Anybody know what a gasoline burn curve looks like? That would be burn rate on the left or vertical axis and time (or maybe crank degrees) on the horizontal or bottom axis.
My guess is that rate is not constant. It will start off slow, increasing as the flame spreads, then reach a fairly constant speed until most of the fuel is expended, then fall off again. It might be something like a bell curve with a flat portion on the top?
If that is true, the initial part of the burn would affect how long it is before the mixture reaches its "constant speed". I think this would affect the optimal start of ignition.
Maybe the small difference in starting conditions is too small to make a measurable difference in ignition timing. That's kind of where I was coming from in terms of relating octane and timing.
Also, I once dyno-ed an engine with race gas and the power kept increasing with more timing, up to 45 degrees before I chickened out. I can't remember what octane it was now. Something between 104 and 110. I now have another engine with the exact same heads and it likes 37 degrees total timing with 92 octane gas. There are other differences between the two engines which affect timing, as well. Still the exact same heads, though. I think the timing was accurate both times. Same timing gun and even the same balancer on the engine. So I don't think you can say a certain set of heads always wants the same ignition timing regardless of other factors.
Maybe directly relating octane to timing is not warranted. But surely burn rate, or the average burn rate, varies between different gasolines?
I'm just throwing that out there for discussion.
Flame away! Get it?