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Waste Spark Ignition System

Bulletin 4000

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All engines that use a waste spark type ignition system


Spark plug electrode wear with waste spark ignition system

One type of ignition system uses an ignition process called waste spark. Waste spark is the method of firing two spark plugs at the same time. This design increases reliability, saves space, has less maintenance, and lowers the number of components needed to create efficient spark.

To verify that your engine may use a waste spark system, locate the ignition coils and determine if the ignition coil has one secondary output or two. Two secondary outputs on one ignition coil will identify the system as a waste spark type system. Another observation would be to confirm how many cylinders your engine has as compared to the number of ignition coils, if you have half the number of ignition coils as compared to the number of cylinders, you have a waste spark system.


A waste spark ignition system fires two spark plugs simultaneously. The spark plugs fire in opposite polarities causing the spark to travel in opposite directions of each other. The coil fires the two spark plugs slightly before one is at top dead center of the compression stroke and the other cylinder will fire slightly before top dead center of the exhaust stroke. The firing event that occurs during the exhaust stroke has limited effect on the engine performance making the spark that occurs a “wasted” event.

The dual firing spark plug event with opposite firing polarity, or spark moving in opposite directions will cause electrode wear in different ways. With this type of ignition system one of the spark plugs will wear dominantly on the center electrode while the other spark plug will show more wear on the ground electrode. When installing spark plugs into an engine using waste spark ignition, dual precious metal electrode spark plugs should always be used. This will substantially increase the lifespan of the spark plugs and cause less stress on the ignition coils. If non-dual precious metal spark plugs were installed into a vehicle with a waste spark ignition system, the drivability of the engine will not be affected at first. Because single precious metal spark plugs are not designed to withstand reverse polarity firing, uneven wear of the center electrode and ground electrode will drastically reduce the life of the spark plugs. Uneven and premature wear can cause misfiring or other drivability issues along with poor fuel economy and increased emission levels.

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