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Wasted spark ignition systems are a type of DIS, which uses one coil for every two cylinders. The coil provides the spark for one of the paired cylinders on the compression stroke and to the other on the exhaust stroke. Because the coil fires the spark plug on the exhaust stroke as well, it is appropriately named 'wasted spark ignition'. In effect, the spark plugs fire simultaneously and twice as often.
One of the two paired spark plugs is always negative polarity while the other spark plug is always positive polarity. Negative polarity means the spark plug's center electrode is negatively charged and its ground electrode is positively charged. Positive polarity is the opposite. Each time the plug fires, a rapid exchange of the protons and electrons occurs, called ionization.
The negatively charged electrons will be attracted to whichever side of the spark plug that is positively charged. The positively charged protons have much more mass than electrons, and thus cause more wear on the electrode they collide with. Hence, one plug will exhibit more wear on its ground electrode, while the other plug will experience more wear on its center electrode. If a spark plug with a precious metal on only the center electrode were to be used with this type of ignition system, there would be uneven wear on half the plugs. Although single precious metal or standard nickel plugs will still allow the engine to run, plug life will be greatly reduced.
Therefore, if a vehicle was originally equipped with dual precious metal spark plugs, replacement with a single precious metal or standard nickel plug may reduce plug life and engine performance.