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#7374
Folkert
Participant

maybe this is an explanation for different advanced/retarded timing:

slow engines like the SCOTT are working like a piston pump – the piston sucks and presses the gases through the engine like in an air compressor.
Therefore more advanced timing for the spark at “high” revs is correct in the view that there is less time for burning the mixture.

In very high revving modern engines the cylinder porting and piston position acts like a trigger for the timing and direction of pressure waves in the induction and exhaust system. These engines are only “pumping” at low (starting) revs and will work at higher revs only because the piston and ports trigger the pressure waves in a much more efficient way than a pump could do. In these engines the actual compression rate is more and more independent from the geometrical compression rate and the direction of pressure waves is independent from the moving direction of the gases: for example there is a pressure wave running back through the exhaust to press fresh mixture near the exhaust port back into the combustion chamber for the next working cycle but the burnt gases are still flowing out of the exhaust´s end. It is obvious that such a “wave machine” will need different ignition settings than a pump at least to limit this effect before the engine desintegrates because of mechanical overload and over-revving.The phase in which both (inlet- and exhaust-)valves in 4 stroke engines are open is caused by the same “wave”-effect.