The purpse of the spark plug is to ignite the air/fuel mixture inside the cylinder. To accomplish this, electricity is applied through the electrode. A spark jumps from the electrode to the ground strap, and if all goes well, a nice controlled burn results in full power to the cylinder, with no hot spots and little unburned fuel.
The plug also serves to conduct heat away from the cylinder. All other things being equal, if you can get heat out of the cylinder, you have a more controlled burn and less chance of harmful detonation. A plug that takes too much heat out of the cylinder, though, will never get up to an operating temperature where the carbon deposits will burn off, and the plug will foul. So you want a spark plug that is "hot" enough to clean itself off, and "cold" enough to prevent detonation.
High turbo boost can increase the cylinder temperatures to a point where the stock plugs cannot remove enough heat, so for high-boost applications, we offer colder range plugs. For cars operating at or near stock boost ranges, the factory-spec plugs will work best.
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