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Originally Posted by shipo
Bingo! You've hit the crux of the matter. The thing is, the extra fuel doesn't cool combustion temperatures per se, it helps slow the flame front, which delays the Peak Pressure Point to further after TDC, that in turn reduces the absolute Peak Pressure and also places the greatest pressure right at the point where it can be the most mechanically useful. Too much pressure too soon, and temperatures skyrocket, and pre-ignition is sure to follow. Now here is the key point, leaning an engine beyond peak torque slows the flame front in the exact same manner (just quicker) as does enriching from peak torque.
A perfect Air to Fuel ratio, called the stoichiometric ratio is 14.7 pounds of air to 1 pound of fuel. As you move away from stoichiometric in either direction, the resultant flame front slows down, and it is that slowing that results in cooler combustion temperatures. As I said before, the trick is to properly manage the combustion enviornment, and that is much easier to accomplish on the rich side because the power curve is much flatter.
The only point I'm trying to make here is that there in no such a thing as "Too Lean" with regard to pre-ignition. I think a better way of saying it is, "For proper engine operation, there is 'Rich Enough' and 'Lean Enough', anything in between and you can kiss your engine goodbye."
Best Regards,
Shipo
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Got it. Makes sense. Learn something new every day. Most auto tuners would have no idea that leaner than stoich could EVER be a good thing under boost. There's an evil cross over spot from around 13:1 to 16:1 that is real nasty.
I suppose in an aircraft engine you'd have a different tune altogether to keep the motor running properly at those leaner mixtures.
Chris.