What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Confusion with Spark Plug Gap

jhiggins

Active Member
OK so I bought a Surefly mag and a while back an employee there (can't remember who) said one of the benefits of the mag is you can gap the plugs to .032. I have Standard aviation plugs and slick harness on my io-390. Today I talked to someone at Surefly and asked the same question, and his position was that there is no benefit to gapping the plugs. I asked what is Surefly's official position, and he responded with "gap them according to your engine manufacturer". He claims the spark energy (45mj) will be the same regardless of gap. So I am asking the "gurus" here for opinions. Benefit or downside? I have noticed the Surefly runs a little bit rougher at mag check (1800 rpm) than the standard slick mag, but still within rpm drop specs. I am in break in right now so I am fixed timing right now 20BTDC.
Surefly gapped .032, the slick gapped .016. I am starting on the Surefly (Left). Regular non impulse Slick on right.
 
Have you been aggressively leaning for taxi & best rpm for mag checks?

If you are still breaking in, you are probably operating at richer settings generally, so there might be increased potential for lead deposits in the plugs which might account for slight roughness?

Did Surefly with 45mj mention how much hotter this would be compared to Slick mag output? If not much difference between the two, tighter plug gaps may calm things down (I seem to remember .019 gap listed in Lycoming literature).

What is factory suggested timing for the 390?
 
Last edited:
Not sure the guy you spoke with knows too much. Spark energy produced is determined, in part, by the size of the gap and in part by the size of the coil windings. The gap controls how much of the available energy is drawn from the coil. However, it is critical that you don't over do it. If you draw more energy, via a larger gap, than the coil was designed to deliver, it will fail prematurely. This is the reason that gap size is so important on a std mag. They have very weak coils and as the gaps grow from wear of the electrodes, it starts drawing more power than the coil was designed to reliably deliver.

Given that the surefly was designed as a "mag replacement," I would be concerned that the coil was sized for a .018" gapped plug and wouldn't go bigger until someone there told me it could handle more. Do they use auto plugs? If so, the manual should state the recommended gap. If not, I would assume .018"

I would be a bit surprised and disappointed if they didn't put in a coil big enough to handle an 032 gap. And if they did, I would not reduce the gap from there. I suspect your rough running on the EI is something other than gap size.

Larry
 
Last edited:
The UREM40E plugs on my O-320 fired by the SureFly are gapped same as the ones on the Slick mag. The Surefly is very smooth during mag checks and has slightly less drop than the Slick.
 
In my discussion with the tech help at SureFly, they recommended setting gap at .022.....
 
Actually, he might. And gaps don't draw energy.

Correct, but they do control how much energy is required to jump that gap and spark. Keep increasing the gap and at some point the coil will not have enough stored energy to create a spark across that gap when the primary circuit is opened. Dwell time plays a role as well.
 
Last edited:
Correct, but they do control how much energy is required to jump that gap and spark.

How much voltage (V), not at all the same as energy (mj), which in the practical sense is volts x amps x how long the arc can be maintained. Note energy is of interest after the gap is ionized.
 
I have installed several Surefly magnetos in Lycoming 320 / 360 and no where does it say to gap the "aviation spark plugs to 0.032".

What I have "heard" is the Surefly magneto has sufficient spark energy to fire across an aviation spark plug gap of up to 0.032".

The aviation spark plug gap should always be what the MFR recommends ~ 0.015" to 0.017" or what ever the latest tech data states.

The selling point behind this statement is that as the gap grows over time, the spark will still be sufficient enough to fire the fuel / air mixture.

This is not always the case with the traditional magnetos, hence the reason to check the aviation spark plug gaps often.
 
Correct, but they do control how much energy is required to jump that gap and spark. Keep increasing the gap and at some point the coil will not have enough stored energy to create a spark across that gap when the primary circuit is opened. Dwell time plays a role as well.

Only voltage determines whether or not the gap is ionized and produces a spark. The energy stored does not come into play until AFTER the spark is produced by the voltage - then the energy gets released, and the quantity of energy determines how hot the spark is, and how long it lasts.
 
....the quantity of energy determines how hot the spark is, and how long it lasts.

Now there's a fine detail I've hunted a while, so far without success. "Hotter" spark....does the temperature of the arc vary significantly within the range of power available with typical spark ignitions? Or is arc temperature relatively constant across a wide range of discharge conditions,?

Put another way, in any given instant following arc formation, does more wattage actually result in significantly higher temperature?

I say wattage, because max voltage is a function of gap. For the same gap, electrode condition, and cylinder pressure, the flashover voltage should be the same regardless of the source ignition type or available amperage. Once ionized, the current path is relatively low resistance, so voltage quickly drops to a more or less steady, lower value for the duration of the spark. The variable is amps.

Anyone have a reference?
 
The aviation spark plug gap should always be what the MFR recommends ~ 0.015" to 0.017" or what ever the latest tech data states.

Do note the very small gaps traditionally specified for aviation plugs assume magneto ignition. It is a compromise, and a nice illustration of "smallest initial gap which will get the job done".

The practical problem with magnetos is that primary voltage is dependent on the angular velocity of the rotor, i.e RPM. At low RPM, primary voltage is low,
thus secondary (plug) voltage is low. It's the reason for the impulse coupler, a device which artificially boosts RPM at the right moment.

Anyway, with our av engines a small gap can be ionized with small voltage, while still providing enough spark exposure to light friendly mixtures at normal RPM. Less friendly mixtures (more rich or more lean) are lit more reliably with wider gaps and more duration. A mag will fire a wider gap at cruise RPM, but the engine may be hard to start, a problem which will worsen with plug wear and conductive deposits.

To the OP's question, can you run wider gaps with a aviation plug? Yes, in the context that the plug doesn't care. The typical Slick mag may not get'er done at cranking speed. An EI, however, has an external primary voltage supply, so it produces full secondary voltage at any speed which allows its trigger to operate.

So, you ask, why not just use a wide gap with this EI? Remember, secondary voltage rises until the gap ionizes, then falls as the coil's stored energy is discharged. If the gap is too wide, the secondary voltage will continue to rise until it pushes the electrons through some other discharge path, which may be a weak spot in the coil or spark plug wire insulation. It is why almost every ignition manufacturer states "Do not fire the ignition without grounding the plug leads."

I know nothing about the coil used in the Surefly. It may or may not have robust insulation properties. What I do know is an old Slick magneto harness is not going to have super duper high voltage insulation, not like some of the spark plug leads available for automotive ignitions. A gap which is no wider than actually needed is simply easy on everything.

Illustrations from the Bosch Automotive Electric/Electronic Systems Handbook:
.
 

Attachments

  • Bosch Figs 2 and 3 Spark.JPG
    Bosch Figs 2 and 3 Spark.JPG
    75.8 KB · Views: 170
Now there's a fine detail I've hunted a while, so far without success. "Hotter" spark....does the temperature of the arc vary significantly within the range of power available with typical spark ignitions? Or is arc temperature relatively constant across a wide range of discharge conditions,?

Put another way, in any given instant following arc formation, does more wattage actually result in significantly higher temperature?

I say wattage, because max voltage is a function of gap. For the same gap, electrode condition, and cylinder pressure, the flashover voltage should be the same regardless of the source ignition type or available amperage. Once ionized, the current path is relatively low resistance, so voltage quickly drops to a more or less steady, lower value for the duration of the spark. The variable is amps.

Anyone have a reference?
I didn’t manage to come up with an article directly addressing the question of temperature, but here is what I could glean from the various papers. The plasma kernal temperature is around 60,000K at initiation, and then decreased to about 6000K for the duration of the spark. Ultimately the peak voltage is limited by the gap and the atmosphere of the spark, and once the plasma kernal is created, the impedance drops drastically and current (power) is limited by the source and things like the impedance of the wires. In the time of the established plasma kernal, the availability of power and the hardware impedance control the plasma (spark) duration.
Also it appears that the atmosphere that the arc is in effects the temperature of the plasma kernal more than the voltage or current.

Practically, 6000K or 5000K won’t make much difference with regards to igniting the fuel, but the impedance of the circuitry and power to produce a spark of longer duration will likely make a difference.

As the makers of early CD ignition found out, once the engine is running the extra spark energy given in one burst only serves to break down wires and plugs.

Key references:

NASA-TM 73833

White Paper, “Plasma temperature of spark discharge in a spark-ignition engine using a time series of spectra measurements“
 
Interesting info guys for sure. I like to take the safest route so for now I will go back to what Lycoming is recommending .016 to .022. It just annoys me that some of these tech support guys are all over the place with their recommendations. Maybe it was somebody in sales I spoke to, I can’t remember
 
Back
Top