ok so the z6 head flows better, but wouldnt you want higher compression from the y8? because yes, the z6 head does have greater combustion chamber volume, but smaller quench pads wich gives greater risk of detonation. The y8 head on the other hand, gets a boost in compression because it has less combustion chamber volume, and squared combustion chambers. a plus to the y8 head is the larger quench pads to help reduce detonation from the high compression when the head and block is mated. so it boils down to either better top end(z6) or somewhat less of a risk of detonation associated with higher compression with the y8. But thats just my opinion. Sorry for posting the wrong info about the head comparison, I was looking at the wrong engine when i was getting my sources!:o