Compression

[Shane oakhill]

hi grumpy and crew,
I have a question about my alleged compression in my 400 sbc. I was told that the final static compression was 10.4.1, I am trying to work out how they achieved this with a -21cc piston and a 64 cc head?the block was zero decked. The srp catalogue says I should be around 9.7 but know that can be manipulated higher or lower.
Cheers shane

 

[Grumpy]

http://garage.grumpysperformance.com/index.php?threads/dynamic-vs-static-compression.727/

keep in mind the piston can,t compress anything until both valves seat, and seal
you might have a 383 with a listed stroke of 3.75" but the valves don,t seat with the cam below, until the piston is almost 1/2 the distance up toward the cylinder head on the compression stroke

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

valves don,t close at BDC, you need to read the cam card for the specs
heres a typical cam card
look at the cam card posted below
notice the intake valve seats near 82 DEGREES AFTER BOTTOM DEAD CENTER
notice the exhaust valve seats near 46 DEGREES AFTER top DEAD CENTER

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

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

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

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/

 

[Strictly Attitude]

10.4:1 sounds like the static ratio but some easy math tells the story and gasket thickness helps

 

[Loves302Chevy]

hi grumpy and crew,
I have a question about my alleged compression in my 400 sbc. I was told that the final static compression was 10.4.1, I am trying to work out how they achieved this with a -21cc piston and a 64 cc head?the block was zero decked. The srp catalogue says I should be around 9.7 but know that can be manipulated higher or lower.
Cheers shane

I used the Wallace calculator and came up with 10.29:1 with a .030 overbore and a .021" thick head gasket that has a bore diameter of 4.160".

To come up with the 9.7:1 figure, 70cc combustion chambers are needed.

 

[Loves302Chevy]

Grumpy, looking at the ISKY chart you posted above, it seems to me that the debate regarding long rods vs. short rods is almost inconsequential. The chart does not say what stroke was used, but the .300" difference between 5.7" and 6.0" rods is negligible at TDC.
From 35 degrees before to 35 degrees after TDC, there is only a .005" difference on either side.
So that is a total of only .010" for 70 degrees of rotation.

 

[Strictly Attitude]

Side loading long rod and .300 in an engine is a mile so is .01 think about it .01 is enough to need a new piston or different bearings in a land where tolerances are aimed in the .004 to .006 land that only is huge if your bore is out that far it could take .030 bore to straiten that out.

 

[Grumpy]

ITS more about the difference in dwell time at TDC between 5.7" and 6" connecting rods,
and the difference in side loading, neither case shows a huge advantage , but remember max piston speed on a 383 will be near 4200 fpm,or about 6700 rpm with high quality aftermarket components like SCAT 7/16" ARP rod bolt rods

now you can look here
http://www.rustpuppy.org/rodstudy.htm

Piston movement was computed by simulating the crankshaft/connecting rod/piston assembly in several precise engineering drawings (DesignCad) and then determining the exact amount of piston movement for each of 256 divisions of one rotation.



The piston movement data was then used as an input vector in a MathCad program to calculate velocity, acceleration, and dynamic forces.



The simulation of an infinitely long connecting rod, which imparts true harmonic motion to the piston, is the starting point.



The motion generated by a finite length connecting rod is quite distorted by comparison. It has much more velocity and acceleration at the top of the stroke compared to the bottom. A graph of the movement is peaked at the top of each cycle and rounded and flattened at the bottom. This is caused by the rod angle increasing and pulling the piston down and adding to the motion caused by the crankshaft rotating down from top dead center. At the bottom as the rod journal slows the angle decreases. This retards the movement of the piston by subtracting the rod angle component that was added at the top of the stroke from the crankshaft movement component at the bottom of the stroke.



Compression and combustion pressures are in opposition to the inertial forces so the top of exhaust and intake strokes generate the largest forces on the rod.





1) Maximum Piston Acceleration



This table is for a 3.75" stroke used in a 400 0r 383 small block Chevy engine.

------infinite rod--------6.0" rod---5.7" rod---5.565" rod

5000rpm 1332G 1749G 1776G 1790G

6000rpm 1933G 2525G 2558G 2578G

7000rpm 2631G 3437G 3482G 3509G



Percent difference due to rod length in above table.

Difference between 6" rod and 5.565" rod 2.34%

Difference between 6" rod and 5.7" rod 1.54%

Difference between 5.7" rod and 5.565" rod 0.79%



This table is for a 3.48" stroke used in a 350 or 305 small block Chevy engine.

------infinite rod---------6.0" rod---5.7" rod

5000rpm 1240G 1600G 1623G

6000rpm 1786G 2305G 2338G

7000rpm 2432G 3138G 3182G





2) Maximum Connecting Rod Dynamic Load (Tension)



This table is for a 3.75" stroke used in a 400 or 383 small block Chevy engine. The forces are based on the weight of the piston and pin assembly and do not include the percentage of force generated by the acceleration of the end of the connecting rod. The reference piston is the stock replacement Silv-O-Lite piston for a 400 engine.



------infinite rod-----------6.0" rod-----5.7" rod----5.565" rod

5000rpm 2249LBS 2938LBS 2976LBS 3000LBS

6000rpm 3239LBS 4232LBS 4287LBS 4320LBS

7000rpm 4409LBS 5769LBS 5834LBS 5849LBS



Percent difference due to rod length in above table.



Difference between 6" rod and 5.565" rod 2.34%

Difference between 6" rod and 5.7" rod 1.54%

Difference between 5.7" rod and 5.565" rod 0.79%





3) Maximum Rod Angularity



This is the angle the connecting rod makes with the axis of the cylinder bore at 90 degrees after top dead center (maximum excursion from bore axis. This measurement is for the 3.75" stroke of the 400 and 383 only.



6.0" rod-----18.21 degrees

5.7" rod-----19.20 degrees

5.565" rod-19.69 degrees





4) Cylinder Wall Load



Percentage of compression and combustion force against the top of piston transmitted to the major thrust face of the piston and then to the cylinder wall.



This table is for the 3.75" stroke.

6.0" rod----32.89%

5.7" rod----34.83%

5.565" rod-35.64%



This table is for the 3.48" stroke.

6.0" rod---30.31%

5.7" rod---32.05%





5) Piston Speed



Maximum piston speed for the 3.75" stroke at 5000 rpm.



Infinite rod---81.68 feet per second, 55.69 MPH

6.0" rod------85.64 feet per second, 58.4 MPH

5.7" rod------86.01 feet per second, 58.6 MPH

5.565" rod---86.20 feet per second, 58.8 MPH





6) Effective Stroke



Because of the mechanical advantage provide by the toggling effect of the rod the shorter rods act as if they were in a longer stroke engine at the top of the stroke. This effect would make the short rod engine rev faster from 2000 to 4000 rpm and the circle track people claim that acceleration out of the turns is significantly improved with the shorter rod. In all other factors the longer rod comes out superior...



Effective stroke as compared to the infinite rod model for the 3.75" stroke.



infinite rod-=- 3.75"

6.0" rod------- 4.20"

5.7" rod------- 4.23"

5.565" rod---- 4.25"



Note that the differences are subtle...





7) Dwell Time



This measurement is of the number of crankshaft degrees the piston is within 0.250 inches of top dead center. It is the subject of much conjecture and controversy in the automotive literature.



This table is for a 3.75" stroke used in a 400 0r 383 small block Chevy engine.



Infinite rod---59.853 degrees

6.0" rod------52.397 degrees

5.7" rod------52.071 degrees

5.565" rod---51.915 degrees



Percentage difference in dwell time between the 6.0" rod and the 5.7" rod is 0.626%.



Percentage difference in dwell time between the 5.7" rod and the 5.565" rod is 0.3%.



Percentage difference in dwell time between the 6.0" rod and the 5.565" rod is 0.928%. (Still less than 1 percent)





This table is for a 3.48" stroke used in a 350 or 305 small block Chevy engine.



Infinite rod---62.188 degrees

6.0" rod------54.929 degrees

5.7" rod------54.605 degrees



Percentage difference in dwell time between the 6.0" rod and the 5.7" rod is 0.593% at the 3.48" stroke.

at first the difference in listed amounts seem nearly meaningless,
but the piston changing directions at nearly 110 times PER SECOND
means the minor differences in stress compound rather quickly and over time theres a measurable reduction in stress and a minor but measurable advantage to the longer rods.
the 1/2%-to-2% difference in the lower stress or slightly longer dwell, etc. due to the longer connecting rod length, and lower side load, longer dwell time and effective use of cylinder pressure per degree of rotation, the longer rod length gains you adds up over time ,
the piston changing directions at nearly 110 times PER SECOND
means the minor differences in stress compound rather quickly, if it was only a .025 effective improvement, times 110 rotations per second thats a 2.75% advantage per second and in theory a noticeable advantage in a 10 second 1/4 mile acceleration contest to say nothing of a 500 lap race!

remember theres almost no effective pressure forcing the piston down on the power stroke after about 35 degrees past TDC , so your power is derived during less than 5% of the engines rotation,(remember theres 720 degrees in a cycle )

related
http://garage.grumpysperformance.co...cranking-pressure-calculator.4458/#post-61262

http://garage.grumpysperformance.co...sure-hurting-your-combo.495/page-2#post-56504

http://garage.grumpysperformance.co...ng-combustion-chambers.2630/page-2#post-54344

http://garage.grumpysperformance.co...ing-parts-and-a-logical-plan.7722/#post-51341

http://garage.grumpysperformance.co...cranking-pressure-calculator.4458/#post-50454

 

[Loves302Chevy]

Thanks. I read the article.

7) Dwell Time

This measurement is of the number of crankshaft degrees the piston is within 0.250 inches of top dead center. It is the subject of much conjecture and controversy in the automotive literature.


This table is for a 3.75" stroke used in a 400 0r 383 small block Chevy engine.

Infinite rod---59.853 degrees

6.0" rod------52.397 degrees

5.7" rod------52.071 degrees

5.565" rod---51.915 degrees


Percentage difference in dwell time between the 6.0" rod and the 5.7" rod is 0.626%.

Percentage difference in dwell time between the 5.7" rod and the 5.565" rod is 0.3%.


Percentage difference in dwell time between the 6.0" rod and the 5.565" rod is 0.928%. (Still less than 1 percent)




This table is for a 3.48" stroke used in a 350 or 305 small block Chevy engine.

Infinite rod---62.188 degrees

6.0" rod------54.929 degrees

5.7" rod------54.605 degrees


Percentage difference in dwell time between the 6.0" rod and the 5.7" rod is 0.593% at the 3.48" stroke.


8) Author's comments:


The data in this report seems to indicate that the differences between the rod lengths are exaggerated in the literature. In many (most) cases claims are anecdotal and represent the vested interests of the suppliers. I have seen no objective dyno testing of rod lengths but keep hoping for one.


There are real gains to be had by going to longer rods but they are small, usually a lot less than 2 percent. However, the hard-core racers are grasping at every tiny bit of performance and can justify the expense. For the more average rodder I would suggest staying with the rod length specified by the factory. Money would be far better spent on improving the heads, cam, and induction and exhaust systems. (and perhaps a supercharger..)

Still nearly inconsequential. Only .626% for the 3.75" stroke, even less for the 3.48" - .593%.
But, that small percentage could translate into longer engine life due to less stresses.
For my money, I would run the longer rod just to be able to have a lighter piston.
Lighter reciprocating weight definitely translates into less stress particularly when multiplied by high RPMs.

Chevy V8 bore & stroke chart

Chevy V8 Crankshaft Journal Sizes

Here's a list of Chevy V-8 crankshaft journal sizes. All journal sizes are given in "STANDARD" sizes. Your crankshaft may have been cut down in size previously by a machine shop. Make sure your crank will work in the block you have. Blocks were made for each crank main journal size. If you are putting a "small" or "medium" journal smallblock crank into a "medium" or "large" journal smallblock block you will need crank bearing "spacers" or use special "thick" bearings available from aftermarket suppliers.

Chevy Smallblock V8 Crankshaft Journal Sizes


Gen.I, "Small Journal"
265...Mains-2.30"-Rods-2.00"
283...Mains-2.30"-Rods-2.00"
302...Mains-2.30"-Rods-2.00"
327...Mains-2.30"-Rods-2.00"



Gen.I, "Medium Journal", includes "Vortec" 305 and 350 thru '98
262...Mains-2.45"-Rods-2.10"
267...Mains-2.45"-Rods-2.10"
302...Mains-2.45"-Rods-2.10"
305...Mains-2.45"-Rods-2.10"
307...Mains-2.45"-Rods-2.10"
327...Mains-2.45"-Rods-2.10"
350...Mains-2.45"-Rods-2.10"



Gen.I, "Large Journal"
400...Mains-2.65"-rods-2.10"



Non-production Gen.I combination, using Gen.I 400 crank in Gen.I 350 block
383...400 crank, Mains cut to 2.45"-Rods-2.10"



Non-production Gen.I combination, using Gen.I 350 crank in Gen.I 400 block
377..."Spacer" or "thick" main bearings with 350 crank-Rods-2.10"



Gen.II, "Medium Journal", includes "L-99" 265, "LT-1" 350, "LT-4" 350
265...Mains-2.45"-rods-2.10"
305...Mains-2.45"-Rods-2.10"
350...Mains-2.45"-Rods-2.10"



Non-production Gen.II combination, using Gen.II 265 "L-99" crank in Gen.II 350 block
302...Mains-2.45"-Rods-2.10"



Gen.III, includes '97-2005 "LS-1" Corvette, Firebird, Camaro
345...Mains-2.558"-Rods-2.10"



Corvette "ZR-1", DOHC, "LT-5"
350...Mains-2.76"-Rods-2.10"


CID BORE STROKE
262 = 3.671" x 3.10" (Gen. I, 5.7" rod)
265 = 3.750" x 3.00" ('55-'57 Gen.I, 5.7" rod)
265 = 3.750" x 3.00" ('94-'96 Gen.II, 4.3 liter V-8 "L99", 5.94" rod)
267 = 3.500" x 3.48" (Gen.I, 5.7" rod)
283 = 3.875" x 3.00" (Gen.I, 5.7" rod)
293 = 3.779" x 3.27" ('99-later, Gen.III, "LR4" 4.8 Liter Vortec, 6.278" rod)
302 = 4.000" x 3.00" (Gen.I, 5.7" rod)
305 = 3.736" x 3.48" (Gen.I, 5.7" rod)
307 = 3.875" x 3.25" (Gen.I, 5.7" rod)
325 = 3.779" x 3.622" ('99-later, Gen.III, "LM7", "LS4 front wheel drive V-8" 5.3 Liter Vortec, 6.098" rod)
327 = 4.000" x 3.25" (Gen.I, 5.7" rod)
345 = 3.893" x 3.622" ('97-later, Gen.III, "LS1", 6.098" rod)
350 = 4.000" x 3.48" (Gen.I, 5.7" rod)
350 = 4.000" x 3.48" ('96-'01, Gen. I, Vortec, 5.7" rod)
350 = 3.900" x 3.66" ('89-'95, "LT5", in "ZR1" Corvette 32-valve DOHC, 5.74" rod)
364 = 4.000" x 3.622" ('99-later, Gen.III, "LS2", "LQ4" 6.0 Liter Vortec, 6.098" rod)
376 = 4.065" x 3.622" (2007-later, Gen. IV, "L92", Cadillac Escalade, GMC Yukon)
383 = 4.000" x 3.80" ('00, "HT 383", Gen.I truck crate motor, 5.7" rod)
400 = 4.125" x 3.75" (Gen.I, 5.565" rod)
427 = 4.125" x 4.00" (2006 Gen.IV, LS7 SBC, titanium rods)

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.