Using high octane fuel in a low compression engine is waste of energy. The combustion process becomes too late because a low compression engine cannot utilize the high octane fuels energy because it is not compressing the air fuel mixture enough and the peak pressure wave of the combustion process becomes too late so think of it this way,The greatest mechanical advantage of a crankshaft is 90 degrees A.T.D.C. and if the peak pressure wave is too late so will the force being applied to the crankshaft and you can only advance the timing so much because the engine will lose top end to try to get the peak pressure wave to arrive near the 90 degrees A.T.D.C. so think of a Diesel engine and the fuel it uses you would have a hard time lighting it off with a match because of it's octane rating being so high but inject it in a super high compression engine and the BTU rating far exceeds a gasoline engine. Using too high of an octane rating on a low compression engine the engine will tend to go flat on the top end.On a two stroke engine the compression ratio should be measured after the piston closes the exhaust port not at B.D.C. the trick is to experiment and find the lowest octane the engine will run on without detonation and still pull hard on the top end and not flatten out there are many factors involved,rod length,combustion chamber shape,air fuel mixture,oil ratio,compression ratio,altitude,ambient air temperature,etc.So a real good way to think about the greatest mechanical advantage in relation to a engines crankshaft is to think about pedaling a bicycle and one can see that the greatest force on the pedal crank is 90 degrees after top dead center.This is the same for the crankshaft of a engine we want the peak pressure wave of the combustion to arrive as close as we can get to 90 degrees A.T.D.C.
