Advanced topic this one
I've got somewhere a model of this I made a couple of years ago. The graph on my site has the incorrect shape, by the way, it is exaggerated bell shape to make a point.
Max speed for gasoline is at AFR 12:1 or thereabouts. Go leaner or richer and it slows down (effectively retarding the ignition)
It is a 3D graph, because mixture DENSITY also affects burn speed. More density (boost, nitrous) more speed. It is the *main* reason you need to retard the ignition under boost or nitrous, despite what most people think.
One example of misguided 'experts': Running lots of NOS and running pig-rich as well. They think that the extra fuel cools down the situation, when in reality it simply slows down the burn speed so that they don't need to retard the ignition. False logic ofcourse, because if they were to lean down and retard the ignition properly they'd be better off. The extra fuel just messes up their overwhelmed spark plugs. :wink: