Spark Plug Heat Range
Heat range is a rating of the temperature at which the tip of the spark plug normally operates. It is a coded measure of how much heat the plug can transfer to the engine block and therefore cooling system. Generally, a greater heat range number means the spark plug has a higher operating temperature and thus is a "hotter" plug. However, as an exception, some spark plug manufacturers assign higher numbers to "colder" plugs.
The number can usually be found on the insulator (white portion) of the plug. Heat range ratings vary greatly. "Hot" plugs have long insulators, transfer heat slowly and operate at higher temperatures. Conversely, "cold" plugs have short insulators, release heat faster and operate at lower temperatures.
SPARK PLUG INSULATOR
Courtesy of Car Care Council
