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.