The specs on the h405cp's say you can get 9.1 with 63 cc heads and 8.66 with 68 cc's, which is what I think my heads are. I just hate to tear into something that runs so sweet. It doesn't use a drop of oil. Guess I have to decide how much of an increase in performance I want. Think I'll bolt in what I've got and maybe add better pistons this winter.