<VV> Re: combustion "speed"

[djtcz at comcast.net]

"I always thought that the higher the octane, the slower the combustion. Now doing a google search the top hits on gasoline "flame speed" and octane, the top hits say that the combustion speed is independent of octane. Could someone amplify that for me please because if true there is no reason to retard the timing when running a lower octane. (my rule of thumb has always been 1 degree per 
octane number so to run 87 in an engine designed for 93 you retard six 
degrees initially. According to what I see on the web, this is not true)"

Kevin Cameron has done a few articles about spark ignition engine combustion over the years in Cycle, Cycle World, and his Sportbike book.
I believe the answer may lie in the description of coumbustion as a complex process that must begin with a delay period when free radical formation takes place as a necessary first step. Kind of like the start of a foot race, when the starter says "ready, set, go."  Hi test has has a longer delay period, so the mixture in the far corners can't start so easily/quickly on its own when pressure and temp starts to rise as normal combustion progresses.
http://www.mech-eng.leeds.ac.uk/res-group/combustion/activities/knock.html


--
Dan Timberlake