One more point. The fact about higher octane fuel burning slower is exactly right. The lower octane fuel cannot tolerate as much timing advance because it burns faster, and the combustion process is too far along before the piston gets to the top. It is important to remember that we want the fuel mix to burn and expand; not explode, and that we want the peak pressure of combustion to occur around the time the piston is passing top dead center. This gives the most power, and is why we light off the mixture before top dead center to begin with. This is easy to visualize while adjusting the timing on a running engine. The engine will increase in speed, (and efficiency) as you advance the timing from a too slow position, and then start bucking and running rough when you get to the too advanced point.

AV8TOR