Spark Plug Gapping

[ionbeam]

The length of the porcelain will determine the temperature of the spark plug. The port velocity, swirl of the combustion chamber, compression, target fuel mix, cylinder head temperature, coil energy, etc will all go into calculating what the plug temperature should be and how wide the spark plug gap should be.

The wider the gap, the harder it is to initiate an ionization path for spark to develop across. Wide gaps require a high voltage coil. On the one hand, a wide spark gap exposes a large ignition source for the fuel/air, but on the other hand the spark can actually be blown out by the fuel/air velocity in the combustion chamber. This is where a multi-spark capacitive ignition system comes in because it repeatedly sparks the plug.

A narrow gap will be easier to generate a spark but the spark will be small and may make it hard to ignite the fuel/air mixture.

Coils that generate the ignition voltage have an output that varies with how frequently it must generate pulses to the plugs. Typically there is a trade-off where the coil kicks ass at slow pulsing, but the output voltage falls off as frequency goes up, or a lower energy output at low frequency pulsing but the energy increases as the frequency goes up. Motorcycles are difficult because of the high RPMs they spin, requiring the coils to perform better at higher pulse rates.

Spark plug gaps can be off 10-15% in a street engine without causing any driveability problems. You don't truly have a spark plug failure until it starts to cause misfire. As a plug ages it looses the sharp edges that promote spark generation. You look at a plug and it looks 'good', but if you look under a microscope you will see what should be 90º edges around the center electrode and the side electrode have become rounded. Sharp edges promote sparks, rounded edges make it much harder to initiate a spark. Sharp edges tend to focus the energy from the coil and provide an easy site for the spark to jump to.

Plugs can only generate one spark path, regardless of how many side electrodes it has or if the side electrode is split. If the center electrode is made from an exceptionally hard material it will hold the sharp edges but the side electrodes are generally made from the same soft material as the threaded metal base, so the extra electrodes or split electrodes provide lots of consumable edges. As the edge wears the spark will walk along the edge of the electrode until it is 'used up', then the spark moves on to the other split tip or the next side electrode. The spark will always want to jump the shortest distance so the primary sweet spot for sparking is directly above the center electrode, the spark needing to move any farther is tantamount to a wider gap.

It is generally a bad idea to try to gap iridium spark plugs because the tiny center electrode is very fragile.