for boost higher then stock a colder plug will slow the burn and allow the car to run more boost with out knock. it's more about the temp of the spark then the intake temp. While the intake temp is directly related to this the spark temp is a little different (in the reasons why you want it cooler) if you run boost higher then 14 PSI I would run the 7ES plugs
Hey Bill, are you certain that the colder plug actually slows the burn?
From my understanding, the only difference between a hotter plug and a colder plug is just that, the cold plug actually operates cooler, ie it transfers more heat to the head. Advantage of using a colder plug is that when boost is turned up, the plug won't get too hot and preignite the fuel. On the down side, if the plug is too cold for the boost you're running, then you're not going to get to the operating temperature of the plug, ie high enough to burn off deposits and your plugs is going to get carbon fouled.
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