[Mgs] Engine maths...and spare time

[PaulHunt73]

That's exactly what does happen, because ignition is anything from 10 degrees upwards BTDC.  It's a trade-off between how much power you lose by too much of the fuel burning while the piston is still coming up - over advanced, against how much you lose with fuel still burning far into the expansion stroke - retarded.  There is a sweet-spot, but that continually changes with rpm and  throttle opening, which is why we have centrifugal and vacuum advance.  Centrifugal increases the advance as the rpm increases and the likelihood of detonation decreases, vacuum advance backs off the timing as the throttle is opened and more mixture goes into the cylinder giving higher cylinder pressures, which increases the likelihood of detonation.

PaulH.
  ----- Original Message ----- 


  Well, it seems to me that if you advance the spark to a point on the compression stroke where the piston is still coming up, the expanding ignited mixture would be fighting the piston, and it would actually reduce power. I’m not sure if that is what you are talking about, though.
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