I never said nitro itself cools the engine, but the richer needle setting does, once you start to go above 20% you need to lower the compression ratio with head shims or will overheat. As fare as varnish is concerned, I've run up to 40% nitro in the Tee Dees and not had a significant/noticeable increase in varnish, varnish is just plain weird, it generally gets worse with humidity, and some days it doesn't? Fore instance this just this last Sunday I taught two youngsters how to fly control line, and we must have put through 20 tanks fuel through a well used blackwidow engine, and when we striped it later in the day it was as clean as a whistle. While a couple of weeks ago I was running in new old stock golden bee and it varnished up after just two tanks, and that was using a lower nitro fuel.