Good info Tech...I was going to respond, but after reading your reply, you said basically everything I was going to. Im surprised how many people put, say, 11/1 pistons in their engine (not this case) and think they have 11/1 c/r. There is so much more to it, and like you said, the cam is probably the biggest determining factor in how that engine will run on what octane. The dynamic c/r (running) is what will make it or break it. There is a calculator I can post here if I can figure out how, that will show the dynamic c/r if you know the other factors, but you also need the rod length, bore, stroke, piston dome or dish, etc.....if you dont have the engine apart, or are building it, it wont do any good to guess. The engine Im building now is at just over 8/1 dynamic c/r, and about 10/1 static. Thats about as high as you want to go to use pump gas. Al, you cant tell too very much by cranking compression, but I would say you are about as high as you want to be for pump gas and a street motor. You really have to know what's inside to be sure. John