Do you have a particular plane you're setting up, or do you just want to know what plugs you already have? If the latter, tell me what engine, prop, plug, fuel and plane you have in mind and we can take if from there. The plug is like a distibutor in an engine so to speak. It's a way to advance or retard the ignition point based on a what you are experiencing now. I, like you took an interest in this very thing awhile back and have some info at home in the archive. Let me know_Bob