The compression calc tells you that. Its always on the low side. I few guys over on B bodies have done this with great results. Don't have any numbers to prove, but Malex over there says he could feel a 2 pt increase in the seat of the pants using these heads and I still believe this head for whatever reason needs high compression 12.5:1 or higher to run strong. Malex with a 505/12.8:1 compression in a B body at 3450# and these same heads goes in the 9's. The heads get bolted on today after I check the piston/valve clearance.