An important aspect of a spark plug is its heat range, which refers to the rate at which heat is drawn away from the spark plug's central electrode. A spark plug with a short central electrode is a cold spark plug because heat has a short distance to travel to the water jacket in the cylinder head. A spark plug with a long central electrode 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 will fracture due to excessive heat and, more critically, will become a hot spot in the combustion chamber that will cause pre-ignition and detonation, sooner rather than later. 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 and once the sparkplug is fouled, it will become less effective and its spark quality will tail off. Therefore, it is best to use a spark plug that is hot enough to prevent fouling, but is not so hot that it will fracture or become a hot spot.
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. On modified cars you can 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 full throttle racing. After driving the car under one driving condition, remove and inspect the spark plugs. 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 spark plug is of the correct heat range. Of course, if the spark plug is too hot, you need to change to a spark plug with the lower heat range, and if the spark plug is too cold, you need to change to a spark plug with the hotter 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 appropriate for all conditions. In our next section we'll look at the spark plug gap.