gasoline engines for the most part use spark timing to time the explosion of the fuel/air mix. But in high performance engines fuel/air mix can ignite early due to high compression, also known as "knocking" this is why they use higher octane gas. ie 116 octane vs regular 87. I like to call r/c engines high performance because of the amount of power potential outof such a small displacement. As for the breaking in period, this is so that the separate parts can adjust to work with each other, things like the piston and either the sleeve or the rings will adjust to small small imperfections in the boring of the cylinder. In 4 stroke engines the pushrods will adjust to the cams and so on... It is impossible to ruin an engine by running it really rich. The onlything that this will hurt is your wallet. By running an engine rich it will run cooler, opposed to running it at peak rpm by leaning it out. In the summertime especially you should increase the oil content a bit and richen up the mix because the added heat of the enviroment and the running of the engine can cause it to cease. As for breaking in an engine with 5% nitro, when you only have 0% and 10%, just take equal quantities of each and mix them.