> Perhaps you mean the change in burn time (i.e with different mixture
> ratios)
> but if not then certainly the burn time occupies a significant portion of
> an
> engine revolution or we wouldn't need ignition advance in the first place.
>

It has always been my understanding that ignition advance is used to
compensate for piston velocity as it approaches TDC. By increasing the
advance as RPM's increase, the intent is to time the spark to occur at the
optimum "instant", literally. The subsequent combustion and pressure occurs
as the piston passes through TDC, creating the power stroke.

doug