What Are Resistor Plugs?

[sparkplugs.com]

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 introduced 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 interfere 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 (CDI) such as used on some Johnson and Evinrude marine engines require a special inductive type resistor (such as a Champion Q-type or NGK Z-type). Use of non-inductive resistor type plugs on these motors can cause misfire and poor performance.