The heads have 57 cc combustion chambers. Used on a stock 239 w/no relief the CR will be ~8.41:1, with a relief ~8.09:1. The CR will increase as the bore size increases. In a 255 (Merc) you're looking at ~8.90:1 with no relief, and ~8.56:1 with a relief. The reliefs quoted above would be .0100"...