Quench Vs Piston To Head Clearance

I think most of this has been coevered but in bits and peices. My own input...

The piston to head clearance is the mechanical distance between the piston top and the chamber bottom. For any engine build.When we talk about quench tight is best but like was said - a lot of variables go into what's "perfect" vs what may not work or what might hit.
The the proximity of the head and piston create the condition for what's called "squish". That's the forcing of the intake charge from the dead space of the chamber over to the valves and spark plug. That does some good stuff: it cools the surfaces of the head and piston, it re-suspends liquid fuel that may have come out of suspension during the intake cycle, it packs the area around the plug and exh valve where it will more readilly ignite, and if effectively lowers the volume of available space the expanding (ignited) gasses have which in turn puts more force on the piston and crank during the initial flame kernel/front development.
That first thing - the cooling affect - that's what "quench" really is. It's not the distance, it's to cooling. The tighter the "quench distance" (or piston to head) the more cooling there is and the more effective the detonation suppression. Obviously the larger the distance, the less cooling, the less mixing, and the less detonation resistance there is. Beyond about .045 or so there isn't much happening.

Getting into specifics of certain applications - An as-cast surface is far from smooth. Any roughness or texture adds volume that reduces the quench effect. Add in the additional volume of the radius of the cast chamber & the space built around the edges of the quench dome of the piston and you can see issues. With a rough open chambered surface and quench dome type piston there is a less defined direction of movement. There is more squishing, but it's more chaotic accross the entire bore width of the chamber and less centered on the direction we want.
Think of it like this - Assume we have a perfect 2'x'2 box. Drop a 1'x2' piece of plywood so one edge of the 1' width edge hits first and then it falls flat into the box (covering 1/2 the floor of the box). The air basically gets squeezed out from the edge that's on the ground, towards the edge that is falling. Now drop a 10"x23" log in there on one side of the box. There is some air that will follow that path and go towards the open 1/2. But a lot will just whoosh around the log too.

I build engines designed to maximize quench. I have built one using the Kb quench domes (a 440). It requires a tremendous amount of work to get any really strong quench effect vs a flat top/closed chamber design. The deepest chamber has to be found on each bank. they all have to be equalized to that depth either by hand or (preferably) by milling to get a flat surface perpendicular to the deck surface (BTW - some chambers are twisted in the castings too so one end of the same side's deeper than the other.) The block is decked to a known height. Pistons and rods are mocked up and each cylinder is checked because some varience in stroke and rod length is normal especially in factory parts. Then you decide on a gasket thickness, then remove the pistons and mill them to get every hole the same hieght so they have the same quench distance. It's blue printing taken to it's next level... and the effect even if you get it perfect is not as good as it would be with flat tops and closed chambers.

Getting it right is important, and there are huge benefits especially in the game of running real pump gasoline and making big power and long service life.