Magnetos and their Disruptive Gremlins

By Spud Miller
Contributed to Vintage Fuel Magazine Vol.2, Issue 6

Perhaps the most pervasive engine component found throughout the nostalgia racing community is the magneto. It provides the wallop to light whatever you happen to be feeding your engine. Whether it's the high cylinder pressures resulting from a supercharger or a steady diet of nitromethane, a heck of a spark is required to keep things running smoothly. This is especially true in the upper RPM range when things are happening fast and there is generally excessive fuel in the cylinders. While the new-fangled ignitions can offer multiple rev limiters, retard controls, and such frills through electronic wizardry, the old magneto continues to provide cheap, reliable, and effective fire-power. When the "magic smoke" escapes from the modern electronic boxes, the old mags with contact points are still zapping the air fuel mixture. Typically, electronic ignitions gradually drop off in output as RPM increases. The magneto is the opposite...the faster it's rotated, the more energy it creates. Perfect for a racing engine! Practical limitations exist of course (heat in the windings, point bounce, coil saturation time, etc.). However, a magneto is a very good ignition system for an extreme sport such as drag racing, especially in the nostalgia ranks where cars may not even have a battery onboard to power a modern ignition.

As we "race the past into the future", magneto hardware is frequently coupled with modern accessories. Many folks find that their shift lights, tachometers, rev limiters, digital gauges, etc. act erratically when put in an environment of severe electromagnetic interference (EMI) such as a magneto generates. LCD displays on some gadgets are unreadable, lights blink when they shouldn't (or not at all), the needle on the tach twitches and goes crazy, and data recorders spit gibberish. Attempting to combine electronic systems from two different centuries can result in major frustration.

Electromagnetic interference is emitted by circuits that carry rapidly changing signals, such as ignition systems. This is especially true of magnetos, which work by spinning magnets and thereby generating a powerful alternating current. The more powerful the magnets, the more EMI that surrounds the mag, permeating all gadgets on board with invisible fuzz. The resulting primary energy of the mag is amplified into much higher secondary voltage by the transformer and then sent to the plugs through the spark plug wires. Each plug wire serves as an antenna that transmits electrical craziness to anything that will listen. Sensitive solid state electronics (transistors, integrated circuits and stuff) do not like this environment at all! Even electronic tachometers that are specifically made for magnetos can experience problems.

So what's a racer to do? Junk the magneto and leave behind the powerful simplicity of yesteryear? Forsake all modern accoutrements and "wing it" like they did in the old days? There can be a happy medium. While some modern devices are more prone to problems than others, steps can be taken to improve the situation enough that a tach or shift light can be used. If you have problems with EMI, you can lessen or eliminate the effects by optimizing component location while employing some shielding and filtering.


A shift light or tach mounted right on the back of the motor is just begging for problems. The farther away from the source of the interference, the better. If you can move the devices to the cockpit, this alone will help. Data recorder boxes can be mounted near the front of a dragster to put some distance between it and the problem area. If you use a magneto signal converter box for your tach, don't mount it near the magneto or coil! Put it as close to the tach as possible and away from the interference. If components are too close to the mag, no amount of shielding or filtering suggested below will likely solve the problems.


A grounded metallic barrier or shield between the source of the interference and sensitive devices can greatly reduce problems.

Make sure that the magneto housing itself is grounded properly. The mag body serves as a shield to help contain and reduce emitted EMI. Also, since the mag is the source of the EMI, a strong, common ground from it to all of your other shielding efforts is imperative. Don't rely on the mag clamp to ground through gaskets, silicone, paint, oil, etc. in order to get to the engine block. Put a ground wire somewhere on the mag housing directly to the engine.

Utilize a ground wire from each cylinder head to positively ground the spark plugs to the mag. Good grounds everywhere help to complete the circuit and prevent EMI.

A good low impedance suppression core wire plug wire is just fine for use with a mag and is a must if you are having problems with interference. Just be sure that it is 50 ohms per foot or less. There are a number of good wires out there that meet this requirement. Of course, any amount of impedance will cause a slight drop in output at the plug, but it really is very negligible. When dealing with peak voltages of thousands of volts, you won't miss a few hundred at all. Be sure NOT to use regular suppression core wire found on street cars. This stuff is of such high resistance, that your mag will easily arc and spark inside the cap...the spark plug may no longer be the path of least resistance! When using even the good suppression wires, keep an eye on the cap and rotor and keep them in top condition to keep the spark going where it should.

Make sure that the case or shell of your tachometer or other device is grounded to help shield the internal components from interference

Use a shielded wire for any ignition signals sent to tachometers or other devices. A tach needs to see a specific waveform in order to accurately count and report the ignition pulses. The interference from the mag will add noise to the signal altering this waveform. As a result, the tach can count false pulses in the signal and give erroneous readings. This can cause a twitchy needle or an unreliable shift light. Shielded instrument cable can work well for this purpose. Attach the shield conductor in the jacket of the wire to a solid source of ground that ends up at the mag body eventually.

Another important thing to shield are the power sources for your devices. The 12V wire running to the device is a big antenna for EMI. You could use a shielded wire, put a braided and grounded sheath over the wire, or run the wire through steel or aluminum tubing that is grounded.

If you use a magneto signal converter box, be sure to place it in a grounded metallic enclosure. A metal "tea box" with a snug fitting lid works well and looks good. Solder or screw a ground wire to the bottom and pass the wires out of it through a rubber grommet. Make sure the lid has metal to metal contact with the rest of the box so it is grounded as well.

If the body panels of your car are metallic, they are a great primary source of shielding for the items in the cockpit. You just need to come up with a way to ensure the panels have decent ground contact when they are attached to the car.


Even if you've shielded things well, some electronic hate will still find its way into the power wires of your devices. Since there is not supposed to be any "signal" here (only +12V), you can filter out the noise. A quick, easy way to do this is purchase a noise filter from an electronics store where car stereos are sold. This small filter is meant to be mounted between the device and the voltage that powers it. It conditions the power and will smooth out any ripples or spikes induced into the system. A single filtering device could easily condition the power for your tach, shift light and mag signal converter box. Don't use a noise filter on the signal wire to the tach! It may remove the useful pulses from the signal and the tach won't have anything to count.

With a little effort, those magnetos can spin in harmony with modern accessories on board. If your shift light starts blinking a morse code message to you at half track, it may be time to purge your race car of it's electromagnetic demons.

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