Detonation by it's very nature is NOT TIMED. A hot spot like a bit of carbon can ignite the incoming charge while the intake valve is open. That will back out the intake real fast. Lean condition will make the hot spot hotter and that will make the detonation issue worse. This all gets worse with higher compression, think about the carbon build up for that. And last but not least, if the mixture is lean the plug will get hotter and cause the incoming charge to ignite while the valve is open. So detonation can have more than one cause. Most of the time it is related to the timing, but it can be just a hot spark tip, rough valve edge, or hot bit of carbon. And lean will make it worse.
