A spark plug’s Heat Range, defined as the measure of how fast the tip of the plug dissipates combustion heat, can range from as low as 500°F to as high as 2,500°F. Manufacturers offer plugs in many heat ranges, from cold through medium hot, to very hot. It is imperative that the plug stay cool enough to prevent pre-ignition or electrode destruction, and still remain hot enough to burn off combustion deposits that form at the firing tip.
A plug that is excessively cold or hot for the application in question will eventually cause the engine to misfire. When working with an engine that is stock or mildly modified, it is often best to use the plug recommended by the car manufacturer. However, if the compression ratio has been altered, a different heat range plug may be required. Many racers rely upon their engine builder for plug heat range recommendations.
If a racer is unsure of the correct heat range to use, it is always recommended to “stay on the safe side” and use a colder plug. A switch in heat range can change the temperature of the spark plug tip by 70-120°F. If the plug selected is too cold, it will eventually foul and misfire, but it will not damage the engine. A plug which is too hot can cause pre-ignition and detonation, often resulting in a severely damaged engine.
