I'm just starting to research building a racing engine. This is based on a 70s - 80s VW 8V watercooled (Rabbit, Scirocco, Golf etc./Reducing Valves) engine. The class rules specify an 11:1 CR and an unmodified combustion chamber so the basic design is pretty fixed.  A flat top piston with the right head milling will give approximately the correct CR.

The stock combustion chamber with a flat top piston will give a pretty big squish area - maybe 70% of it opposite the spark plug and 30 "behind" the spark plug.

Finally to my question - what would be a reasonable "squish gap" - ie clearance between piston top and head face?  And should it be uniform or tighter opposite the spark plug?  

My previous race engine building experience was with unlimited CR engines with very short stroke and more open wedge chambers where we were fighting for CR and got everything as close as we dared.  But that's not the case here - there should be no problem getting the max allowed CR.

I can "visualize" - ie gut feel - that possibly there is such a thing as too much squish from a turbulence point of view.  Also I would think that a very tight squish would produce some pumping losses.

Obviously the optimum would be discovered empirically on the dyno.  And more time and $$ than I've got.

Anybody got an opinion or any rules of thumb as a good starting point?

The standard rule that I have heard, for iron block V8 engines, steel rods, moderate rpm is .040 inches.  More if you rev very high, have aluminum rods.  Less if you don't rev very high, have tight piston to bore clearances.  Above .045 you start to lose effectiveness, less than .035 it can get dangerous to the engine.  I have never heard of anyone complaining about having too much squish (too little gap) as long as they didn't actually kiss the heads.

On a Golf moror, you should be able to run tighter than a typical American V8, but I don't how much. About 0.032" comes to mind, but that is an unsubstantiated vauge memory.

I have never heard of not enough squish either, and all my best engines showed some signs of soft contact between pistons and heads, like maybe just a zero clearance touch during an over rev.

This1 Is Slightly Confused & Way Out Of Field Here But Wonders, Wasn