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Old 05-20-2021, 09:14 AM
pa38112 pa38112 is offline
Join Date: Mar 2008
Location: Clarksboro, NJ
Posts: 988
Default Why replace auto plugs

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?
2004 RV6A Flying
2002 RV6A Flying
1978 PA38-112
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Old 05-20-2021, 09:40 AM
BillL BillL is offline
Join Date: Sep 2007
Location: Central IL
Posts: 6,141

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.

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Old 05-20-2021, 09:44 AM
Jimzim Jimzim is offline
Join Date: Mar 2009
Location: Arvada, CO
Posts: 146
Default 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.
2015 RV7,
Lyc Thunderbolt 0-360 A1A
Bendix FI
Hartzell BA CS
Dual Pmags
Dues gladly paid
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Old 05-20-2021, 10:07 AM
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reak reak is offline
Join Date: Oct 2005
Location: Loveland Colorado
Posts: 32

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 thatís what the did in the war or some other silly reason.
Brad Reak
RV7A Flying, N789W
Loveland, CO
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Old 05-20-2021, 10:33 AM
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bjdecker bjdecker is offline
Join Date: Jun 2005
Location: Georgetown, TX
Posts: 833
Default 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.
Brian Decker
RV-7 (Flying)
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Old 05-20-2021, 10:35 AM
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DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 10,209

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

Credit Bosch Automotive Electric/Electronics:

Dan Horton
Barrett IO-390
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Old 05-20-2021, 10:38 AM
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MacCool MacCool is offline
Join Date: Aug 2020
Location: central Minnesota
Posts: 676


And they are cheap. I don't see any value in using iridium plugs. Others feel YMMV.
RV-9A, 2011, bought flying
IFR certified
AFS 5400/3500, G5, 430W, some other stuff
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Old 05-20-2021, 10:43 AM
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Ironflight Ironflight is offline
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Location: Dayton, NV
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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.
Paul F. Dye
Editor at Large - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)
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Old 05-20-2021, 11:18 AM
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Steve Melton Steve Melton is online now
Join Date: Apr 2011
Location: Cincinnati, OH
Posts: 3,362

I use massive plugs. the first set lasted 700 hrs. 200 hrs on the second set. they looked great at condition inspection.
Steve Melton
Cincinnati, OH
RV-9A, Tip-up, Superior O-320, roller lifters, 160HP, WW 200RV, dual impulse slick mags, oil pressure = 65 psi, EGT = 1300F, flight hours = 900+ for all

Simplicity is the art in design.
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