Understanding Spark Plug Heat Range
One of the most misunderstood aspects of spark plugs – is the heat range. One misconception is that the heat range is related to the spark temperature or intensity. Another common misconception is that the spark plug is designed as a heat sink to “remove” heat from the combustion chamber. These ideas are both false. The spark plug is heated during combustion and must dissipate that heat to the cylinder head at a certain rate to avoid overheating the ceramic firing end. The spark plug heat range only indicates the rate that the spark plug dissipates its firing end heat to the engine.
A hotter heat range spark plug has an insulator design with a longer heat flow path to the metal shell of the plug. As a result, more heat stays in the ceramic firing end and less is dissipated to the engine. A colder heat range spark plug has an insulator design with a shorter heat flow path to the metal shell of the plug. As a result, less heat stays in the ceramic firing end and more is dissipated to the engine. For a spark plug to function properly, it must have a tip temperature high enough to burn off carbon deposits (self-cleaning) and avoid fouling, while remaining low enough to avoid overheating the ceramic firing end and pre-ignition.
For most vehicles, the factory recommended heat range is sufficient; however, on some modified or special-use engines, alternative heat ranges may be necessary. Often hotter heat ranges have been used to address a fuel delivery or oil consumption problem. Installing a hotter heat range plug will reduce the pre-ignition safety margin, so it is better to correct the mechanical or tuning issue instead of changing the plug heat range.