detonation issues???

Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member
Hi guys, I NEED some advise here... I have a 350 sbc motor that has been bored out 40 over, 6" piston rods, dished out piston heads at 20cc, 3.750 stroke crankshaft, my deck clearance is -.006, Trick flow 175 cylinder head chamber size is 56cc, mild Elgin 923 cam, and an .042 head gasket... I run these numbers on a compression ratio calculator and I come up with around 10 to 10:1.1 compression ratio... I performed a compression test on all 8 cylinders and I am getting numbers between 210-235lbs of pressure between all 8 cylinders. My question is, should I be concerned about that much compression on a 383 stroker running on pump gas??? Seems like I should be around 200 lbs per cylinder... This is NOT a race car what so ever, its a street car daily driver with some power that I desire... Any and all answers of help would be VERY much appreciated...

https://www.memoparts.com/img/cms/Documents/Piston Failue.pdf
your piston compresses nothing until both valves seat,
then the difference in volume above the piston, as it moves upward after bdc, as it continues to move up toward tdc between the piston at that point valves seat up to tdc where the volume trapped in that area defined by the recess or dome in the piston, the thickness of the compressed head gasket, the distance the piston is above or below the block deck and the combustion chamber volume will be the area the compressed gases are forced into.
obviously you have hundreds of options and potential combos, every component choice you make
will effect the other potential choices.
your current listed combo is near 10.2:1 static compression
your current Elgin 923 cam specs are noticeably too mild for that higher compression ratio 383 sbc

  • Valve Lift .421/.444
  • Advertised Duration 278/288
  • Duration At .050" Lift 204/214
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if your experiencing detonation issues that are cured by swapping to higher octane rated fuel, and you would prefer to use the lower octane , less expensive fuel, you should adjust your cars ignition advance combination , so that its advance curve has either less initial timing, or delaying the mechanical advance vs. rpm with some stiffer springs, or a combination of both might reduce the pinging under load at 2500-3500 rpm where its most commonly seen,. Does this detonation or pinging, only occur at WOT? If not, limiting the vacuum advance with a stop, or using an adjustable vacuum advance unit and raising the amount of vacuum required vs. the amount of vacuum advance might be warranted also and installing a lower temp rated t-stat and adjusting the engine fuel/air ratio a bit richer may also help..


I would have asked about your car weight trans gearing rear differential gearing and converter stall speed, before suggesting a cam correctly matching your application, for a daily transport, but generally a mild 383 sbc designed for daily transportation is generally matched to a cam with about 210-220 degrees of intake duration, @.050 lift and a 110-112 LSA and a 9:1-9.3:1 static compression, aluminum heads with a 72-76cc combustion chamber and a 190cc-200cc intake port size, 1.5/8" long tube headers prefered but 1.3/4" long tube will help.
a popular choice would be the brodix 180cc-200cc or similar, Dart, profiler edelbrock or summit, or trick flow heads
1.6:1 roller rockers matched too the correct springs and correct clearances generally helps, a 2600-rpm -3000-rpm stall converter, and a 3.36:1-3.73:1 rear gear with an over drive transmission on a daily driver is a good compromise in power, mileage and performance.
the guys have a point, best power and in many cases durability, is generally found at nearer 12.7:1 f/a ratio, I don,t ever remember your engine getting near that lean while we discussed getting it tuned up?
yeah I know a few of you would rather gargle broken glass scrap than read links and sub-links but trust me if I tell you in the long term, youll gain a wealth of info you need to use to build an exceptional and durable engine, the secret is mostly in getting each component working to its maximum efficiency and in maximizing durability, you won,t win many races or enjoy owning the car if it spends most of its life being repaired or waiting for replacement components to arive, and in many cases simply thinking things through, and selecting the best quality matched components you can afford, and carefully installing them with the correct clearances and lubrication and cooling goes a long way to reaching that goal.
if you, as the engine builder, have a choice.
ID suggest you always give up that 5% in peak power potential,you might get by running on the ragged edge , if you can gain 10%-20% increased durability, by not pushing things to just at the point the parts are likely to fail, and knowing that point takes either experience or knowledge gained by watching others fail. and in many cases thats an option if you fully understand exactly how and why things are intended to function.
THINK THINGS THROUGH!
porting that intake port wall paper thin , pushing the rpm's you your constantly bouncing the valve train into valve float, or not having consistent oil flow on the critical components might seem like a route to gain an edge in power needed to win races, its much more likely to see you drain your checking account trying to do expensive repair work when components get pushed to the point of catastrophic failure

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well I'll assume you previously read the links on engine building,and quench, and you have at least tried to build a well balanced combo with reasonable quench,and tried to match the cam duration and lsa to the engines compression and intended power range, and you selected a fairly well matched cam timing and reasonable compression, but at this point in the tuning ,your still having indications your getting into detonation.
keep in mind that keeping reasonably consistent and as low as practicable , combustion chamber temps are a huge factor in avoiding detonation issues, having an auxiliary oil cooler and a trans fluid cooler with a powered fan, and the proper fuel/air ratio and ignition advance curve along with matching your cars engine dynamic compression ratio to the available fuel octane can go a long way toward avoiding detonation issu
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so what your trying to do is reduce combustion chamber temps,increase your fuels octane, reduce your fuel/air temperatures, or reduce your combustion chamber pressure or speed up combustion

http://garage.grumpysperformance.com/index.php?threads/carb-tuning-info-and-links.109/


something like this on the milder side
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something like this on the wilder side
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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
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yes your very likely to see detonation issues with that much 210-235 lbs compression psi. if you run into crap quality fuel, and it will require high test octane in warmer weather the 56cc combustion chamber is too small for a 383, you should be using a 75cc with that combo if you intend to run pump octane gas
your current listed combo is near 10.2:1 static compression
you should be closer to 9:1 static compression, to use crappy pump octane fuel to avoid detonation issues

https://speedmaster79.com/tools/engine-compression-ratio/
http://www.wallaceracing.com/dynamic-cr.php

you would need a cam with about 68 degrees of intake valve seat timing to work with that 10.2:1 compression an crap pump gas.
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=88&sb=2

and that cam directly above ,certainly is not designed for a daily driver, or swap to the 75cc combustion chamber heads, with the current cam, MAKES MORE SENSE
there is , without any doubt, at least... theoretically, long bloody scratch marks on the pavement,
as I'm dragged kicking and screaming into the computer/digital era,
but you really can,t dispute the facts and the facts are,
that there are now ways to set the car engine ignition timing that were just not all that easy to duplicate with purely mechanical controlled ignition advance curves.
between knock sensors that see and react to detonation, coil on individual spark plug ignition, crank sensors etc.
and computer controls that allow you to select and set the degree of ignition advance,
and sensors that read the fuel/air ratio hundreds of times a second, and allow the injector pulse duration to be adjusted to compensate if you have the correct matching software,
at every few hundred rpm check points, its no longer a question that if you select the correct components you can pull power levels from engines that were previously very difficult to achieve.

http://www.hotrod.com/articles/ccrp-0512-ignition-advanced-basics/

http://www.hotrod.com/articles/set-ignition-curves-create-optimal-performance/

https://www.cbperformance.com/product-p/2013.htm

https://www.msdperformance.com/products/ignitions/street_and_strip/parts/6530

http://documents.msdperformance.com/6530.pdf

http://www.superchevy.com/how-to/engines-drivetrain/sucp-0103-msds-electronic-ignition-system/

https://bangshift.com/general-news/...ble-ignition-can-make-anyone-look-like-a-pro/
http://garage.grumpysperformance.com/index.php?threads/dynamic-vs-static-compression.727/

http://garage.grumpysperformance.co...ing-a-383-sbc-combo-planing.12168/#post-71932

http://garage.grumpysperformance.co...ing-parts-and-a-logical-plan.7722/#post-71181

http://garage.grumpysperformance.co...g-e85-in-your-old-muscle-car.2141/#post-25201

http://garage.grumpysperformance.co...temps-detonation-resistance.12842/#post-66668

http://garage.grumpysperformance.com/index.php?threads/383-information-overload.11137/#post-50659

http://garage.grumpysperformance.co...ow-to-reduce-its-likely-hood.9816/#post-49741

http://garage.grumpysperformance.co...clecar-street-racers-friend.14941/#post-84290

http://garage.grumpysperformance.co...-calculators-and-basic-math.10705/#post-50173

http://garage.grumpysperformance.co...octane-for-compression-ratio.2718/#post-35582

http://garage.grumpysperformance.com/index.php?threads/detonation-damage.2883/#post-31941
 
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