How to gap a sparkplug
Firing tip configuration
As a rule of thumb, the more the spark gap is exposed to the air/fuel mixture, the easier to initiate combustion. Better combustion equates to more horsepower, improved throttle response and higher fuel economy. Unfortunately there is no one "best" tip configuration for all engines. Each plug manufacturer offers a variety of designs each with its own advantages and disadvantages. The only thing we found the plug techs agreed on was that for performance issues, a fine wire center electrode will provide a stronger spark from the existing voltage. However, fine wire electrodes will not survive in many modified motors. Fortunately, there are a variety of firing tips for you to choose from. After you have narrowed your plug choices according to your shell construction features and heat range. you will see each of the remaining choices have their firing tip described and pictured.
The term spark plug heat range refers to the speed with which the plug can transfer heat from the combustion chamber to the engine head. Whether the plug is to be installed in a boat, lawnmower or racecar, it has been found the optimum combustion chamber temperature for gasoline engines is between 500°C850°C. When it is within that range it is cool enough to avoid pre-ignition and plug tip overheating (which can cause engine damage), while still hot enough to burn off combustion deposits which cause fouling .
The spark plug can help maintain the optimum combustion chamber temperature. The primary method used to do this is by altering the internal length of the core nose, in addition, the alloy compositions in the electrodes can be changed. This means you may not be able to visually tell a difference between heat ranges. When a spark plug is referred to as a "cold plug", it is one that transfers heat rapidly from the firing tip into the engine head, which keeps the firing tip cooler. A "hot plug" has a much slower rate of heat transfer, which keeps the firing tip hotter.
An unaltered engine will run within the optimum operating
range straight from the manufacturer, but if you make modifications such as a turbo, supercharger, increase compression, timing changes, use of alternate racing fuels, or sustained use of nitrous oxide, these can alter the plug tip temperature and may necessitate a colder plug. A rule of thumb is, one heat range colder per modification or one heat range colder for every 75100hp you increase. In identical spark plug types, the difference from one full heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber.
The heat range numbers used by the various manufacturers are not universal, by that we mean, a 10 heat range in Champion is not the same as a 10 heat range in NGK. Some manufacturers numbering systems are opposite the other, for domestic manufacturers (Champion, Autolite, Splitfire), the higher the number, the hotter the plug . For Japanese manufacturers (NGK, Denso), the higher the number, the colder the plug .
Do not make spark plug changes at the same time as another engine modification such as injection, carburetion or timing changes as in the event of poor results, it can lead to misleading and inaccurate conclusions (an exception would be when the alternate plugs came as part of a single precalibrated upgrade kit). When making spark plug heat range changes, it is better to err on the side of too cold a plug . The worst thing that can happen from too cold a plug is a fouled spark plug, too hot a spark plug can cause severe engine damage.
Air fuel mixture
Theoretically the ideal air fuel ratio for gasoline motors is 14.7:1(14.7 lbs of air to 1 lbs. Fuel), however due to cylinder wall wetting, the fuel ratio is increased to 12.2 to 1. Ratios higher than this may be too lean and can contribute to pre-ignition or detonation. Ratios lower may be too rich and cause fouling although some favor a richer mixture due to a slightly slower burn and increased charge cooling (cooling from the air/fuel mixture on intake).Source: www.garbee.net