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 spark plug gapping, which is simply a matter of bending the ground electrode so that it is closer to or further from the central electrode. On a spark plug with a single ground electrode, this can be accomplished quite easily with a gap gauge to widen the gap or a light tap on a hard surface to close the 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 air/fuel mixture will come into contact with the spark and the easier it will be to ignite the air/fuel mixture. However, a bigger spark plug gap requires more voltage to create a spark that can arc across the gap between the central electrode, that is connected to the coil via the HT leads, and the ground electrode. Similarly, when the combustion chamber pressure is increased more voltage is required to arc the spark plug gap. Insufficient voltage will result in a failure to create a spark across the gap and may be noticeable as a misfire. However, misfires are not always noticeable, especially at high rpm but it will have an adverse effect fuel consumption and on engine power and performance.
A narrow spark plug gap would require less voltage to spark, but the spark might be too small and weak to ignite and consume the fuel-air mixture in the time available for the ignition phase of the Otto cycle. The result is a failure to effectively convert the chemical energy in the fuel-mixture to mechanical energy, which is how engine power is produced. Thus, engine power and engine performance will not be optimized. However, it's not simply a matter of increasing the spark plug gap and the output voltage from the ignition coil to improve power as 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 suite the performance of your engine and your driving style.
As a rule, a properly gapped spark plug will burn hot without being too wide at high rpm to cause a misfire. 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. The spark plug itself, and the residue that forms on it, would indicate whether the gap is too big or too small. A light brownish discoloration of the tip of to porcelain insulator indicates the proper operation of the spark plugs with the gap being ideal or close to ideal for the most recent engine speeds. Thus, to check the spark plug gap at high engine speeds, you'd need to run at full throttle and immediately turn the ignition off without allowing the engine to idle. But ultimately, you'd need to run your car on a dynamometer 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. Also note that it is not advised to adjust the gap on a multi-electrode spark plug as this will affect the proper operation of the spark plug.