Detonation resistance .040" vs .045" quench

[BradH]

Street/strip combo w/ 4.38" bore & zero deck height w/ flat-top pistons:
11.50 CR w/ .040" gasket & quench
11.35 CR w/ .045" gasket & quench

Any benefit from an octane requirement perspective with slightly wider quench + slightly lower CR? Or is it more likely to tolerate the same octane due to the tighter quench, despite the slight CR increase? Thanks - Brad

[groberts101]

Street/strip combo w/ 4.38" bore & zero deck height w/ flat-top pistons:
11.50 CR w/ .040" gasket & quench
11.35 CR w/ .045" gasket & quench

Any benefit from an octane requirement perspective with slightly wider quench + slightly lower CR? Or is it more likely to tolerate the same octane due to the tighter quench, despite the slight CR increase? Thanks - Brad

Some of it depends on the squish pad shape/sizing.. but in general.. the trend towards higher mixture speeds pays off by improving octane tolerance and subsequent reduction in spark advance needed to make a similar power number. Keeping the crevice volumes on the lower side(higher top ring placement/reducing excess gasket to bore OD sizing) also helps. Even reductions in a pistons valve notch volume can add to the overall effect in cumulative manner.

And what some fail to remember is the affect tighter combustion space volumes have on the intake/exhaust port/valve seats velocity profile at any given valve lift point. The secondary benefit is that manifold vacuum can go up which further improves the carbs circuitry and boosters becomes more responsive as well.

[groberts101]

duplicate

[BradH]

When II hit the Submit button, the response I expected to get was "it depends...". Yeah, too many variables for a definitive answer.

In my case, the last combination was 10.8:1 CR and could run on straight 93 E10 w/o any issues. The .7 CR increase was not the original intent, but it is what it is. I'm not sure at this point where the octane "needs" to be, but I'll be running a blend of 93 E10 w/ some grade of race gas to provide a safety margin for the tune until that's better known.

[David Redszus]

For a given SBC engine with identical bore, strike, rod, rpm and squish area ratios, a change in
squish clearance from 0.040" to 0.045" would result in a reduction in squish velocity from
30 m/s to 27 m/s.

Cycle to cycle variances probably exceed that difference.

[Tuner]

My experience in the parking lot at the Holiday Inn has been tighter is always better, sometimes remarkably so. Taylor (MIT) found .005" per inch of bore diameter to be optimum, with consideration for dynamic conditions, heat expansion and material elasticity, stretch with RPM. GM and Smokey Yunick advised ".035" to .040", minimum preferred" and Bill Jenkins concurred. Perhaps Warp Speed can tell us what clearance is used in current 'Cup' engines.

[groberts101]

Another thread brought up a good point I forgot to add here as well. Higher compression will also cause the exhaust slugs to hit the pipes harder and increase velocity to help improve overall scavenging effects. Probably even makes the exhaust port think the collector is slightly smaller as well.

Realistically we're getting down further towards gnats-ass sized differences here.. but cumulative affects can easily add up to bigger overall gains than some may have originally anticipated. And I do know for fact that the gains can be even more pronounced when the compression ratio increase "fixes" less than idealized combinations of parts.. such as too big carb venturi's, large'ish camshaft and/or bigger overlap period, too big a CSA induction, larger primary/collector sizing then the intended peak power calls for. More compression can really make a bigger difference after the tune is redialed in on some combo's and bring higher average power across the entire rpm operating range.

[Powertrip]

11.50 CR w/ .040" gasket & quench
11.35 CR w/ .045" gasket & quench

I would bet that the octane requirement is the same between the two choices, giving the win to higher compression. You can always go less compression with chamber mods later if you find that you can't tune it to live with the higher compression.

My next build will be pushing the compression limit with pump gas, just to see what I can get away with.

[Newold1]

My advise is start with minimum clearance and see what you can get away with being careful to check and stop detonation. If it's a little to tight a simple increase in head gasket thickness will allow you to lower things slightly and still stay at a good working maximum.

Also remember that camshaft design (intake closing) timing can be adjusted also to help with lowering detonation potential. Dynamic

[digger]

would it fair to say that with a smaller squish pad its most likely better to close up the squish clearance a bit more? (within safe limits of course)

[modok]

Go with .045
If the rest of the engine is as big as the bore size, i think .040 may not be enough.
In theory more than ,040 does lose some squish, but in reality pistons rock, and parts flex, so .045 on the bench is less when you are actually rev it.

Or, if you like, make it tighter till the pistons kiss the heads and then back off .010

[BradH]

3.75" stroke --> 452 ci; steel rods & 7200 RPM max

Engine has been run successfully with .040" and no signs of insufficient clearance.

[BradH]

Can't get too hung up on this and need to collect the rest of the parts; ordered .040" Cometics to get things moving again w/ re-assembly. Thanks - Brad

[modok]

probably good.. Not as much stroke as i was imagining.

[Bazman]

Low 030's is not too tight on my application.... Well, I cannot add much science to the question but I can give you a real world example that is outside the normal limits.

I picked up a 2013 LS3 a few years back that came out of a wreck right off the showroom floor so was as new. I wanted to twin turbo it for street duty, track days, and closed road events - one of which lasts a week.

I added a small cam and changed head gaskets to 040 Cometics thinking I'd get a nice tight quench and resist detonation despite the small rise in compression. I did this because I was stupid and did not know that LS3's routinely have pistons 0.009 out of the hole.... giving me a quench perhaps as tight as 0.031.

Several 1000km later I have done multiple events and road kms at up to 663rwhp. The only problems I have had is a blown intake pipe losing boost and heat on track days or long closed road events... an issue created by having a front mounted intercooler and since improved significantly by adding an under hood cowling that directs air down in front of the radiator.

So I can say that it has not had any issues at perhaps as tight as 0.031, at around 11:1 on 93 octane at 13.7lb boost. It does not detonate despite spending quite some time over 6000rpm.... I think fuel cut out is 6500, and a lot of 4th gear pulls out of turns from 50mph (3.9 gears) because it can out accelerate a Maclaren (that was trying) without changing down (not a 720 mind you).

Had I known it was that tight I would have been too scared to try it, but it works.

Looking to build a forged motor next and will change rods to Callies Ultra H 6.125" when I add some dish pistons. No idea if they stretch more or less than factory LS3 rods. Should I go to a safer 040 just because lolz ...or should I stick with the 040 head gasket and quench in the low 030's? I may turn that motor to 7000rpm so probably just answered my own question. Thoughts?