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At the moment the spark jumps the gap it causes a high frequency burst of energy, this is known as RFI (radio frequency interference). This is why resistor spark plugs were introduces in the mid 1960's. Placing a resistor within the spark plug suppresses the RFI. Without resistor plugs in your car you can experience static on your radio as well as interference with other sensitive electronic equipment. Some later model vehicles as well as newer Powersport engines must use resistor plugs for a proper "talkback" to the electronic ignition.
Outboard marine Capacitive Discharge Ignition systems (CDI) such as used on some Johnson and Evinrude marine engines require a spark plug with a special inductive type resistor (such as NGK Z-type plugs). This is because these type of ignitions systems produces less voltage; use of non-inductive type plugs can cause misfires and poor performance.
For automotive CDI high output ignition systems (such as Mallory, MSD, Crane and Accel), refer to "Resistor plugs for race engines" in the ADVANCED TUNES section of this website for information relative to modified motors.
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