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I'll quote Smokey Yunick for you: I’ve also heard there are some builders who claim an engine will make more power if you let the “piston smack the heads a little.” I think this sort of talk is mechanical nonsense and I would not recommend it under any circumstances.
It is hard to recommend a desired quench clearance that will be correct for all engines, but I think 0.038-inch of clearance between the quench surfaces is an absolute minimum in an engine with a 4.00-inch bore and about 0.007-inch of piston-to-wall clearance. However, if the piston clearance is greater, say 0.009- to 0.010-inch, this may not be enough. A safe figure for all around performance is 0.040-inch. Anything up to about 0.045-inch is probably okay, and I wouldn’t worry about a little extra clearance as long as it was close to these recommendations.
But remember, if you let the quench clearance open too much – to 0.060-inch or more – you’re going to lose power. This much opening at the quench will allow excessive amounts of the intake charge to remain in the quench area as the piston reaches top-dead center, and especially if the piston has a high dome to obstruct the spread of the flame front, these gases may not be properly combusted during the ignition phases. This significantly reduces combustion efficiency.
Bill Jenkins likes to use .010” beyond ‘smacking clearance’ & that overly tight squish contributes to pumping losses. .050"-.060" static for a steel rod oval track motor.
Aaro, Parker, Hueppchen, and I all agree on this and have demonstrated it many times over.
I don't know who Mr.Hueppchen is, but I seriously doubt that Aaro or Parker could show a single HP difference between the same engine running .030" or .040" of squish clearance.
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