Heat range

Refers to the operating temperature of a given style of spark plug. Plugs are made to operate at different temperatures depending upon the thickness and length of the porcelain insulator as measured from the sealing ring down to the tip. In this way it transfers heat from the combustion chamber to the cylinder head. The speed at which it transfers heat is considered in terms of hot and cold plugs. A hot plug transfers heat slowly, causing the plug to operate at a higher temperature. A cold plug transfers heat more quickly, thus operating at a lower temperature. If a plug is too cold, it will foul. If it is too hot, it will cause preignition.