As a rule of thumb, the higher the nitro content the cooler the plug. If your running 30% nitro you'll want to run a med. to med. cold plug. Run a plug that is too far down on the cold scale and you'll have trouble with idling and getting it tuned properly. Running a plug that is too hot and you'll have detonation problems. If the detonation is severe enough, it will burn a hole in the piston. You can also fudge a little in either direction depending on the weather conditions. I prefer to run a med. plug because the engine idles and tunes good plus it doesn't cause detonation problems