An important aspect of a spark plug is its heat range, which refers to the thermal conductivity of the spark plug or the rate at which heat is drawn away from the spark plug tip, i.e., the insulator tip and the spark plug's central electrode. The insulator material, the length of the insulator tip and the alloy material of the center electrode affects the operating temperature of the spark plug, but its heat range is affected by the size and shape of the space between the insulator and spark plug's shell. A spark plug with a short insulator tip around its central electrode is a cold spark plug because it has better thermal conductivity. This is because the heat caused by the combustion process has a shorter distance to travel from the spark plug's tip to the water jacket in the cylinder head. A spark plug with a long insulator, on the other hand, is a hot spark plug because the heat takes longer to dissipate into the water jacket.
What makes the heat range of the spark plug important is the reliability and longevity of the spark plug. A spark plug that is too hot could fracture due to excessive heat but, more critically, it will become a hot spot in the combustion chamber causing pre-ignition and detonation that is often referred to as engine knocking. However, a certain amount of heat is required to prevent the spark plug from fouling. A cold spark plug will be prone to carbon deposits and fouling. The carbon deposits create an insulation barrier around the spark plug's electrodes that makes the spark plug less effective and the quality of the spark will tail off as the carbon deposits increase. Therefore, it is best to use a spark plug that is hot enough to prevent the buildup of carbon deposits and fouling, especially at low RPM or idling, but is not too hot that it will become a hot spot at high RPM and maximum power.
Selecting the Correct Spark Plug
As you probably realized already, different driving conditions that result in different temperatures in the combustion chamber will require spark plugs of different heat ranges. Fortunately a spark plug that meets most driving conditions for a stock production car has already been identified by the manufacturer. However, the situation is different for modified cars, where the difference in driving conditions will be more extreme. Here, you might not find the perfect spark plug but will have to settle for one that, while it is not ideal, won't foul during cold start and idle, and won't cause pre-ignition under full load.
A good rule of thumb for modified cars, as a starting point, is to begin with spark plugs that are one step colder for every 100hp increase in power. You can then check if you are using spark plugs of the correct heat range by inspecting the spark plugs after driving in different conditions, such as stop-starting, cruising, and wide open throttle racing. However, ensure that you ignition timing is correct before checking your heat range as ignition timing affects combustion chamber temperatures and, consequently, the temperatures the spark plugs are exposed to. On to the testing: after driving the car under one driving condition, shut the engine, remove and inspect the spark plugs. It is important to turn the engine off immediately, especially if you testing at high RPM or wide open throttle conditions, rather than allowing the engine to idle as that may mask deposits on the spark plug. If the spark plug electrodes are covered by soft sooty black deposits, then the spark plug is too cold. However, the soft sooty black deposits can also indicate that your air/fuel mixture is too rich; and if the deposits are moist, it means that oil is finding its way into the combustion chamber. If the porcelain insulator of the central electrode is white or brittle, and/or there is excessive erosion of the electrodes, then the spark plug is too hot; though this can also be an indication that your ignition timing is advanced too far, that you air/fuel mixture is too lean, or that there is a leak on your intake manifold. If the spark plug electrodes exhibit grayish to light brownish deposits then the spark plug is of the correct heat range. It might also be useful to have a little sparkplug holder for each cylinder. This could simply be a piece of cardboard with holes in it, each marked with the cylinder number. This will allow you to identify if there is a problem with one of your cylinders.
Of course, if the spark plug is too hot, you need to change to a spark plug of a slightly lower heat range, and if the spark plug is too cold, you need to change to a spark plug of a slightly higher heat range. Anytime that you change to spark plugs of a different heat range, you must to test all driving conditions again to ensure that the spark plug is the best fit for all conditions.
Factors influencing spark plug temperature
There are a number of factors that affect the operating temperature of the spark plug and consequently affect required heat range. This includes the air/fuel mixture, ignition timing, cylinder pressure, etc. Some of these, such as air/fuel mixture and the ignition timing, must be addressed before you can determine the correct heat range for your car.
- A lean air/fuel mixture, with too much air, leads to an increase in cylinder-head temperatures while a rich air/fuel mixture would lead to an increase in fouling.
- Ignition timing that is too far advanced leads to higher cylinder head temperatures while ignition timing that requires more advance leads to lower temperatures as the air/fuel mixture may not be completely burnt.
- Cylinder pressure, which can be affected by increasing the compression ratio or by fitting turbochargers, superchargers or nitrous injection, increases the cylinder head temperatures and will thus require a colder spark plug.
- Prolonged high RPM, wide open throttle, high load engine operation increases cylinder head temperatures as it reduces the time available for the dissipation of heat. A colder spark plug would thus be required.