Hmm, maybe my hangover caused a bit of an over reaction, apologies..
Whilst Pre-ignition and detonation are seperate things they can be and often are linked together and one will often cause the other.
My comments about RPM are really based on a fixed pitch a/c since MAP is actually a bigger concern, but you have no direct indication of it without a CSU. Higher RPM actually REDUCES the tendancy to detonate due to reduced time for the pre-flame reactions, but a high load will INCREASE it because of the increase in engine temps and end gas pressure.
The higher cylinder temps caused by running at a leaner mixture are an aid to detonation compared to the much lower temps you would get with an excessively rich mixture. A mixture of around 13.5:1 has been proven to be the ratio most likely to cause detonation. If the mixture is leaned too far, then the slower speed of the flame front can cause all sorts of issues, especially as less heat is converted to mechanical energy and the cylinder temps will rise rapidly.
For geeks only: for every part you lean the mixture by, the octane needs to increase by 2(RON+MON)/ONR
(RON: Research Octane Number, MON: Motor Octane Number, ONR
ctane Number Required)
I hope the noise you hear in the car is not the piston skirts hitting the cylinder liners since you would have a very second hand engine very quickly. I have seen (heard really!) 'pinking' in engines that have a very short 'skirt' and haven't detected an unusual amount of wear on the liners post strip down which is what I would expect.
However ,what you say is absolutely right, and there can be quite alot of movement caused by an off centre detonation, though this is often catastrophic rather than the annoyance that you hear.
The detonation resistance of the fuel is as you say the octane rating, when running on a higher octane you can reduce the squish and increase the compression ratio without fear of damage. Something I have found to my cost when running an old car at 13:1 on normal 95 octane. The mixture leaned out and pop went a very expensive bit of tuning work. New head please!(and a lot of other bits!)
The important numbers aren't just the number given ie: 100 octane but the difference between RON and MON, the 'sensitivity.'
As a rough guide for a carburetted engine I'll reproduce some figures I have for octane ratings required compared to compression ratios and then to thermal efficiency.
Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %
I hope this makes sense and I apologise if I have been overly technical in my use of language. This is a hugely complicated subject, and to go into it in great depth would require far more typing than I can be bothered with and enough text to bore 50 people to death with.