Many people believe that spark plugs fire instantly. This is partly true because they fire in milliseconds, although if one looks at an oscilloscope pattern you will see much more than a single instantaneous firing event. Many things also occur that you cannot see even with the oscilloscope. Part of what you cannot see, but can in many cases hear, is the noise that is picked up in the speakers of your car stereo. This is called RFI, or Radio Frequency Interference.

    Spark Plug Firing Voltages:

    When the breaker points or solid state ignition unit (switching device) interrupts current flow in the primary ignition circuit and induces current flow into the secondary windings of the coil, there is an instantaneous voltage spike. (as seen in the illustration at right in position A to B). This represents the voltage required to overcome the spark plug and distributor rotor gaps. Once the spark gaps have been bridged, the secondary voltage required to sustain the spark across the gap is much less and drops (as seen in position B to C above). The spark continues to arc across the gap at more or less constant voltage until the arc is extinguished (at position D above). This is due to coil energy drop in that it can not sustain the spark any longer. During this arc duration (Spark Duration), the plug actually fires several times. This is caused by high frequency oscillations in the primary and secondary windings of the coil, which continues to induce voltage spikes. They continue and slowly diminish (positions D to E above) even after they are no longer strong enough to sustain spark. All of this takes place in roughly one thousandth of a second.

    With our race designed ignition units, they concentrate their efforts on sustaining spark duration as well as limiting the voltage drop after the gaps have been bridged. Most aftermarket ignitions concentrate on giving us 20° of spark duration (crank degrees) as well as much higher spark energy output. A high performance coil helps this out, but the Capacitive discharge and digital ignition units assist in storing and delivering this power through the coil more efficiently, faster and give the ability to achieve higher RPMs more safely and efficiently in fuel mixture burning. The coil is only the pawn of the ignition trigger or control unit. The coil is the real workhorse and takes most of the abuse ... make sure you use a good coil.

    The Cause of RFI:

    If we were to slow down the oscilloscope to perhaps 0.00000025 seconds and greatly expand the pattern (as pictured at right), we would see that what appears to be consistent from position C to D in the the first illustration above is actually a series of extremely high bursts of energy. These energy bursts are discharged at the same frequency band as radio and TV frequencies. It is these bursts that make your car radio snap - crackle - and pop ... as well as just about anything electronic including telephones, aircraft control towers and heart pace makers by causing static and interference.

    Sources of RFI:

    Automotive ignition systems are not the only things that spew RFI into the atmosphere. Lawn mowers, snowmobiles, ATV's, tractors, power lines, traffic control devices, etc. all do it. One publication refers it to "electronic air pollution". As many of us know, we live in a sea of constant electromagnetic waves.

    Any time you have a flow of electric current you will have a magnetic field. Coils, relays, switches, solenoids, generators, servomotors all affect communication equipment, electronic circuits and computers. The higher the voltages, the more critical this becomes. Anytime you have the spark jump a gap or a contact, you have a miniature radio transmitter.

    RFI Standards:

    Back in the 1930's, engineers recognized that RFI could be a nuisance. As the years, testing and technology advancements went by, it turned into an even greater problem. Especially with the advent of high-tech communications systems, computers and electronic engine control devices. The Society of Automotive Engineers (SAE) decided to set up standards for measuring as well as the control of RFI. These are called "EMI Standards" or the more technical name for radio static of Electromagnetic Interference (EMI).

    The current standard for EMI was adopted in 1961 and is known as J551. It limits RFI at frequencies between 20-1,000 MHz. All spark plug manufacturers must adhere to it. The most common method used to suppress RFI is to install a resistor in series with the spark plug's center electrode.

   Other ways that control RFI include:

• The metal fenders, grille and hood of your car. These provide a shielding affect which absorbs much of the RFI emitted from your ignition components. Plastic and composite body panels are basically transparent to RFI and provide little to no shielding.

• The use of capacitors, silicone grease at connections, proper grounding of all circuits and routing wires to reduce electromagnetic interference are all helpful in reducing RFI.

• The use of carbon impregnated secondary wiring (plug wires) and resistor spark plugs have the most impact when reducing and controlling RFI. In our race cars with our high output ignitions, it is best to use a specifically designed plug wire for our applications. These are usually the what is called "Spiral Wound" style plug wires.  The construction of these wires starts with a Ferro0-Magnetic impregnated inner core, helical wrapped copper alloy conductor, a high dielectric insulator then a heavy fiberglass braid. Wrapping this is a 8mm to 10mm silicone jacket. Also, secure connections of the plug wire's terminal ends are mandatory along with secure fitting boots.