Quote:
Originally Posted by nkaminar
My old single cylinder BSA motorcycle had dual plugs. It was modified. The coil had two ends. Instead of grounding one end of the secondary, it was lead to a second high voltage connection. One end of the coil when to one plug and the other end went to the other plug. They both fired at the same time.
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This method was common on new cars a few years ago; one coil for two cylinders. Interestingly, the spark jumps from the center electrode to the ground electrode on one plug, and from the ground electrode to the center electrode of the companion cylinder. Since the electrode that the spark jumps to wears faster, Ford (and maybe others) used plugs that had platinum on only one electrode. Half were platinum on the center electrode and half were platinum on the ground electrode. We just used double platinum tips when replacing the plugs. This system produces a "waste spark", that is,when two cylinders fire at the same time, one is on the power stroke and the other is on its exhaust stroke. The plug that fires on the exhaust stroke is called a "waste spark".
Now most cars have the coil-on-plug system and each cylinder fires independently.