SDPC Tech Tips: Quench Clearance
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- เผยแพร่เมื่อ 5 ก.พ. 2025
- Thinking of running a thinner head gasket to give your engine a little bump in compression? Piston to valve clearance isn't the only thing you need to consider when making these changes. Nick goes over Quench, what it is, how it can help, and what to look out for!
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The part he left out isn't the minor bump in compression. It's the effect tight quinch has on detonation and between .060 and about .100 is the most prone to detonation. Sbc's ran .060 and low 8:1 compression to prevent detonation on cheap gas. By tightening quinch to less than .045 it allows you to run higher compression with the same gas without detonation. It also helps mix the fuel mixture in the chamber. I suspect that tight quinch helps prevent detonation by not allowing fuel to sit on the hot spots at the edge of the chamber or possibly creating slightly lower pressure in the quinch areas.
As I understand it, quench ends near 0.060". Somebody out there is smarter than me and can probably prove that wrong, I suspect [science does that]. A proper cylinder head bowl/chamber combo provides swirl on filling, but its quench that adds a soft roll motion to the compressed air/fuel mix right near spark to help flame front perform better. A lazy flame front will not burn as efficiently, plus peak cylinder pressure will be a little off.
Quench i believe helps promote swirl in the combustion chamber which i believe helps to take heat off the exhaust valve which is where detonation will typcially occur first. Also, air has a large thermal conductivity, so tighter Quench reduces the thermal resistance between the cylinder head and block helping to more effectively remove heat and prevent detonation. Further, not using tighter quench loses you power and efficiently bc the higher compression makes the engine more efficient and produces more power. This also should help to allow running a leaner 14 to 15 AFR with lower octane fuel in cruise to get better mpg. I'm not sure but I believe piston rock especially with forge pistons also has alot to do with the quench limits apart from mechancial strain from higher RPMs which limits the safe quench to .035 to .04. Also, piston type such as recess dish, flat, or dome pistons to my understanding is very important since stock dish pistons have terrible quench area so you have a good quench height based piston to deck clearance and gasket thickness, but the dish is recess .06in and effectively your quench is missed at over .060.
My lq9 after being decked had a deck clearance of -0.018 which is 18 thou out of the hole. I was set on ordering a .040 head gasket. Had I done that my quench would have been 0.022 quench which would have caused the piston to smack the surface of the head at high rpm (huge issue!). Im glad I double checked and went back to really understand this. It saved me a lot of headache and my pocket thanks me for it.
I was 25 thou overdeck and used .40 gasket. Rev limit 6500. No issues . 1.6L sohc
I did a super late model engine and when I freshened it up after a season the pins were loose and I could see microscopic marks on the tops of the pistons. I figured I had too little quench space and got the pistons pin holes resized and .003 bigger pins and a .003 thicker head gasket and got the quench to where it should have been. I was lucky it didn't destroy the engine and could be fixed.
Do the math... Forged Steel expands about 0.001 per inch & Aluminum about 0.002 per inch.
- Rod length 5.7" to 6.0" x 0.001 = 0.006"
- Piston pin height 1.1" to 1.4" x 0.002 = 0.003"
- Rod bearing cap may stretch = 0.002"
- Insulating carbon layers on piston & cylinder heads 0.005" each = 0.010
Total expansion / stretch = 0.0021"
- Deck height of cylinder block from crankshaft bearing insert expands providing
some level of mitigation to heat expansion of rods & pistons. 8" = 0.008"
- Quench height of 0.030" was not a problem in SB Chevy engines I built 35 years ago.
If you are wiping out rod bearings, it's either excessive bearing clearance or oil starvation.
Rod bearing clearance o 0.0005 on the rods & 0.0007 on the mains works well with 20W50 Synthetic Oil.
Just for kicks, one observation: people always measure piston vs deck with the engine right side up. I do. So... you're "zero decked"? Turn that puppy upside down and see how many tenths out of the hole you are. 5, 8, 11...more? - stacked bearing clearances will show themselves. And we haven't gotten into crank flex, rod stretch, or thermal expansion for a running engine yet. This is why most stop at 0.038" quench when its possible to reach for 0.035".
some good shit!! love these tech tips. big help in building my little 5.3
Awesome information. Thanks
A trend in higher performance applications is to reduce quench. Pro Stock and Comp eliminator have found that they make more power, have a larger tuning window, and less detonation issues when quench is reduced. Softened chambers are being used for this...
I didn’t get it with this “ softening “ . On one side you try to make the quench as small as possible and on the other , in the same time you increase it with the “ softening” of the quench surface of the head . Also , looks like they
apply softening mostly to high boosted engines heads .
7👍's up SDPC thanks again for taking us all alone with you
Interesting information. Cool video.
On Gen. 1 SBC's I have the machinist deck the blocks so the pistons will be down .010" in the hole and I run a .030" thick head gasket. Never built a LT1 or LS motors.
I run a 15 tho to 65 tho it depends on rod, bore size, bearing clearance bore size.... its completely different for every different engine and where it has to be used
My ls3 mesured .007 out of the hole
Thanks for the great lesson
SBC I am building has .020 at TDC and .015 head gasket. Total quench is .035 I would not go any lower than.035
And if you run that .015 thick head gasket, the best way to get it to seal is to deck the block...which means you can lose that zero deck. Plus the Gen One block uses five bolts around the cylinder bore to seal, the LS uses four bolts.
Awsome info thank u i started out on genone small blocks also thank u u guys are awsome!!!
Factory small and big block chevy's came with steel shim gaskets.
True smog ERA L82 SBCs came with 0.018” shim gaskets. That doesn’t mean that come rebuild time that the deck surface is still perfectly flat. (My 14.4k mile L82 had some high spots on the deck.)
The OEM deck finish doesn’t meet the smooth RA values that FELPRO recommends for that 0.015” shim gasket, but they do admit that a spray of copper coat can help deal with minor surface imperfections allowing them to be used.
@@adamarndt7617 With iron heads and under .003 with a straight edge you're fine. I use permatex super 300 for sealer,
What if you run the piston 5 out of the bore on a 383 6in. rod ?
Safe is to factory specs! Factory tolerances need the extra wiggle room. A after market pc with proper machining will benefit drastically with a .030 gap on wedge style chambers. But realistically...l.s. platforms builders are using factory clearances." Play it safe" is for those guys.
So where can we find the safe guidelines for the L33 5.3 engine? Or are all the LS engines the same as what you said here?
You will have to measure it yourself every block will be different
I have a 406 sbc and for some reason my piston to deck clearance is .053. So I end up with .075+. So I need new pistons?
Yes
Nice
You are incorrect about the engine deck having to be flat to run a .016 felpro shim head gasket. I just rebuilt a 1966 283 and it had these head gaskets and and it was a stock bore 3.875". The engine was still sealed and running when we rebuilt it. The deck was so far off and warped it was at TDC to deck clearance of .034 in one hole and another was at .057. we checked tops and bottoms of pistons deck clearances as well as the middles the numbers above are the middles. We found the top and bottoms to show up to .007 of warpage twist, on top of the slant. It was no where near straight. Oh and I miked the compressed shim head gasket and it comes out to .016.
Steel rod .035 safe .030 .028 close ran .035 not problems on 7500 rpm sbc 550 to 600 Hp bearings look great could have reused on rebuild up grade .035 safe if rods hit then you are ready blown up
How much Quench do you recommend for a boosted forged LS?
Perfect or none
Quench,,
Extinguish or put out a fire
Use squeeze
Quench should be perfect or nothing on boosted stuff
Race Proven Motor sports has been around for over 16 years. You should take NOTES!!!!
Fastest engine we had on a spec engine was running the piston in to the head.
.036 is where it's at , higher rpm .038 ... 7500+ .040. .. aluminum rod .60
Who is buying these thin gaskets & figuring quench but not measuring piston to deck hight?
People with an abundance of paper weights.
I put .027 Cometics on a 5.2 Magnum..had .048 gaskets..I carefully measured my deck height...043 in the hole...no quench whatsoever lol...reduced quench from. 091 to .064
More on the term blueprint your engine
Blue printing an engine means that you're going back to the exact specifications and everything is done by exact measurements, the piston, cylinder, lifter bore, all bearing clearances, is all clearanced to spec...
thats what i thought
Pump gas and quench is a completely different subject. If your running pump gas, don't squèz the quench.
Why, cause I said so.
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Aw man, I hoped YOU were be the guy who would actually define quench. You did not. You defined some theory about the clearance and it's benefits, but you did not define QUENCH. Quench means: "To Cool" So I ask you, what's being cooled in a RUNNING ENGINE??? (There's only one answer) Cheers.
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