Spark Plug Heat Range

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Sep 21, 2006
Sangre de Christo Mtns of North Central NM.
I never understood what this really means. When one plug is "hotter" than another, what does that really mean? Is it that the spark is bigger or lasts longer during the ignition and expansion cycle?

Thanks in advance for info.
A spark plug's heat range has no relationship to 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 measurement is determined by several factors: the length of the ceramic center insulator nose and its ability to absorb and transfer combustion heat, the material composition of the insulator and center 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 center 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 center 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 ont he 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 rpms 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).
Is there any relationship between spark plug heat range and fuel octane; e.g. can changing plug heat range enable an engine to run efficently on lower octane fuel?

That was a lot to digest and must have taken no small amount of time to organize and write out. I thank you for the time and clarity of the post.
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