A spark plug’s heat is a measure of the spark plug’s ability to remove heat from the combustion chamber. The heat range measurement is determined by several factors: the length of the ceramic centre insulator nose and its ability to absorb and transfer combustion heat, the material composition of the insulator and centre electrode material.
The insulator nose length is the distance from the firing tip of the insulator to the point where insulator meets the metal shell. Since the insulator tip is the hottest part of the spark plug, the tip temperature is a primary factor in pre-ignition and fouling.
Whether the spark plugs are fitted in a lawnmower, boat or a race car, the spark plug tip temperature must remain between 450°C-850°C. If the tip temperature is lower than 450°C, the insulator area surrounding the centre electrode will not be hot enough to burn off carbon and combustion chamber deposits. These accumulated deposits can result in spark plug fouling leading to misfire. If the tip temperature is higher than 850°C, the spark plug will overheat which may cause the ceramic around the centre electrode to blister and the electrodes to melt. This may lead to pre-ignition/detonation and expensive engine damage. In identical spark plug types, the difference from one heat range to the next is the ability to remove approximately 70°C to 100°C from the combustion chamber. A projected style spark plug firing temperature is increased by 10° to 20°C.
The firing end appearance also depends on the spark plug tip temperature. There are three basic diagnostic criteria for spark plugs: good, fouled, and overheated. The borderline between the fouling and optimum operating regions (450°C) is called the spark plug self-cleaning temperature.
The temperature at this point is where the accumulated carbon and combustion deposits are burned off.
Bearing in mind that the insulator nose length is a determining factor in the heat range of a spark plug, the longer the insulator nose, the less heat is absorbed, and the further the heat must travel into the cylinder head water journals. This means the plug has a higher internal temperature, and is said to be a hot plug.
A hot spark plug maintains a higher internal operating temperature to burn off oil and carbon deposits, and has no relationship to spark quality or intensity.
Conversely, a cold spark plug has a shorter insulator nose and absorbs more combustion chamber heat. This heat travels a shorter distance, and allows the plug to operate at a lower internal temperature.
A colder heat range can be necessary when the engine is modified for performance, subjected to heavy loads, or is run at high rpm’s for a significant period of time. The colder type removes heat more quickly, and will reduce the chance of pre-ignition/detonation and burn-out of the firing end. (Engine temperature can affect the spark plug’s operating temperature, but not the spark plug’s heat range).
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 too cold there is risk of fouling during extended periods of idling and low.
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.