A common misconception about the terms hot and cold as they apply to spark plugs is that hot refers to higher levels and cold refers to lower levels of voltage.

Turns out the terms are all about heat – or the lack of it.

A hot plug operates hotter than a cold plug, explains Sam Steadman of Autolite, thanks to the fact that the length of its center electrode or firing tip is covered with more ceramic insulation (see the illustration at upper right). “Heat range is a measure of how fast a plug’s firing tip dissipates combustion heat,” he adds.

A cold spark plug has less of the length of its center electrode covered with insulation. This allows combustion heat to easily travel from the firing tip to the plug’s shell and then into the cylinder head where it dissipates. The insulation on the tip of a hot plug is longer, which requires combustion heat to travel farther before dissipating.

Other factors affecting heat range are the thermal characteristics of the insulator and the bond of the insulator to the plug’s shell.

The balance between hot and cold is crucial to tip performance, Steadman says. The firing tip must operate hot enough to burn off combustion deposits. But a tip must stay cool enough to avoid preigniting the fuel-air mixture as well as to prevent eroding the side electrode.

Spark plugs are designed in many heat ranges to suit a wide variety of engine applications. Switching between heat ranges can change a plug’s operating temperature by 70°F. to 120°F.

“If you are unsure of the correct heat range to use, always start with a colder spark plug,” Steadman recommends. “If it is too cold, the plug will eventually foul and misfire, and while this results in rough operation, it won’t hurt the engine.”

Selecting too hot a plug can lead to preignition and detonation that can severely damage an engine.

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