Insofar as the shape is concerned, a "figure-8" is considered the best chamber shape for two-valve heads with a spark plug in the middle.

That seems to be the common wisdom about that, but not everyone would agree. The guru I use says 8 is a fine shape if you don't mind reduced VE and how the combustion pressure transfers less efficiently to a *round* piston (his emphasis).

This is about the point where it gets beyond my current understanding of the theory, but I have yet to find anything the guy has told me that didn't produce the results he said it would. Come take a ride behind the motor with the head I showed in the pic, and you won't be surprised -- you'll be shocked.