Ok let's see if this helps. In the pic, look at the red X. That is the quench area. You cannot have that with an open chamber head......without a specially made quench dome piston. Essentially a piston with a dome on the "wrong" side if you will, unlike the piston in the pic. By making quench distance small (between .035 and .045") this does lots of stuff. It creates turbulence and that makes for a more complete burn and results in lower combustion temps. Which in turn allows more compression on pump gas VS the same engine without quench.
