Re: Piston-to-head: what's ideal? What's safe?
Many of us shooting for pump gas use only, usually have a static compression ratio number as the traget we shoot for. Nobody wants to drive a car on the street, and have to pay $9, $10, or $12 per gallon for race gas because the engine in question pings all over the place on 93 octane pump gas. Unless of course you're a Jay Leno type who earns a seven figure yearly salary. So ofcourse the static compression ratio target number has to be considered when we're choosing the piston-to-head gap to use, since it will ultimately become one of the variables that determines the static compression ratio that your combination will end up with.
When you're starting a build completely from scratch, you'll have more control over that, since you can decide on dome height of the pistons, and head chamber volume too. But be it a partial build, or a build up from scratch, the piston to deck space will be a factor. However, if it's true that having a good "squish" action going on inside the chamber during combustion, increases efficiency, and reduces the potential for detonation, ("pinging") then striving for the ultimate or the best piston to deck space IMO should be one of our goals, (for pump gas builds in particular).
I suppose that what we need to add to the mix is what Scott mentioned about certain applications perhaps requiring, or being optimized by slightly different squish heights too. But from what I'm hearing from some of the examples given, (especially from Jeff) indicate that this isn't an exact science, and/or that a larger squish space than the accepted .035"-.040" might very well be optimal for many pump gas applications. But this is just me speculating. Lets face it, it would be pretty tough for most of us, (if not all of us) to spend the money and the time to perform back to back tests on the dyno and at the drag strip, as well as on the street, on different piston to wall clearences just to see what works best for pump gas use.
Even if we had nothing better else to do with our time and money, any conclusions derived from such tests would still only apply to the specific vehicle combo that ws used, unless perhaps multiple vehicles at various race weights, with multiple rear gear ratios were used. Which makes such an endeavor even more unrealistic. But it still serves as some food for thought when the details of different combinations are shared, and what end results were brought about by those combinations. If nothing else, it can sometimes help us explain why certain combinations that we might have tried in the past might not have worked the way we had planned.
70 Chevelle SS clone (632 CI powered).