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Spark Plug Life

Spark plugs operate in severe conditions. Spark plugs require regular maintenance to provide desired engine performance. The service life of a spark plug is directly related to the voltage required to produce a spark in the electrode gap. As the voltage required to produce a spark increases, the service interval is reduced and the overall service life decreases. Higher voltage reduces the service life of components such as spark plugs, wires, and transformers.

The following six main factors affect the voltage required for spark plugs:

Air-fuel ratio - When the air-fuel ratio is near stoichiometric, the voltage required to produce a spark is the lowest. As the air-fuel ratio becomes leaner, the voltage required becomes higher.

Compression ratio - As compression pressure increases, the required voltage increases.

Ignition timing - As ignition timing is retarded, the voltage required to produce a spark will increase.

Electrode wear - As the spark plug electrode wears and becomes more rounded, the voltage required to produce a spark will increase.

Fuel BTU - Lower BTU fuel reduces the spark plug service life.

Electrode gap - The wider the spark plug gap, the more voltage is needed to produce a spark.

All of the factors above, except for compression, can be adjusted somewhat on most gas engines. As compression increases, the required voltage increases. A cleaned and regapped plug requires more voltage than a new plug. As the gap increases on a plug the voltage required for spark increases dramatically. The required voltage can exceed the arc over voltage of the plug. Arc over voltage is the amount of voltage required to produce a spark between the electrode and the ground strap. Fuels that cause high levels of deposits can cause compression to rise well beyond normal. Higher compression increases the voltage required closer to the arc over voltage.

The service life of the spark plug varies for different applications. The service life can be affected by various factors and operating conditions. 

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