Thanks Trevor, your a wealth of information. I get it now. While both valves are slightly open the backpressure helps keep the new fuel&air that's coming in the cylinder from escaping out the exhaust vavle. Get that part, now I'll have to think about what your saying about the affects of overlap at higher RPM's. Is it that the exhaust flow at higher RPM's helps suck the fuel&air into the cylinder from the intake, or did I get that wrong also. Wish I could re-pay you for your time explaining, but New Zealand is kinda far away. I'll buy more Kiwi to boost New Zealands exports
