[psalt]

That is is way cool diagram. I have never seen such a thing.
Is Zb pre-ignition or detonation? I am thinking Zb is detonation.
It looks like the combination of early ignition that builds heat and pressure is kind of multiplied by an increase in pressure from the piston being closer to TDC until where the mix auto ignites or flash's instead of burning in an orderly manner.
I would suspect pre-ignition would show a spike in pressure before TDC but not sure.
I think I read that we want peak pressure at about 14 deg after TDC for peak HP. That would be Za it looks like.
With Zc the mix is still burning and adding pressure after the other two times but the piston retraction is such that the the piston dose not see as much peak pressure. This is also the condition where more exhaust heat is created as the gasses are still expanding when the exhaust cycle is started.
Can near ideal timing be measured from EGT's. That is is it the low point between detonation and late ignition? Or maybe we can start w late timing and start advancing until temps start to taper as they get cooler?



This is the standard Bosch diagram of detonation, it appears in many of their books, the little Blue Handbook and Gasoline Engine Management. Z b (2) is detonation (knock), the normal 2ms burn rate is accelerated by the spontaneous explosion of unburnt fuel, causing the abnormal pressure spike. It is a bit of a cartoon and the actually pressures are different. The point to understand is that knock is not normal burning, it is an uncontrolled explosion with no benefits. The increase in pressure does not result in higher output, too much work is lost BTDC. This is why power drops once the knock limit is reached. The lower output of retarded timing occurs because of the mechanical advantage difference and a loss of heat. EGT rises as ignition is advanced, falls once knock occurs, but is not a safe reliable indicator for tuning. Peak CO2 can be a good indicator of ideal ignition timing. With all hemi headed engines with wide VIA's and domed pistons (like a 911) it is safe to assume they are knock limited on pump fuel. The "ideal" timing can only be found using the highest octane race fuel, then they are retarded (detuned) for what ever octane fuel will be used, plus a safety margin. The peak output for N/A hemi head road race engines was in the 50's when they ran 14:1 compression, domed pistons and 50 BTDC ignition timing. The reason this was possible was they were burning butyl alcohol, methanol, benzene and nitro instead of gasoline and the fuel acted as a coolant to prevent detonation. Corn politics aside, E85 could be a 911's best friend.