A "hot plug" has a long insulator nose which exposes more surface area to the combustion gases. This keeps the plug temperature higher overall which is ideal for stop-start city driving conditions.

A "cold plug" has a shorter insulator nose, which minimizes the amount of surface area exposed to the combustion gases. Cold plugs are typically used in racing conditions because of its ability to transfer heat out of the cylinder chamber quickly. Most turbo, supercharged and nitrous oxide applications use this type of plug because of the tremendous heat they generate. A colder plug can also minimize the risk of pre-ignition and detonation, however if the plug is to cold there is risk of fouling during extended periods of idling and low-speed operation.

The heat range number located on the insulator indicates the temperature range of each DENSO spark plug. The lower the number, the hotter the plug; the higher the number, the colder the plug. Information about reading plugs can be found at the Find My Part - Identify DENSO Spark Plugs page.

Heat Dissipation in a spark plug. Of the 100% heat generated from the combustion, 20% is absorbed by fresh air from the intake of the following stroke. 58% of the heat is absorbed by the walls of the cylinder head which hold the spark plug in place. Twenty percent is absorbed by the insulator and side walls of the plug, the remaining 2% being absorbed by the spark plug wires.