ORIGINAL: B.L.E.
I believe it would more accurately be described as pre-ignition, which is not the same as detonation although pre-ignition can often lead to detonation.
It has been my observation that a glow engine that is too lean behaves a lot like a model diesel engine that is over-compressed and a spark ignition engine that has it's ignition timing over advanced. This leads me to think that we actually use mixture adjustment to fine tune the ignition timing of glow engines.
B.L.E.,
Pre-ignition is an ignition caused by a hot-spot within the combustion chamber, like a sharp edge on the piston, or the head, or carbon deposits. In a spark-ignition engine, it can even be the electrode of the plug.
In a glow engine it is nearly always the glow-plug coil that initiates the ignition.
So too hot a glow-plug would simply be an overly advanced ignition point. And yes, this can cause the fuel-air charge to detonate, as the piston continues to rise and compress it, after the advancing flame-front had hit it...
About the needles being the ignition timing control, I have been saying it all along, in several past threads here.
With the compression ratio being a given value, as is the fuel you filled the tank with, the prop load you are using and the glow plug you installed, the needle is the only thing that allows you to change the ignition timing.
With a leaner mixture being easier to ignite and faster to burn, it is clear what must be done to increase and optimize the ignition advance. Just screw the needle in a bit.
Using too hot a glow-plug (or too much nitro) requires you to use more fuel, to delay the ignition, reduce the advance.
This fuel is just wasted, just leaving you more gunk to clean off the plane after the flying day.
...Not to mention that you had to pay for all that fuel.