What exactly is a spark plug “heat range”?
The heat range has nothing to do with the actual voltage transferred through the spark plug. Rather, the heat range is a measure of the spark plug’s ability to remove heat from the combustion chamber. The heat range is determined by the insulator nose length and its ability to absorb and transfer combustion heat, the gas volume around the insulator nose, and the materials/construction of the center electrode and porcelain insulator. In identical spark plug types, the difference from one heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber. A longer the nose on a spark plug forces the heat from the tip to travel farther before it is absorbed by the cylinder head, which reatins more of the heat in the plug tip – making the plug “hotter” than a similar plug with a shorter nose. Engine temperature will affect a spark plug’s operating temperature, but not the plug’s heat range.
