Spark Plugs - Heat Ranges & Construction

A plug's heat range is its ability to transfer the excess heat from the insulator tip to the cylinder head. The speed of this transfer is commonly described by the term "hot plug" and "cold plug."

A "hot plug" means that the heat transfer is slow, causing the plug to operate at a higher temperature. A "cold plug" has a faster rate of heat transfer, thus it operates at a cooler temperature. In other words, a hot plug has a low heat range and a cold plug has a high heat range. That being said, it's easy to see how heat ranges can be confusing.

Plugs are available in different heat ranges to accommodate the operating conditions of different engines and driving conditions. A plug must operate hot enough to stay clean (i.e., not foul) and cold enough to prevent pre-ignition (premature ignition of the fuel-air mixture). If pre-ignition were not controlled, engine performance would drop and the plug would eventually destroy itself by overheating