Rob,
I know that I'm paddling against the current with my perceptions, but I still believe that the heat range of the plug should be selected based on the combustion temp that is is present so that it does not foul or heat to the point of melting or causing a "hot spot" that will cause pre-ignition. Race cars make more power with more heat in the combustion chamber (heat is energy), but race cars always require colder plugs than a street car. At low rpm a hotter plug is better because it will keep the electrode cleaner (not fouling) but race cars operate at high rpm which brings in more cool fuel which produces more energy when it's ignited. The transition from cold to hot is what produces more power. Cold fuel mixture is denser, thereby allowing more molecules of fuel in a given mass of air. Power is derived when that fuel-air mixture is ignited by a spark. I don't think that if the electrode of the plug is hotter or colder, it is going to increase the heat produced by the ignition of that mixture. I believe the spark itself ignites the mixture and the mixture produces it's own heat, regardless of how hot the plug electrode is.
If a hotter plug produced more combustion heat (which equals power) everyone would be running hotter plugs, but race cars require colder plugs. The best way to tune a race car's plugs is to start with the coldest plug you can find, then gradually increase the heat rating until the porclain is nearly white, without a trace of aluminum spots on it.
As I said, I know I'm rowing upstream on this debate, but that's how I see it.
Verne
