I think rocker geometry issues start to show past .630???? The rockers start to run out of sweep IIRC. Im not going to make HP claims, flow numbers are never consistent from one guy to another, but BES and Thitek have claimed flow numbers of ported Apache heads that rival W8/W9 stuff that is MAXED. If you have no heads at all and want HP just start with the Thitek's IMO.