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Auto Plugs with Mags

Gerald Loyd

Active Member
I never hear about anyone using auto spark plugs with magnetos. Why not use them. A set of 8 auto plugs cost a lot less than one aircraft plug.
 
My Volkswagen powered aircraft used auto plugs and a mag, I think I heard somewhere that the electrode needed to be larger to keep a more consistent gap due to the close gapping of plugs used with a mag and the electrode wear. Im sure we will get a real answer here soon.
 
This guy makes a setup for what you are looking for.


As far as I know if you gap the plugs to the magneto requirements and keep the lead cleaned out, the auto plugs should work fine.
 
many have Engines with 4 auto plugs on an Electronic Ignition. But the other 4 Aviation plugs on the Magneto.
 
Not uncommon in racing engines to run magnetos and auto plugs, I have many times.. But, the heat ranges are different, the application is different and the plug life is short. Aviation mags have far less output than auto racing mags. Racing mags can jump a much larger gap. I've seen them jump over a foot on the test stands. The smaller auto plugs would be more susceptible to lead fouling with an aviation mag, not so much with an EI. Not a lot of pros to the cons.
 
I purchased the components and instructions from G3i which is linked above, and ran 18mm auto plugs from a standard Slick mag. Worked flawlessly. If you have any questions, I can try to answer them, but it?s a pretty simple conversion.
 
Thanks guys. I have all the components to do it, but didn't know if anyone was using auto plugs with mags . Unless someone gives me a good reason not try it, I'll go ahead with the project.
 
Auto plugs with mags

My experience with the G3I setup has been positive for many years. Both the -7 and the cub have Bendix mags. These two planes have @600 hours with this setup and have a steady diet of avgas with no plug fouling or any other problems. I use the Denso IK27 iridium plugs gapped at .022-.023. I change them at 200-250 hours but I?m not sure that is necessary. $8.50 for an iridium plug sure beats $30 for a regular massive electrode aircraft plug.
 
If you go with all of the G3i parts to make the ignition system, you should gap the plugs per G3i recommendation. If you only use the wire harness conversion, you gap per magneto specifications which will be a smaller gap.
 
I ran the Autolite 386's gapped around .030
Changed them about every 50 hours, but not because they were functioning poorly: just because they were cheap.
 
KV vs's Gap, 1 foot?

Jump a foot, really, maybe in a vacuum chamber? It takes 60K average to ionize 1.00" @ standard pressure & temperature. X12, do the math
Thomas


Not uncommon in racing engines to run magnetos and auto plugs, I have many times.. But, the heat ranges are different, the application is different and the plug life is short. Aviation mags have far less output than auto racing mags. Racing mags can jump a much larger gap. I've seen them jump over a foot on the test stands. The smaller auto plugs would be more susceptible to lead fouling with an aviation mag, not so much with an EI. Not a lot of pros to the cons.
 
I used the G3 ignitions instructions to modify one of my bendix s-1200 mags. The other ignition is a CPI from SDS. I run NGK BR8ES plugs on both ignitions, .018 gap on the ones for the mag and .032 on the ones fired by the EI. I swap the plugs out every 100 hours after I perform a compression check and cylinder borescope.

I now have over 200 trouble-free hours and I'm on my third set of plugs. I dont throw them out because they are worn, damaged or fouled. They're so cheap it just makes sense to replace with new after they are removed for the scheduled maintenance. The plugs fired by the mag look the same as the ones for the EI except for the gap.

Before this I ran a similar setup on an O-360 in my Glastar before going to dual pmags. I had successful service for over 400 hours.

BTW the G3 instructions are cheap insurance for getting the mod done correctly.
 
Yup.. Believe it or not. I used to get Vertex mags beefed up at Don Zig Magnetos for race engines. After the mag was finished it's put on a test machine that clamps the mag and rotates the shaft. The 8 wires went to 8 spark gaps that had a pull handle on one set of gaps. They start the machine and pull the handle to test the output of the mag. The difference between a stock Vertex and a Don Zig Vertex was amazing. And yes, it would jump a foot gap right out in open air. Looked like a Jacobs ladder. Aviation mags..... Well, maybe an inch or two.
 
Hello Thomas

Thanks for sending the G3i module. I installed it this afternoon and got a good mag check. I didn't get to test fly it because of weather. I'll do it in the morning & call you.

Gerald
 
I purchased the Bendix 'Old Style' ignition harness plate (has gland nuts holding the wires) from an eBay ad, it easily adapted to 8.5mm MSD spiral core wire & 90 degree boots.
It fired BR8ES plugs gaped to .016 just fine.
 
High resistance

Some of the auto plugs, such as the BR8EIS have very high resistance, about 5K ohms. Isn't that hard on the mags, especially the coil?

The Autoilite 386's are not resistor plugs, and do not have high resisitance.

Vic
 
Some of the auto plugs, such as the BR8EIS have very high resistance, about 5K ohms. Isn't that hard on the mags, especially the coil?

The Autoilite 386's are not resistor plugs, and do not have high resisitance.

Vic

The 386 IS a resistor plug. In fact, after much research, I could not find anoher 18mm plug that was a resistor plug. Having tried non-resistor plugs with resistor caps, I can assure you that you would know if you had a non-resistor plug installed. I had a lot of noise, even with the resistor caps and went back to 386's. The 18mm plugs are sold mostly for vintage tractor applications and most don't have electrical systems and therefore the limited demand for 18mm resistor plugs. resistor plugs rob a bit of energy in exchange for reduced electrical interference.

Resistors are not hard on the mags. In fact, they reduce the spark energy a bit and make life easier on the mag. Plug gap dictates the power drawn from a coil and it is the large gaps that try to draw more power and damage the coil. If you run large gaps (>.018) on a mag, you are significantly increasing the likelihood of an early coil failure. The standard aviation plugs are also resistor plugs, they just use a lower resistance than auto plugs (1K vs 5K) due to the low energy output of the aviation mags.

Larry
 
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Larry is correct.
Autolite 386 is a resistor plug. I use them with a Bendix mag on top of an O-290. I set the gap .016, the same as aircraft spark plugs to protect the mag. The engine runs fine.
They run 'WHITE' which is considered a bit hot on the heat range. They are always clean, so maybe I should have put them on the bottom?
Anyway, I continue to seek other 18mm auto plugs, the current one I have (but have not tried yet) is Champion 526, which is RD15Y a resistor, extended nose version of the D series. D21 the hottest, D6 the coldest. Here is a link I found in my quest to verify heat ranges of 18mm auto plugs:
https://www.gsparkplug.com/ignition/spark-plugs/18mm-diameter
Too bad the Autolite 386 isn't on there for comparison.
I have considered using resistor caps on non-resistor plugs to take advantage of the heat range variety of the 'other' 18mm plug offerings, but the resistor caps I received all fit 7mm wire, and I use 8.5mm MSD Superconductor suppressor wire.
Maybe this wire offers enough inductive reactance too suppress spark noise, I haven't tried non resistor plugs with them yet.
I would like to find if Champion makes an RD9Y, I like the idea that they would continue to improve the D series plugs adding resistor and extended nose.
 
I found 386's to be hot plugs, and would eventually erode.

L14U's are equivalent to the Autolite 386's. Pretty much the same story.

Autolite 373's would be worth a try, even though they are non-resistor plugs.
https://www.rockauto.com/en/moreinfo.php?pk=95867&jsn=254

Denso M22's would also be worth a try. Price is right:
https://www.rockauto.com/en/moreinfo.php?pk=261545&jsn=288

Worth noting I've been NOT USING any sort of antiseize on spark plugs on the airplanes I maintain for several years now. They spin out easily and cleanly. Antiseize, even the **** Champion stuff, gums up threads.
 
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