What is RFI in Your Ignition System and How Do You Defeat It?

Short for Radio Frequency Interference, RFI is a prevalent issue that causes a number of serious problems. RFI can oftentimes be difficult to detect and extremely irritating.

A Familiar Sound

Typically identified as a humming or whistling sound emanating from the car’s front end, RFI often interferes with ignition, racing radios, engine control units, and computers. In fact, enough RFI has caused computers to crash during dyno pulls. In addition to that annoying hum, RFI can lead to slower starting and overall poor performance.

Occasionally it can be due to a bad injector and/or a defective wiring harness feeding them. We typically saw this on older diesel engines that used a shuttle system. The most common cause is an overloading of the ignition system. A counter-electromotive force pulse is generated when the coil field collapses and the supply current is removed. This pulse jumps a spark across the plug’s gap, which generates the irritating radio frequency interference. 

Sampling a set of LiveWires is a simple way to reduce RFI.

Radio Frequency Interference Prevention

It’s typically because of high-voltage ignition—ie. performance and racing coils—that we have to deal with radio noise interference. Due to the increased output of a high-performance ignition system, related components such as the spark plug wires, cap, and rotor, spark plugs, or related components can be affected. A thorough inspection of these pieces, as well as any necessary replacements, will help solve the problem of RFI.

To get a better idea of how to prevent this issue, we spoke with Steve Davis, President/Owner of Performance Distributors.

“First, make sure you are running at least 8mm spiral core plug wires, which are readily available in the performance market—having nice insulation prevents voltage leakage—our LiveWires are worth a look. Make sure your boots on the end of the plug wire are sealed properly to the wire. This prevents the RFI caused by voltage leaking out of the plug wires.”

Figuring Out HEI

One area to inspect is the HEI cap (if you have this style of ignition). The HEI cap utilizes a capacitor attached to a wiring harness next to the ignition module inside the distributor. Capacitors reduce voltage leaks—also known as spikes.

“Make sure that none of your plug wires are no closer than a 1/2 inch from the 12-volt power wire that plugs into your HEI cap and coil assembly. If they are too close to one another, that proximity often causes some RFI. Also, ensure the screw that secures the capacitor/wiring harness is tight if you are experiencing RFI. Performance Distributors’ DUI (HEI) distributors utilize a more efficient chip style capacitor compared to the older canister-style units.”

In addition, an HEI coil is surrounded by an iron frame with three wires protruding from it. The center lead is for the ground. When the connector on the distributor is attached to the terminals, the metal frame is grounded to the distributor housing. Any electrical charge that has accumulated on the coil’s metal frame will drain off onto the ground. This, in turn, prevents arc-over and radio noise.

Save yourself the headache and make sure the right parts are in place. With a few tips and decent equipment, RFI can be avoided—and so can tinnitus and another contributor to road rage.

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About the author

Tommy Parry

Tommy Parry has been racing and writing about racing cars for the past seven years. As an automotive enthusiast from a young age, he worked jobs revolving around cars throughout high school, and tried his hand on the race track on his 20th birthday. After winning his first outdoor kart race, Tommy began working as an apprentice mechanic to amateur racers in the Bay Area to sharpen his mechanical understanding. He has worked as a track day instructor and automotive writer since 2012, and continues to race karts, formula cars, sedans, and rally cars in the San Francisco region.
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