Waterbug,
You wont hear detonation. Detonation will leave a sand blasted finish on the head on the top of the piston. Usually when it does this it will blow the plug. Depending on how hard the engine is working it may take the element out completely. If this happens you should take the head off to check for the sand blasting.
If you have the compression high enough you will need to keep the engine sightly rich to stop it from detonating. Running slightly rich with this setup is not really a problem as the engine still makes plenty of power and torque. My K-90 is set up so critical on the compression that it will detonate if I go lean, but the detonation causes the plug to get so hot the centre is blown clean out. I have done this 3 times so far.... :
If you do have detonation you should use use 800 grit wet/dry sand paper and WD-40 or similar on a flat surface to lightly polish the flat surface of the head. This is so you can see if it detonates again.
Craig is correct about running minimal squish clearance for best power. For something like the 18, that could be about 8 thou. For larger motors slightly more is needed and I run all my 90's and 100's at 11 thou. An effective squishband prevents the mixture from igniting out towards the edge of the piston. This area is one of the main sources for detonation, which is combustion starting from a point other than the glow plug, and coliding with the combustion from the plug creating damaging shock waves and very high pressures. If the squish band is working properly (by having a small clearance) the compression can be higher without detonation than if the squish clearance is too high. More compression equates to higher torque and power! I use about 7% castor oil in my fuel (with another 13% synthetic of course!) which will burn leaving a light brown film on the top of the piston and in the head. Generally there is a ring with no brown around the outside of the piston, which indicates that the fuel is not burning there and thus the squish band is working.
I helped a friend set up a CMB 45. It was detonating. I checked the head clearance, took OUT a shim, and the engine not only stopped detonating but ran and idled much better as well!
The downside of running on the limit of compression is it does not take much of an upset to also cause the engine to detonate again. Blow a water line. Detonation. Little too lean. Detonation. It keeps you on your toes! But also maximises power. ;D
Personally I have worked out in testing what compression ratio I can use and be on the edge with using 25% nitro. I measure each engine I get, based on that compression ratio I make a head to suit which also has the squish clearance I want without using head shims. Not a simple process tho....
Nitrocrazed racing: How did this all get so hard?....
