Detonation resistance .040" vs .045" quench

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Re: Detonation resistance .040" vs .045" quench

Post by rfoll »

Powertrip wrote: Tue Jun 05, 2018 2:30 pm
BradH wrote: Mon Jun 04, 2018 1:20 pm 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. :wink:
I'm wondering how you determine you can't get away with it before something gets hurt.
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Re: Detonation resistance .040" vs .045" quench

Post by PackardV8 »

Tuner wrote: Tue Jun 05, 2018 12:17 pm 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.
We square deck all our blocks, so we regularly build tight piston-to-head on low RPM cast pistons; however, .020" on a 4" bore would scare me.

Since this is mostly a performance/race forum, is that .005"/inch of bore at all applicable? Most here are already testing the limits of "dynamic conditions, heat expansion and material elasticity, stretch with RPM."

Several previous threads here recommend .035"-.040" is a workable minimum, which is .009"- .010"/inch of bore for higher RPM and forged pistons.
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Re: Detonation resistance .040" vs .045" quench

Post by CGT »

Bazman wrote: Mon Aug 20, 2018 3:49 am 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?
I usually measure most LS's at .006 - .007 out of the hole. I have several "all aluminum" ones out there with .036 Cometics on them. No problems. My personal car stock ls3 shortblock, .036 Cometics...15lbs of boost.
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Re: Detonation resistance .040" vs .045" quench

Post by David Redszus »

BradH wrote: Mon Jun 04, 2018 1:20 pm 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
Static compression numbers have little meaning when attempting to predict combustion properties.
What is needed are the dynamic compression numbers so that compression temperature can be calculated.
When comp temp is used in conjunction with inlet air temperature, we have a good chance at predicting combustion autoignition of the specific fuel being used.

Similarly, squish clearances without Squish Area Ratio numbers do not have much value. With a low SAR, a tight clearance can be used and will produce the same velocity as with a high SAR and wide clearances.

If we have TCR, SAR and IAT, and RPM, we have a reasonable chance at predicting the onset of autoignition.
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Re: Detonation resistance .040" vs .045" quench

Post by Bazman »

CGT wrote: Mon Aug 20, 2018 12:08 pm I usually measure most LS's at .006 - .007 out of the hole. I have several "all aluminum" ones out there with .036 Cometics on them. No problems. My personal car stock ls3 shortblock, .036 Cometics...15lbs of boost.
Wow, you may be as low as 029"... good to know.

I may stick with the 040's then when I go forged and change rods. With the longer but billet rod and shorter piston I can't see it stretching significantly more than stock... but I've been wrong before
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Re: Detonation resistance .040" vs .045" quench

Post by pdq67 »

Through the years, I have read where some guys have ran as tight as to kiss the head and pistons because upon tear-down, there are tell-tail(Sp?) witness marks.

Me, no way, about .040" or so is fine due to stock 1st Gen. SBC being usually .020" to .025" DITH and they all pretty-much through the early years came with .020" steel shim head gaskets. And I do like Felpro's #1094, .015" thick shim head gaskets. (Again, SBC here.)...

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Re: Detonation resistance .040" vs .045" quench

Post by CGT »

Bazman wrote: Mon Aug 20, 2018 4:10 pm
CGT wrote: Mon Aug 20, 2018 12:08 pm I usually measure most LS's at .006 - .007 out of the hole. I have several "all aluminum" ones out there with .036 Cometics on them. No problems. My personal car stock ls3 shortblock, .036 Cometics...15lbs of boost.
Wow, you may be as low as 029"... good to know.

I may stick with the 040's then when I go forged and change rods. With the longer but billet rod and shorter piston I can't see it stretching significantly more than stock... but I've been wrong before
Well, I haven't tried it on a cast iron block ls yet. Gotta be .005 or so growth on the aluminum block. I have had under .030 on sbc's iron head iron block though.
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Re: Detonation resistance .040" vs .045" quench

Post by Bazman »

CGT wrote: Mon Aug 20, 2018 4:31 pm
Bazman wrote: Mon Aug 20, 2018 4:10 pm
CGT wrote: Mon Aug 20, 2018 12:08 pm I usually measure most LS's at .006 - .007 out of the hole. I have several "all aluminum" ones out there with .036 Cometics on them. No problems. My personal car stock ls3 shortblock, .036 Cometics...15lbs of boost.
Wow, you may be as low as 029"... good to know.

I may stick with the 040's then when I go forged and change rods. With the longer but billet rod and shorter piston I can't see it stretching significantly more than stock... but I've been wrong before
Well, I haven't tried it on a cast iron block ls yet. Gotta be .005 or so growth on the aluminum block. I have had under .030 on sbc's iron head iron block though.
Lol, you like to push the envelope. A tuner I know and his son went into a race class with a class restricted motor - not much elbow room for mods. I told him about tight quench and he took it to heart and made a miscalculation and ended up at 0.027 instead of .037... won his class loz. Pistons were clean as a nut but were touching lightly above 5500rpm. That was on a 3000c straight six all iron.
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Re: Detonation resistance .040" vs .045" quench

Post by rustbucket79 »

Piston to wall clearance plays a huge role on what will work and what won't, some times with expensive consequences.

I've built a few engines that showed some piston to head contact upon refresh time. Zero indication it was happening, engines performed as expected. I would never gamble contact with a cast or especially a hypereutectic piston.
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Re: Detonation resistance .040" vs .045" quench

Post by pdq67 »

It's kinda funny because imho, a stock, steel-strutted, cast piston tends to be pretty tough for what it is!

This said, I won't run hyper's due to them turning into aluminum gravel when they break.

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Re: Detonation resistance .040" vs .045" quench

Post by user-23911 »

pdq67 wrote: Tue Aug 21, 2018 4:13 am
This said, I won't run hyper's due to them turning into aluminum gravel when they break.

pdq67

If you tune it so it won't knock, the pistons won't break.

It's all about the tune.
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Re: Detonation resistance .040" vs .045" quench

Post by Walter R. Malik »

BradH wrote: Mon Jun 04, 2018 1:20 pm 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
As little clearance as possible without actual piston to head contact.
Finding that point with each combination is not gonna be easy unless you are lucky.
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Re: Detonation resistance .040" vs .045" quench

Post by tenxal »

I've run as close as .028 w/o issue. Not ideal, but the decks were wonky and that's where a couple ended up. On a recent fresh build, .032 was the number. A nice, tight bore fitting .027 head gasket and the piston .005 down achieved that goal....SBC, .005 piston-to-wall, 7100-7300 rpm.

On a NA build, running more than you need is leaving money on the table.
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Re: Detonation resistance .040" vs .045" quench

Post by Bazman »

That's good info right there, thank you.
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Re: Detonation resistance .040" vs .045" quench

Post by tenxal »

As an aside, if there was a thinner quality gasket that fit the bore, I'd opt for the piston down further in the hole. At this point, the gasket tightly fitting the bore gets us more than than we loose with the .027 gasket/-.005 deck lash up.
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