How to Bench Test a Magneto

Author Photo
By Dr. David Cave

Using a bench spark plug gapped to four times the engine plug gap.

by Dr. David Cave
Some of the equipment used in these experiments.

When a high-tension ignition engine won’t start, it’s common practice to pull the spark plug, lay it on the engine, turn the flywheel, and look for a spark. If it sparks, I often hear, “Maybe the mag is weak and won’t fire the plug under pressure.” If that statement is correct and pressure effects ignition greatly, what can I do to ensure the magneto is OK?

The word extreme is often attached to pressure in that saying. If there is extreme pressure, ignition has occurred, and there is no longer a need for a spark. The pressure when spark is needed is the pressure of compression. I have measured many engines over the years, and they all have had compression ratios between 3:1 and 4.5:1, with the majority around 4:1. Compression ratio is the total volume inside the cylinder when the piston is at the bottom divided by the volume that remains when the piston is at the top. The universal gas law conveniently says the pressure goes up as the compression ratio goes up. In other words, the pressure at the top in a 4:1 compression engine will be four times that at the bottom. The confusing part of that is pressure gauges are calibrated to read zero at the bottom. They neglect the atmospheric pressure of 14.7psi. Worst case then, at top dead center, the pressure is four times the atmospheric pressure or 4 x 14.7 = 59psi, but your gauge will read 44psi.


photo by: by Dr. David Cave

Figure 1: A gauge measures the Hercules compression pressure.

Measuring the actual pressure is not easy. Most compression gauges have modern spark plug metric threads while many old engines have pipe threads. After a pair of brass adaptors were made (see Figure 1), I measured the pressure in the cylinder of four engines. A 3/4hp Hercules with a 3:1 compression ratio produced 30psi. John Deere engines have a compression ratio of less than 4:1. A 1.5hp engine produced 44psi while a 3hp produced 40psi. A Stover with unknown compression ratio produced 50psi.

But all engines run with ignition advance; typically they fire 20 to 30 degrees before top dead center (BTDC). With advance, the piston has traveled about 90 percent of the way to TDC, giving a perfect 4:1 compression ratio engine a pressure of about 40psi (gauge) at the time a spark is needed. At spark time, it’s likely that an unrestored engine with “good compression” will have a pressure similar to a car’s tire, 35 to 45psi.

Most often, a weak magneto is to blame for low voltage. So, what does 4:1 compression ratio or 50psi do to the required voltage at the spark plug? A very old physics equation, loaded with conditions and restrictions, says the voltage required to arc between two flat electrodes is 76 volts, multiplied by the gap in thousands of an inch, multiplied by the pressure; V = 76 x gap x pressure. The nice thing about that expression is pressure is measured in atmospheres, which is exactly the compression ratio. In a 4:1 compression engine, the voltage required to make a spark (at TDC) goes up 4 times. For example, a perfect 4:1 engine (neglecting advance) with a plug gap of 0.025 of an inch requires 1,900 volts (1.9kV) at no pressure and 7,600 volts (7.6kV) at pressure to create a spark.

That sounds like a lot but in actuality anything but a totally dead high-tension mag can deliver that much voltage. The problem is when that voltage needs to drive current for some period of time. There needs to be enough voltage to jump the gap with enough current to create a hot channel lasting long enough to cause combustion.

All high-tension magnetos have two phases they go through in a single spark cycle. The first is the rather long energy collection phase called dwell. Energy, in the form of high current and low voltage, is stored in the magnetic field around the primary coil. In a Wico EK, this phase begins when the armature begins to leave the magnet poles and ends when the points open. It is important to understand that this stored energy will be the spark. Think of a basket into which the primary coil chucks energy during dwell or this first phase.


photo by: by Dr. David Cave

Figure 2: The setup used to put a spark plug under pressure.

The second phase begins when the points open, the low voltage/high current stored energy is converted to high voltage/low current, and is delivered to the spark plug. The second phase ends when the stored energy is depleted and the spark ceases. During this phase, energy is snatched chunk by chunk out of the basket and converted to spark. The time the spark arcs is called the plug burn time and ends when the energy basket is empty. Energy is the product of voltage (V), current (I), and time; (t), E = V x I x t. A high-tension magneto cleverly uses a little voltage and a lot of current over a long time to load the energy basket, then converts that to high voltage/low current over a short time, emptying the energy basket quickly. When I say “long time,” I’m using an electronic clock. A Wico EK typically gathers and stores energy for 20ms (0.02 second), then creates a spark that lasts 1 or 2ms (0.002 of a second).

figure-3a-wico-ek-zeroFigure 3A: A reading of a Wico EK driving a 3095 plug under zero pressure.

To investigate the effect of pressure, a steel tube was made with pipe threads for the spark plug and a plug for the air source. A 3095 spark plug was gapped to 0.025 of an inch and put under pressure. An unrestored Wico EK using a spring trip, for consistency, was connected to the spark plug (see Figure 2). The spark was monitored with an oscilloscope, looking at the spark plug wire between the magneto and the spark plug as well as looking at the primary coil. Although I could not see through the steel tube, the electrical signals on the plug wire and the primary coil are very clear indicators of what the spark plug was doing. Your auto mechanic would check your car’s ignition the same way.

figure-3b-arc-timeFigure 3B: The plug under 16psi reads 1.25ms arc time.

What is expected to happen? The collection of energy by the Wico EK in phase one is unaffected by the pressure, therefore the spark energy is the same for all pressures. The energy collected is primarily set by the magnet strength, the rate at which the armature leaves the magnet poles, the number of primary coil turns, and the core material – none of which change as the plug pressure increases. Since energy is the product of time, voltage, and current, the time must get shorter as the required spark voltage goes up under pressure. In other words, a spark plug under 50psi will burn energy at a faster rate than the same spark plug under 30psi. With a given amount of energy available in the basket, the 50psi spark won’t last as long as the 30psi spark. Figures 3A, B, C, and D show the results. In Figure 3A, the Wico EK is driving the 3095 spark plug under no pressure and results in a plug burn time (arc time) of about 2.75ms (0.00275 of a second). Figure 3B shows an arc time of about 1.25ms when 16psi is applied. Figure 3C shows that 29psi gets less than 1ms of arc time. At 45psi, in Figure 3D, the arc time is near 0.5ms.


photo by: by Dr. David Cave

Figure 3C: The plug under 29psi gets less than 1ms arc time.

I would expect a weak magneto to collect and store less energy in the basket in the first phase (dwell) and would have an even shorter spark duration. Figure 4 shows the same magneto, purposely degraded, driving the same spark plug under 0psi of pressure. The plug burn time has dropped from 2.75 to about 2ms. I might suspect this degraded magneto to give an arc near zero duration at 50psi.


photo by: by Dr. David Cave

Figure 3D: The plug burn time at 45psi is near 0.5ms.

Bottom line: A weak magneto may appear to deliver a decent spark on the bench, but under the pressure of compression, the spark duration may become too short to cause combustion. As an aside, automotive folks like to see plug burn time in the region of 2ms (0.002 of a second) and will tell you that 0.5ms (0.0005 of a second) will likely cause misfires. Those times may seem incredibly short, but the spark channel remains hot longer, and your eye has image retention allowing you to see the spark.


photo by: by Dr. David Cave

Figure 4: A slightly degraded magneto driving the spark plug
under zero pressure.

I have shown that all but dead magnetos can deliver the necessary voltage to arc. The issue is doing so with enough current for enough time to cause combustion. As demonstrated on the bench, the plug pressure goes up, the arc duration decreases, and a weaker magneto results in even shorter spark times. At that point, it seemed a real test was necessary.

A Wico EK was mounted on a Stover engine (2.5hp CT-2) developing 50psi and a Hercules (3/4hp) that developed 30psi peak. The magnetos were degraded, step by step, until the engines could no longer be started or kept running. At that point I had a magneto that failed to run both a low and high compression engine smoothly.

The article “Understanding the High-Tension Ignition System,” in Gas Engine Magazine’s February/March 2021 issue, points out all high-tension ignition systems use the same electrical schematic. There are several common problems that will cause any high-tension magneto to be weak: a weak magnet, a sluggish or slow trip, low RPMs, shorted primary coil turns, improperly timed points, and excess primary coil resistance. From the external, these degradations all look the same because they all reduce the energy that is collected and stored in the energy basket in phase 1. In other words, a weak magnet and high primary coil resistance will behave the same. As they all use the same schematic, I chose to use a Wico EK for these experiments because an easy way to degrade an EK is by adding primary coil resistance (see Figure 5). This coil resistance can be added step by step until the engine runs rough or stops. After the experiment, the resistor can be removed with no harm to the magneto. Figure 6 is the Stover under test.


photo by: by Dr. David Cave

Figure 5: A Wico EK with a string of resistors to be patched in.

After degrading the magnetos step by step, I had a magneto that would not run a 50psi engine with a 0.025 of an inch plug gap and another magneto that would not run a 30psi engine. The interesting thing is, as my automotive friends suggested, both the Stover and the Hercules stopped running smoothly when the spark duration got down to about 0.5ms (0.0005 of a second).

It became obvious the reason a weak magneto won’t run an engine is not that it doesn’t produce enough voltage under compression, but that it doesn’t store enough energy in the basket to maintain the spark long enough to cause combustion. The higher voltage required under pressure causes the rate of energy consumption to go up.

After the mathematical manipulation of several energy equations, knowing that the energy basket was the same size and that more than 0.5ms was needed for combustion, I could see gap and pressure were reciprocal. This should mean that a four-times increase in pressure in the engine would correspond to a four-times gap increase on the bench. If the magneto will create a spark under 4:1 compression, it should create a spark four times the engine plug gap on the bench with no pressure.

Spark, however, is a very complex phenomenon. When your eye sees a spark, it is actually many short duration sparks coming back-to-back (see Figure 4). It is only the first one of those that follows the 76 x gap x pressure equation. After the first, the channel heats up and the gas atoms become ionized and the voltage drops. This would imply that a bench plug might not need to be gapped four times for a 4:1 compression engine.

When the ignition was retarded for starting, the magneto needed to be stronger than in the advanced position. When retarded, the piston is closer to TDC with more pressure at spark time. On the bench, the weak magnetos that would not run an engine were tripped with an increasing bench plug gap until the spark ceased. There was little difference between a 0.5ms spark and no spark. For the 50psi engine, retarded with a 0.025 of an inch gapped plug, the equivalent bench plug gap was 0.074 of an inch. The same conditions were run with the engine plug gapped to 0.037 of an inch, and the bench plug equivalent was 0.119 of an inch. In both cases, the bench plug was nearly three times that of the gapped plug.

Some caveats: At these magneto degradation levels, the engine was barely running, was difficult to start, and was sensitive to the air fuel ratio. Sparking a plug on the bench gapped four times your engine plug gap should ensure that your magneto will start with retard and run a high-compression engine smoothly.

After this study was completed and written up, a lower compression (40psi), throttle-governed Cushman that could not be retarded – due to mechanical issues – was run through the same process. Those results, coupled with the Hercules, suggest that for lower compression engines with no ignition retard, a bench plug gapped three times the engine plug is sufficient.

For those who might question adding resistance to the primary coil, the 50psi engine experiment was run again. In this run, an EK magnet was carefully degraded, resulting in similar results. Also, for those who question if the problem is voltage, rather than time, when the 50psi EK magneto was pulled off the engine, it was still – even in its degraded state – delivering 12,000V (12kV) pulses when it didn’t need to drive current for some period of time.


photo by: by Dr. David Cave

Figure 6: A Stover running with resistance added to the primary coil.

The run-of-the-mill unrestored EK used on the 50psi Stover engine needed to be degraded to 35 percent of its original strength before the Stover stopped running. The fact that lower compression engines would run on even weaker magnetos leads me to believe that cylinder pressure is seldom the reason an engine won’t run.

I test all magnetos on the bench with a plug gapped to 0.125 of an inch. Also, when I pull a plug out of an engine to see if the magneto is working, I grab the 0.125 of an inch gapped plug. At 0.125 of an inch I believe I cover engines with up to 0.035 gapped plugs and 50psi compression.

Dr. David Cave is a regular contributor to Gas Engine Magazine and can be reached at jdengines@cox.net.

Published on Oct 6, 2021

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