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 available 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 those car parts. 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 air 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 previous section we looked at the spark plug heat range and how to determine the most appropriate heat range for your particular engine. Now it's time to turn to the spark plug gap.
The spark plug gap, along with the combustion chamber pressure and the ignition timing has a direct bearing on the amount of voltage you require from the ignition system. The bigger the spark plug gap, the more voltage you require to have the spark arc across the gap. The same applies when the combustion chamber pressure is increased. The spark plug gap also has a bearing on engine performance. The bigger the spark plug gap, the more air/fuel mixture will come into contact with the spark and the easier it will be to ignite the air/fuel mixture. However, it's not simply a matter of increasing the spark plug gap and the output voltage from the coil. Firstly, there is a limit to the amount of voltage the ignition system can handle and, secondly, there is an optimal spark plug gap that will best performance for your engine and your driving style.
Ironically, the car manufacturer's recommended spark plug gap is not optimal! The recommended spark plug gap is designed to be adequate for cold starting and smooth driving on a car that is in need of an engine tune up. If you drive your car normally and tune the engine regularly, you can increase the spark plug gap by about 0.010" for better performance and better fuel economy. However, if you drive at full throttle most of the time, you should reduce the gap by about 0.010" for better performance. Ultimately, you'd need to run your car on a dynameter to find the best spark plug gap, and the right ignition timing for your engine.
Remember that when you increase the spark plug gap you need more voltage from the ignition coil to create a spark across the spark plug gap. We'll discuss ignition voltage at a later stage. When a greater voltage is required to create a spark, cold starting and firing fouled spark plugs become more difficult. Therefore you should ensure that your secondary, high-tension ignition wiring is at least 8 mm in diameter, and that it is always clean, dry and in peak condition.