there are a lot of in betweens but basically,you bring the head closer to the piston you increase compression because you are making the combustion chamber smaller.when compression is higher more heat is produced,as was mentioned here nitro has some very good quality's for making power.i ran an asp and in summer used two head shims because of the ambient temperature.in winter they can tolerate 5 to 10%.as far as hearing detonation in an engine i honestly can say i have never herd it in one of these small engines and partly due to the fact that we fly them such a distance away.there is a distinct difference in pitch when wide open and leaned out properly i depend on a tach to make sure the engine is running close accordingly to prop rpm's.if you have a high compression engine and can get no % nitro use it.in my case i cant find in the Hobie shops in this area o nitro so i added shims to my engine as recommended (asp90)and run 10% due to the fact that our club buys it in bulk and we get a good discount.