Heat range refers to the spark plug's ability to transfer heat out of the electrode, through the body of the plug, and into the head. When you run a "hotter" plug that means the plug will retain more heat in the electrode and in power adder applications can lead to pre-ignition and/or detonation. The opposite effect is if you run a plug that is too cold. This type of plug will not get hot enough during normal driving and will collect more combustion deposits and will eventually fail. The key is to start with a plug recommended for your application and learn to read plugs to determine if you should move up or down in heat ranges. Also, see the thread about side gaping your plugs as there is some good little tips and tricks in that thread. Good luck.
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