I agree, Phread, a colder plug in a high rpm engine will help prevent pre-ignition because it is able to shed enough heat between combustion cycles; otherwise it would still be glowing hot enough that the fuel would flash spontaneously as soon as the fuel charge entered the cylinder. Likewise going to a colder plug with a higher nitro content would serve to counter the "timing advance" effect of either higher nitro or hotter plug. There was a great article in RCM, or maybe MAN, a couple months ago that talked at length about glow plugs and heat ranges.