I do not have anything more scientific than what I've witnessed on the dyno and in the certain customers cars. I know that a rowdy street engine with aluminum heads WILL tolerate more static compression before things like running on, hard starting and pinging start to show up. With iron heads cranking compression of 190-195 psi is on the edge with pumpgas while the same engine with aluminum heads can tolerate 210-215 psi cranking before the same symptoms show up. Can't comment on the power difference on the dyno 'cuz I have never had the opportunity to test the same head but cast in two different materials. Believe what you wish and I was a naysayer too years ago until I saw what I saw. J.Rob
