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Most automotive and power sport applications require the use of a resistor type spark plug to reduce RFI and properly communicate with electronic systems. However, CDI (capacitive discharge ignition) systems have a much lower voltage output. So while a resistor of some sort is still required, a standard resistor plug would be overkill, therefore, an inductive resistor is used.
In an inductive resistor design, a coiled nickel wire is inserted into the stem and held in place by a spring. The coil acts as an electromagnet, inducing a magnetic field around the center stem, creating a natural “field” resistor. This type of resistor creates around 40ohms of physical resistance, compared to the 1000-5000ohms of resistance in typical monolithic resistor plugs.
A standard resistor plug, or non-inductive resistor type, should never be used in an engine that calls for an inductive resistor type. Use of a standard resistor plug can create an open circuit within the plug, ending its life.