No ford played around with the compression when they changed to the PI heads. I cant remember if they changed the dish size of the pistons or the size of the combustion chambers of the heads but Duceanahalf is right it will put the compression to about 10.5 to 1. So in performance terms you actually make more power swapping a non PI motor to PI specs. That is actually one reason you gain so much more horserpower from doing the PI swap.