colder plug just means that it will disspate heat in the tip faster.
So if you're running lots of boost, the combustion chamber gets hot. If the plugs tip gets too hot, it can cause detonation, so colder is safer for high (higher than stock) boost.
Under normal driving, cold plug will foul, so thats why it's not a factory item. Some foreign cars run hotter (for better gas mileage) and they use colder plugs to make up for it. I personally am running densos ik16 and have never had a prob. They were recommended to me by Remmy @ corsport
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