OK lets give this a thinking about !
Lets start with he fact that pre-ignition and detonation are two diferent things , pre-ignition is the ignition of the charge it the cylinder normaly by glowing embers of carbon and the charge burns in the normal way producing a relitivly slow and controled flame front.
Detonation on the other hand is the explosion of the charge in the cylinder in this case the burning of the charge is uncontroled and almost instant.
As you say this produces in cars "pinking" the tinking noise that you hear when the engine is under load but this is were I have to differ with you and it is my opinion that the noise is partly due to the un-even loads on the top of the pistion resulting in the piston skirt hitting the side of the cylinder.
Pre-ignition can result in detonation because if the charge is ignited early the flame front and the piston rising result in the un-burnt charge in the cylinder furthest from the point of ignition detonating due to the pressure / temp rise hence the uneven loads on the pistion.
The nub of what I was driving at was why is this most likely when the mixture is lean ?.
As you may have seen Avgas grades are quoted as 80/87 , 100/130 , 115/145 most of these grades have been phased out and it is unfortunate that 100LL that we all know should be designated 100/130 but those numbers were already taken.
The numbers refer to the octane rating of the fuel , the larger the number the better the detionation resistance characteristic of the fuel.
Why two numbers ? this is because the detionation resistance is dependant on the mixture , the first number is the octane rating at max lean mixture and the second number the octane rating at full rich mixture.
From this you can see that leaning an engine at relitively low RPM ( after all most engines will not make max RPM on the ground due to the prop pitch ) and therefore low piston speed and decreaseing the fuel octane rating by leaning the mixture can result in detonation due to the combination of the lack of detionation resistance in the fuel , the slow progress of the piston up the cylinder relitive to the speed of the flame front that has been set off by a spark that is fixed for cruse RPM , this results in a pressure/ temp rise that detonates the part of the charge furthest from the spark plugs .
I will leave the thermal shock issue on the cylinder heads alone but is must also be a factor when cleaning the plugs by "burning off".
Last edited by A and C; 8th February 2004 at 06:11.