spark and MAP

[Edward Hernandez R]

"I can see the burn time being a function of a/f mixture (rich mixture
=>reduced burn time).I don't understand why rpm alone will cause the
flame burn time to vary unless there's a significant contribution from
heat of compression. What's the involved mechanism?"

First, rich mixtures do not burn faster. True, just slightly rich of 
stoich is where you get the fastest burn rate. Burn rates drop as you 
get leaner or richer than that, something to consider since most DIY 
A/F ratios at WOT are not tightly controlled, especially with SD based
systems(oops, there I go again). This means your spark advance changes
not only with rpm and load, but with A/F ratio. Control, that is to 
say, fix A/F, and you can eliminate one variable.

The reason that burn rates rise with rpm is that charge motion 
increases, and the faster the mixture is swirling/tumbling around, the
 faster it burns. There are lots of things affecting burn rate, but 
above a certain rpm, charge motion is dominant. Another variable is 
heat, as you guessed, which also rises with rpm. It isn't dominant, 
but it does make for faster burn rates.

Incidentally, high swirl/tumble combustion chambers with their fast
 burn rates tend to keep the fire lit after you spark it. This is one 
of the biggest design criteria for lean burn engines. It's hard enough
to light these lean mixtures(and very rich ones), but half the problem
is preventing the fire from going out. That's usually the #1 reason
that certain engines have a rough idle: slow moving charge, slow burn
rates, fire may get lit but then it goes out.