[Jeff Chambers]

It is only a rule of thumb, and a rule that only considers the change in compression....nothing else. I've often thought of it as being way to conservative myself, but who am I to argue? I think it seems pretty reasonable if you're considering a stock motor going from 9.0 to 10.0, but it would really seem to fall apart if you think of a Hot Street motor that is pushing 17.0:1. You'd never get to 700 hp just by taking a stock motor from 9.0 to that 17.0. More often then not, you're doing other things at the same time that you're raising the compression to better optimize the combination so the gain due to the change in compression gets washed out. Oh, but I digress. At any rate, I've got several engine building programs that seem to concurr with the 3 to 5% rule. I just ran the numbers for a stock 225hp motor going from 9.0 to 10.0 and it give a new peak hp rating of a little more than 232hp. That's only 7hp or a 3% change.