When it comes to spark plugs, different examples do different things. But there’s a common thread when it comes to plugs: They must ignite the air-fuel mixture in the combustion chamber (igniting the air-fuel charge) all with some manner of efficiency. There’s a catch: There is no perfect spark plug for a particular application. Some power plants run extremely hot (i.e.: nitro burning race cars). Because of the temperature extremes found within something like a nitro funny car engine, a very cold spark plug is required. On the opposite side of the coin, the crew cab dually you drive regularly might have a very cold running powerplant. The type of spark plug utilized is just the opposite — it must be relatively hot. In simple terms, that’s the definition of spark plug heat range.
The “heat rating” of the spark plug (which is nothing more than a description of the thermal characteristics of the plug) reflect the ability to transfer combustion chamber heat from the firing end of the spark plug into the cylinder head. The range of temperatures from idle to maximum RPM determine the heat range of the spark plug. From a design and construction point of view, it is the length of the insulator nose that determines the heat range of the spark plug.
Cold spark plugs normally have a short heat flow path. This results in a very quick rate of heat transfer. Additionally, the short insulator nose found on cold spark plugs has a small surface area, which does not allow for a massive amount of heat absorption. On the other hand, hot spark plugs feature a longer insulator nose as well as a longer heat transfer path. This results in a much slower rate of heat transfer to the surrounding cylinder head (and consequently, the water jacket).
So what’s wrong with using spark plugs that are too cold for your engine? Simple. The powerplant will load up, carbon will form and the spark plugs will need constant attention. If the spark plugs are too hot, it will begin to glow. The result is of course detonation which can easily split a cylinder wall and ruin a cylinder (along with the piston). Of course, these are extreme cases, but even a spark plug that is slightly too hot will inevitably fail over a period of time. In essence, the combustion chamber temperature of your engine dictates the heat range of the spark plug.
Next issue, we’ll discuss the right way to pick plugs for your engine. Stay tuned!