Today's spark plugs use resistors to eliminate radio frequency interference (RFI) which can affect on-board computer systems, AM and FM radio reception, TV broadcasts and even airplane communications.

Each time the spark jumps across a gap, an electromagnetic field is created that can interfere with radio signals. By placing a resistor in the spark plug, we can substantially reduce RFI. Furthermore, the use of non-resistor plugs with today's complex computer systems can result in driveability problems, loss of performance and can even cause the computer to store trouble codes.

At one time, resistors were the weak link in spark plugs. The old technology was to make resistors out of carbon and epoxy. Just like a spark plug wire, if excessive heat or an electrical arc developed inside the plug (not at the electrode), the resistor could quickly be damaged and cause a misfire.

Does the standard resistor have effect on the required voltage?

About 99% of today’s cars are equipped with resistor spark plugs. Most of the standard spark plugs and all the longlife and iridium power and our Brisk spark plugs have a built in resistor of 5k Ohm. This has no effect on the available voltage at the spark plug. Often people fear the word resistor because it makes them think the voltage and current is limited.

The resistor that is fitted in the spark plug is 5k Ohm. This is nothing compared to the resistance of the air or the air/fuel mixture in an engine. There is no measurable effect on performance.

The resistor is necessary to prevent problems with engine management systems, cellular telephones and datalogging equipment. The spark plug is the best place to put the resistor, because this is the closest you can get to the spark itself.

The resistor value of a resistor spark plug and resistor leads and caps is less than 0.01% of the total resistor value of the air/fuel mixture inside the engine.