get all the power you are paying for, right? Exactamondo grasshopper!

An 11 ½:1 compression ratio pump gas street engine at sea level is not uncommon, and that is with a warm street cam and aluminum heads. With a 250º @ .050 cam and aluminum heads you may be getting to 12 ½:1 compression ratio area.

When we design engines, short blocks or engine kits for our customer, we take all these factors into account to get the most “bang for the buck” for each customer, regardless of how much they complain. This is because they read in ______ magazine where every engine on pump gas should be 9 ½:1 compression ratio. But that’s not for our customers.

 

[87vette81big]

Skim read fast Grumpy. Gave basic program values.
Read in depth later today.
Lots of arguements on other boards what dynamic compression calculator is best but no proof.
I learned to check with a Compression Guage with all 8 spark plugs out.
Engine up to operating temp.
Record peak cranking compression using starter motor.
There used to be charts to go by .
Determine pump gas safe or not.
Goes back 20 years now.

 

[87vette81big]

Its actually Dynamic Stroke that matters in the online dynamic compression calculators Grumpy.
It can be longhanded calculated on paper.
Nice Find.
I will calculate after dinner with the Olds 425 & a Few Poncho Engines.

I assume all these online dynamic compression ratio calculators assume Aluminum heads used.
Ideal water temps of 170F or less.
Ambient dry bulb air factor of 65-85 F.
VE is anyone's guess.
Iron heads give an engine better thermal efficiency as long as octane requirements are met at all times. Only guys that use Iron heads still are NHRA Super Stock Musclecar racers & Entry level Street Stock Dirt Tracks. Class Rules.
A few BBC Guys use iron heads yet still with good sucess at 900 HP 540ci levels.