What I have found....and this is just me....is that the as cast bias of the intake.port is not conducive to very high flow numbers....perhaps the PS heads were ported on the factory bias and thus the reason they were limited to around 400....I in no way claim to be a better cylinder head buy than anyone, let alone the PS guys, I just try different things to see what works rather than rely on conforming my design to what everyone else says works.....