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Why replace auto plugs

pa38112

Well Known Member
I run spark plugs in my car for 60,000 with no issues. Why is it that everyone recommends replacing auto plugs every year? The only answer I have heard is "They are cheap", and that does not tell me much. Not to mention, that the Iridium plugs I run are not all that cheep.
What about an Aircraft application would necessitate replacing plugs after 150 hours?
 
Longer exposure to high temps, result in higher temp of all the heat transferring components. Higher power is higher cylinder pressure, and higher peak temps, in spite of the adiabatic flame temperature as it just adds to the temperature of compression.

By example, Non-combustion compression pressure of an idling Alfa Romeo with 10:1 Cr is only 40 psi. Just install a compression tester, start the car and read the pressure, discharge several times. Might not work on EFI with OBDII but it does on an engine with a carburetor.

Also, all materials oxidize and more so at elevated temperatures. Even iridium. All this means that an engine that runs an average of 75% load will have higher life average plug temps than an engine running at 15% load factor. Remember the Arrhenius equation/function - - longer times at higher temps will erode the plugs at a highly non-linear rate. There is less electrode mass of the auto plugs and heat transfer paths are narrow.

Anyway - - It would be a fun experiment to see how long they last in your plane compared to massive electrodes.

Weird information: The average speed of an automobile is about 37 mph. An over the highway truck (Class 8) is 57 mph.
 
Why indeed...

FWIW, a buddy of mine says he has run the same set of iridium plugs in his 0-320 for going on five years (not sure how many hours, he flies around 50 per year I would guess.
Anyway, no problems so far. Pmags.
Jim
 
This is a good of question. I replace my auto plugs at the condition inspection and the old plugs always look fine, actually brand new. One time I installed a new dud plug and another time I dropped a new plug and had to replace it. So what does all this mean? I suspect that I am wasting my money. Like so many other things in aviation we do them because that’s what the did in the war or some other silly reason.
 
Mind the gap...

I think replacing the plugs every year is wasteful.

I would, however, inspect the plugs - re-gap as necessary, check the resistor value, and then replace them when necessary...

Iridium plugs wear better than Platinum, but are a little touchy to set the gap.
 
IKH27 at 213 hours. No real wear to the center electrode, but the ground was eroding away and increasing the gap. Decent illustration of what Bill described.

Increasing gap increases voltage rise ("K" in fig 2) before ionization. Weak coil insulation or an old plug lead may allow that higher voltage to reach ground before the plug can ionize the gap and form an arc. You feel it as a misfire. With good insulation everywhere, plug gaps can get pretty large and still work. Bottom line? How long you can run a plug depends on the quality of the other components in the system, probably more so than the plug itself. It is unlikely to be the same fleetwide

Plug%20IKH27%20213%20Hrs%20Wear.jpg


Credit Bosch Automotive Electric/Electronics:

Bosch%20Figs%202%20and%203%20Spark.JPG
 
Lead.

And they are cheap. I don't see any value in using iridium plugs. Others feel differently...so YMMV.
 
I don’t automatically replace them at the annual - I do it based on how the electrodes look and a check of the gap, based on information like Dan presented. When I only had my RV-8, and flew it 300 hours per year, I probably would replace them every year. But with multiple airplanes, it is rare that any of our planes gets more than 150 hours between condition inspections - so I look at how many hours, and the condition of the plugs.

What I DON’T do is waste time trying to clean and regal them - if they appear bad, I just pull new ones out of the stash. With four Lycomings using BR8-ES plugs, I keep about four dozen on a shelf - and that lasts for a long time.
 
I use massive plugs. the first set lasted 700 hrs. 200 hrs on the second set. they looked great at condition inspection.
 
yes, that's the photo. I don't have any data but I would guess spark plugs are like most electrical components where the failure rate is higher for infant mortality. once you have a plug that lasts 100 hrs, chances are that it will last until the electrodes wear beyond use. plugs can be used for a long time: less plugs, less chance for infant mortality, maybe less cost overall (aviation vs auto), less environmental impact. clean and rotate them at each oil change or every other oil change, or more.
 
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yes, that's the photo. I don't have any data but I would guess spark plugs are like most electrical components where the failure rate is higher for infant mortality. once you have a plug that lasts 100 hrs, chances are that it will last until the electrodes wear beyond use. plugs can be used them for a long time: less plugs, less chance for infant mortality, maybe less cost overall (aviation vs auto), less environmental impact. clean and rotate them at each oil change or every other oil change, or more.

Yes, but if you are using most of the EI systems out there (instead of mags), the spark plug connectors aren’t really designed to work very well with aviation plugs. I would also be using aircraft massive electrode plugs if I was still using mags - you have to match the plugs to the ignition system. And yes, you CAN use aviation plugs with EI, and automotive plugs with mags...but both of those configurations are a bit of a kluge....
 
The performance difference between massive vs fine-wire aviation plugs might be noticeable in that the the fine-wire version is likely to have a bigger gap/hotter spark. In the auto plug category, I suspect it's negligible, and I've seen that to be true based on decades of tweaking snowmobile engines. In the auto category, Iridium, IMHO, is likely to last longer but provides little, if any, performance advantage. OTOH, I see no disadvantage to using standard-electrode auto plugs if they're going to be pitched every 25-50 hours. As a cost percentage, a set of 4 standard-electrode auto plugs is about the same as one quart of Aeroshell.
 
I replaced my iridium plugs for my EI at about 300 hours. No mis-fires but the ground wear shown by DanH was noticeable.
 
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