I believe the static ratio (without getting into cam timing) is about 9.5:1. The actual compression pressure will vary according to when the intake valve closes, because no pressure can begin building until both valves are closed. That's the reasom we use such high static ratios on race engines. The actual cylinder pressure isn't much more than stock, but when we use up almost half the compression stroke with a long-duration cam, it's necessary to squeeze the remaining cylinder volume tighter to get the same pressure as with a stock cam. If you don't change cams along with the higher-compression pistons, it's possible to get very high cylinder pressures, and lots of detonation, or an engine that needs premium fuel all the time. I'm very spooky about a stock-cam engine with static compression above 9.5:1. The most recent 250 I built with 307 pistons also got a .050" head milling job, uses a stock cam, and doesn't detonate. I believe we're right on the verge of too much compression on that one, though!
Jerry
