Originally Posted by
DarkBlack
I think what confuses most people is making the mistake of confusing static compression ratio #'s, with effective compression. They are 2 different things. Published compression ratios are only a static numeric ratio of the cylinder volume at BDC, compared to TDC.
Effective compression is the real resulting compression that results mostly from the valve timing, and with diesels, the injection timing, and or turbo addition. All engines have intake valves open during initial piston upward movements, allowing pressures, and vacuums to be fed back through the intake valves. I general, the higher the intended operating speed, the longer the intake and exhaust valves are held open, and the more time that they are open at the same time.
It's real obvious with gas engines. This is why a static 8.5:1 engine with a low rpm "torque" cam may require preimium fuel, where an 11:1 static engine, such as a motorcyle engine with a cam designed to run up to 11k rpm's can run on 87 octane, even though it has a static 11:1 ratio.