Some more good info on plugs and heat ranges:

https://www.crossfireforum.org/forum...tml#post475399

To summarize: as a rule of thumb you should go with one step colder plugs for every 75-100hp you've added via mods. Also it is better to err on the side of a colder plug than a hotter one. Too hot of a plug can cause detonation and major engine damage. Too cold a plug will just foul quicker.