Specially designed wirewound resistors are used as noise suppressors in automotive ignition systems for reducing RFI or Radio Frequency Interference caused by electrical discharges. These resistors are usually placed in the leads and or caps of spark plugs and in the rotor of the distributor.

A gasoline engine generates high frequency electromagnetic Interference or EMI. This is commonly referred to as RFI or Radio Frequency Interference that comes primarily from the high-voltage side of the automotive circuit. At these places, the ignition system produces sparks at the coil that converts the battery voltage into high-voltage pulses. These pulses appear at the distributor, which routes the high voltage to the appropriate plug. Here, the spark ignites the air/fuel mixture in the combustion chamber producing the power that drives the crankshaft. Diesel engines do not have spark plugs as the air/fuel mixture is compressed to ignite and hence, diesel engines produce negligible EMI/RFI.

The high-voltage ignition pulses have a very rapid current change that generates an electromagnetic field around the ignition system. When electricity bounds through air, it passes through the air molecules, ionizing some of its atoms. As these atoms de-ionize, they release a tremendous amount of RFI. Although the frequencies are random and appear only for fractions of a second at a time, they affect almost any type of electronic device installed nearby to some degree.

Not only do these disturbances interfere with telephone and radio communications, they can even disrupt engine functioning and ABS control electronics. This type of interference sounds like a huge amount of crisps, crackles and rattles in radio receivers in communication systems.

International legislation requires manufacturers to reduce these disturbances to an acceptable level. That means the RFI must be reduced to a level so that there is no appreciable interference with the functioning of receivers not on the vehicle itself. Interference Suppression Regulations describe the RFI damping characteristics that manufacturers are required to follow, for example, VDE 0874 to 0879, CISPR or Council Directive 72/245/EEC, and usually differs from country to country.

Manufacturers usually track down the sources of RFI and limit it either at its source or filter it out before it can reach the instruments. The simplest and easiest method of prevention is by installing resistive spark plugs, resistive leads or ignition suppressor resistors. These contain internal impedance to dampen unnecessary emissions from the ignition system. Some manufacturers resort to redesigning the grounding circuit or installing feed-through/bypass capacitors.

Conventionally, spark plug leads usually carry a resistance of 6 to 15 Kohms per meter, and that makes them poor transmitters of RFI. However, electrical ignition systems may be sensitive to varying resistances in the spark-plug leads due to different lengths and can give mixed signals to the control module. Therefore, it is preferable to have solid-core wires with noise-suppressor resistors screwed onto brass fittings at the ends. This helps to maintain an equal resistance on each cylinder.

Use of noise suppressors is the best solution for reducing RFI. These resistors are designed for specific ignition systems and have the finest damping characteristics that do not cause disturbances to the ignition pulses. It usually suffices to place the resistors in the rotor of the distributor, in the spark plug caps or in the leads.