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How often replace auto plugs

plehrke

Well Known Member
Patron
I have the SDS CPI system running my upper NGK plugs. I have 2 years and 150 hours on the plugs, is it time to replace? How often are people swapping out the NGK plugs? Every so many hours? Every so many condition inspections?
 
Phillip, most seem to replace conventional auto plugs at each annual.

I've been running Denso iridium IKH27's (long reach) 200+ hours, which has meant swapping four at each annual, alternating top and bottom. The drops (30~40 RPM) don't change noticeably with time. Close inspection shows some ground electrode material loss, but almost none to the iridium fine wire.
 
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With the EICommander you can see the condition of the auto plugs and at around 100 hours +/-, they start to degrade. Engine performance seems to be unaffected.

We sent a bunch off to a PHd professor to look at and he found the ceramic insulators were starting to deteriorate.

For around $20 a set, I replace mine every year which is about 130 hours.

One guy I know replaces his every oil change.
 
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I fly about 125 hours a year, and replace my auto plugs each condition inspection, regardless of condition. They are too cheap not to.
 
100 hr

I have the SDS system also. It runs smooth as silk; however, at a little over 100 hours on the Denso iridium IKH27's plugs I noticed a slight roughness. After reading That Bill had tested sparkplugs from the Pmag system I decided to change out the plugs. The engine ran smooth again. I now just change them out every 100 hours, that is usually every 6 to 9 months. It helps having a son that gets a Denso employee discount:)
 
BR9ES is a colder NGK plug than what most people use. I stopped replacing them as they never have shown signs of erosion.
 
BR9ES is a colder NGK plug than what most people use. I stopped replacing them as they never have shown signs of erosion.

Correct and that is exactly what our Professor found. However, it was the insulation that was starting to fail.

That's why I recommend people don't use high dollar iridium plugs, the metal part isn't the issue.

For the price, it is cheap insurance just to replace them once a year.
 
I gap and ohm every new set. I replace all plugs every condition inspection unless it's been a really slow year on that particular airplane.
 
Correct and that is exactly what our Professor found. However, it was the insulation that was starting to fail.

That's why I recommend people don't use high dollar iridium plugs, the metal part isn't the issue.

For the price, it is cheap insurance just to replace them once a year.

Let's back up a moment Bill.

The professor found reduced resistance. Where was this reduced resistance...the center electrode ceramic, or the ceramic body?
 
Let's back up a moment Bill.

The professor found reduced resistance. Where was this reduced resistance...the center electrode ceramic, or the ceramic body?

Dan,

I would have to dig in my old computer to find his report (and we are leaving for the weekend) to answer if it is the center insulator or the external one.

Here is a description of the behavior that prompted us to send a set off:

The P-mags report the current flow the EIC. A shorted or fouled plug would display as a maximum scale reading (100). A broken or open plug lead would display no bar graph (0). (We set the reported values to a 100 point scale for display purposes.)

When new NGK BR8ES plugs are installed, we typically see a value between 70 and 80. As they accumulate flight time, that value drops off and readings of 40 are not unheard of when 100+/- hours are recorded.

Obviously the plugs resistance is going up, limiting current flow. What I haven't done, is to Ohm used plugs and compared them to new ones because I believe the EIC reporting function.

The odd part is I could not detect any change in engine performance; either roughness or reduced performance. The only indication I had was what the EIC was reporting. Replace the plugs and the bar graphs to back up.
 
Hmmm...'Trust, but verify' (?)

Ever try pulling the cap, removing and re-installing the plug, and rechecking the EI's reading? A simple resistance check, new & after, could be useful data, too.

IIRC, running a 'colder' plug makes conducting tip heat out to the head easier. Would that reduce the heat load in the internal resistor in the plug, extending life?
 
Hmmm...'Trust, but verify' (?)

Ever try pulling the cap, removing and re-installing the plug, and rechecking the EI's reading? A simple resistance check, new & after, could be useful data, too.

IIRC, running a 'colder' plug makes conducting tip heat out to the head easier. Would that reduce the heat load in the internal resistor in the plug, extending life?

Yep, done that with regard to pulling and resetting the plug wires but have not pulled a plug to check it's resistance, as I said. No change was noticed.

Also, heat doesn't seem to impact the readings. The numbers do move around but the general trend is down. Meaning that one one flight, from startup to shutdown, the numbers are fairly consistent. Over multiple flights, the numbers trend downwards.

I have not hit the cylinder head with a laser thermometer but I imagine that once the plugs are up to temperature, which might happen slightly faster than the head, the readings are pretty stable.
 
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I was thinking more about how fast the internal resistor degrades ('cold' resistance rises) ; not whether heat affects real-time readings. Does NGK publish min/max resistance specs for its plugs, like a/c plugs?
 
I would have to dig in my old computer to find his report (and we are leaving for the weekend) to answer if it is the center insulator or the external one.

Have fun! It will wait.

Here is a description of the behavior that prompted us to send a set off:

The P-mags report the current flow the EIC. A shorted or fouled plug would display as a maximum scale reading (100). A broken or open plug lead would display no bar graph (0). (We set the reported values to a 100 point scale for display purposes.)

When new NGK BR8ES plugs are installed, we typically see a value between 70 and 80. As they accumulate flight time, that value drops off and readings of 40 are not unheard of when 100+/- hours are recorded.

Obviously the plugs resistance is going up, limiting current flow. What I haven't done, is to Ohm used plugs and compared them to new ones because I believe the EIC reporting function.

The odd part is I could not detect any change in engine performance; either roughness or reduced performance. The only indication I had was what the EIC was reporting. Replace the plugs and the bar graphs to back up.

Hmmm. That's the functional opposite of what you previously wrote, which was...

We sent a bunch off to a PHd professor to look at and he found the ceramic insulators were starting to deteriorate.

and

However, it was the insulation that was starting to fail.

I'll wait for the report.
 
I pulled the factory original NGKs out of my 2002 BMW a few years back with 120,000 km on them. Still looked almost like new. Resistance measured very close to the new ones I put in to replace them. This is a coil on plug setup where the plugs are trapped deep down inside the valve cover with no air circulation at all. This car has been tracked and driven hard its whole life. I've owned it since new.

I've installed thousands of NGK plugs so far in my lifetime and seen only one bad resistor which was on a new plug, out of the box. Other people's experiences may be different but be sure you are not using counterfeit plugs. Those are circulating from cheap sources.

I'm not sure about mags and other EIs but our stuff will reliably fire plugs in a naturally aspirated engine with completely missing ground electrodes. A little bit of plug wear is of no concern with SDS so I don't think you need to be tossing them at 100 hours.

Are other folks on here finding resistors going bad in NGK plugs?
 
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The P-mags report the current flow the EIC. A shorted or fouled plug would display as a maximum scale reading (100). A broken or open plug lead would display no bar graph (0). (We set the reported values to a 100 point scale for display purposes.)

What is EIC?
 
I have the dual SDS ignition and replaced my original iridium NGK plugs after 300+ hours during my condition inspection. No engine running issues at all, the ground electrode had started to taper/wear a smidge. I?ve got over 100 hours and a condition inspection on the new ones, Perhaps in a couple hundred hours I?ll replace the iridium with the standard ES plug and see what happens to save a few bucks.
 
So what is the mechanism that makes a Lycoming eat auto spark plugs. I can understand lead fouling, but why would an 8.5-1 engine "wear out" plugs faster than an 11-1 auto engine?
 
Running at 60-75% power continuously, while the car is running at ~10-15% power continuously?
Higher heat load due to both higher effective combustion pressures (higher MAP, power), and more difficult to shed heat due to 300-400 degree heads, instead of 180 degree heads?
 
They don't.

I replace auto plugs on customers aircraft regularly, after a couple hundred hours the gap has grown significantly from wear even on the iridium. I think iridium can go around 200 hrs while the std ones last about 100 hrs before you start seeing wear.

I heard somewhere it is the leaded fuel that does it, makes sense as I grew up working on cars and used to change plugs every 10k mile back when we had leaded auto fuel.
 
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Back when we had leaded auto fuel, most cars didn't have EI or fuel injection. Usually carbs and distributors.

I know cars operate at lower power (and MP) on average, but I would think the much higher compression ratios would more than make up for that. I would think the combustion pressure and temperature are much higher in the car. The coolant may be 180F, but the combustion Temp is much higher.

But that is just a guess.

I will be running SDS EFI/EI. I think I will use one set of plugs until I notice a change in how it runs when I do the "mag check". The other set I will change at a regular interval. I will also be running at fairly low power settings, and LOP the vast majority of the flight.
 
I replace auto plugs on customers aircraft regularly, after a couple hundred hours the gap has grown significantly from wear even on the iridium. I think iridium can go around 200 hrs while the std ones last about 100 hrs before you start seeing wear.

Absolutely Walt. I'm just saying the observed wear (loss of material from the electrodes, thus increased gap) is perfectly normal. High energy ignition systems should ionize across very large gaps without difficulty. It does load the plug wire insulation. As the plug gap gets larger, peak voltage goes up.

In fairness, I should note that not all EI's perform the same in this regard, even with the same coil. E-Mag purposely limits dwell or amperage (I'm not sure which, but it's in my notes somewhere) to the coil primary. The result is lower peak voltage and/or duration. Brad's position is enough is just right, and more is just a power sink. The limitation may or may not be related to internal generator output.

As Ross noted previously, his system will fire across a very large gap. Robert Paisley has long posted an energy comparison for a system that was, at that time, an SDS. I imagine the current System 32 also has significant coil output.

I heard somewhere it is the leaded fuel that does it, makes sense as I grew up working on cars and used to change plugs every 10k mile back when we had leaded auto fuel.

Lead and carbon are conductive deposits on the ceramic center insulator. The key factor here is voltage rise. Rise time is fastest with CDI, followed by EI, with points triggered bringing up the rear. Fast rise time means not so much leakage across the ceramic. Think of a flood of electrons arriving at the gap, with some leakage down the ceramic. If the flood arrives slowly, the crest never gets very high...in this case, high enough to jump the gap.

In the good ole days we ran points ignitions (slow rise and not much peak) and leaded fuel (more conductive deposits), and cared for plugs a lot.

Let me say that if an owner wants to change plugs at every annual, well, fine. As noted, it does lower insulation loads on the plugs wires, and there is some lead contamination, just like those cars when we were kids. I do not think there is safety related reason to do so, no "insurance" as some have expressed. If anything, it is removing a part that is known to be operational, and replacing it with a part which is unknown.

From the Bosch Automotive Electrics/Electronics Manual ("CDI" = capacitive discharge, "TI" = EI, "CI" = points type):

Bosch%20Fig%2037%20Voltage%20Rise.JPG


Bosch%20Fig%2037%20Voltage%20Rise.JPG
 
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If anything, it is removing a part that is known to be operational, and replacing it with a part which is unknown.

So you don't replace your tires until you get a flat? Brake pads before you have a failure? How do you determine when your engine oil has become non-operational?

Yes, I know what your point was - but you're painting with a mighty broad brush with that statement.
 
Plug life depends on several factors, ignition type and brand being among those. Most of our customers are changing standard plugs at 100-200 hours, depending mostly on hours flown between inspections. Those using Iridium plugs are doing so generally in the 200-300 hr. range.
 
Yes, I know what your point was - but you're painting with a mighty broad brush with that statement.

Quite true. Humor another stroke.

The "replace at annual" common practice has little to do with actual plug condition. Rather, it's an outgrowth of the annual cleaning ritual we all practiced with massive plugs and magnetos. We feel like we're not doing the "proper" maintenance if we don't unscrew them for a look. At $3 each, it's not worth cleaning them, and replacing them feels good.

It's cultural. And woe to the guy who admits he doesn't do it. Unclean! Unclean!

Seriously, the better survey question might be "How many hours have you gone on a set of auto plugs, with EI, before noting an abnormal drop, or roughness?"
 
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Seriously, the better question might be "How many hours have you gone on a set of auto plugs, with EI, before noting an abnormal drop, or roughness?"

I agree. With EIs that have a powerful spark, you may never notice any running differences even with .080-.100 electrode gap. People often ask me what the plug gap should be and I generally say with SDS, it doesn't matter as long as it's more than .025. With other EIs maybe you do have to worry more and should replace plugs more frequently.

It's no big cost or trouble to replace auto plugs at annual or at the 2nd or third one. I'd simply do them on condition. When the gap is noticeably larger or the electrodes rounded off, might as well replace them then.

There is no fixed interval that fits every engine or ignition system.
 
With the EICommander you can see the condition of the auto plugs and at around 100 hours +/-, they start to degrade. Engine performance seems to be unaffected.

We sent a bunch off to a PHd professor to look at and he found the ceramic insulators were starting to deteriorate.

Bill, were you able to find the professor's report?
 
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Quite true. Humor another stroke.

The "replace at annual" common practice has little to do with actual plug condition. Rather, it's an outgrowth of the annual cleaning ritual we all practiced with massive plugs and magnetos. We feel like we're not doing the "proper" maintenance if we don't unscrew them for a look. At $3 each, it's not worth cleaning them, and replacing them feels good.

It's cultural. And woe to the guy who admits he doesn't do it. Unclean! Unclean!

Seriously, the better survey question might be "How many hours have you gone on a set of auto plugs, with EI, before noting an abnormal drop, or roughness?"

I love this, Dan :D

Maybe sometime when I have the cowl off for a conditional inspection, I'm moved with the bowels of compassion and say to my (probably still serviceable) plugs, "I AM willing. Be clean!" :cool: No worries, right?
 
Replacing at annual might be cultural and possibly for ease of mind.

When I first started doing my own annuals, I changed brake pads at each annual regardless of how much meat remained on them. The spark plugs were too expensive to just replace so I cleaned those. Two or three annuals gone and I started questioning this practice, so I decided to see if I can get another six months or more from the brake pads. It went to the next annual and the pads were still good. Long story short, it turned out that every three years was the right time to replace the pads.
I clean my auto style spark plug and test them, If no issues are observed, I clean them and reuse them instead of contributing to the land filed for no good reason.
 
It's cultural. And woe to the guy who admits he doesn't do it. Unclean! Unclean!

Seriously, the better survey question might be "How many hours have you gone on a set of auto plugs, with EI, before noting an abnormal drop, or roughness?"

I'd admit to this one. With 1 P-Mag on my O-320, I'm running 4 Iridium plugs on the bottom of the engine so that I DON'T have to ever clean them. I just pull them out and throw them away at oil change.

Yes... I throw them away every ~50 hours. It could be something symptomatic of my airplanes set up, but $20 for 4 new plugs is cheap enough to me. My first flying job was towing banners along the Maryland and Delaware coasts. In the heat of summer, the boss man had us changing oil and cleaning plugs every 25 hours which was sometimes twice a week. (we flew 7 days a week, sometimes 8 hours a day). I feel like I've cleaned a few plugs in my time already... $20 for a fresh plug every few months seems worth it to me.

As for performance:
I recently installed a newly overhauled Carb (amazing difference for me) and 1 P-Mag around the same time. With new plugs, at altitude (10-13k), I can easily run smooth LOP up to almost 100 degrees LOP. Maybe 85-90 degrees, its too rough. After about 20-30 hours, I can barely get LOP without it acting rough. As I type this, it occurs to me that the plugs are probably just dirty. I subscribe to the Mike Busch method of leaning my engine on the ground (and in the air) so I believe I am minimizing the lead build up. So the last 20 hours before an oil change, I don't get my higher altitude efficiency. But a monh or so and $20 later, the efficiency has returned along with my RV Grin :)
 
I'd admit to this one. With 1 P-Mag on my O-320, I'm running 4 Iridium plugs on the bottom of the engine so that I DON'T have to ever clean them. I just pull them out and throw them away at oil change.

Yes... I throw them away every ~50 hours. It could be something symptomatic of my airplanes set up, but $20 for 4 new plugs is cheap enough to me. My first flying job was towing banners along the Maryland and Delaware coasts. In the heat of summer, the boss man had us changing oil and cleaning plugs every 25 hours which was sometimes twice a week. (we flew 7 days a week, sometimes 8 hours a day). I feel like I've cleaned a few plugs in my time already... $20 for a fresh plug every few months seems worth it to me.

As for performance:
I recently installed a newly overhauled Carb (amazing difference for me) and 1 P-Mag around the same time. With new plugs, at altitude (10-13k), I can easily run smooth LOP up to almost 100 degrees LOP. Maybe 85-90 degrees, its too rough. After about 20-30 hours, I can barely get LOP without it acting rough. As I type this, it occurs to me that the plugs are probably just dirty. I subscribe to the Mike Busch method of leaning my engine on the ground (and in the air) so I believe I am minimizing the lead build up. So the last 20 hours before an oil change, I don't get my higher altitude efficiency. But a monh or so and $20 later, the efficiency has returned along with my RV Grin :)

4 Iridium for $20 is a great buy, where do you buy yours. My last Iridium (long reach) was at $11 the cheapest.
 
I have two pmags and replace the top plugs on odd years and the bottom plugs on even years. I fly about 100 hours per year. Seems to be working well after two years. John
 
My problem?
I like spark plugs! I change them just to try different ones! I have a tool chest drawer full of plugs I want to try.
This year's condition inspection replaced the BR8ES plugs (running perfectly) with Champion RD15Y 18mm resistor plugs with extended tip - gapped to .032". These fire the bottom plugs on my RV-8 via an Emag. The top plugs are fired by a Bendix 200, they got new Tempest UREM37BY extended tip plugs..
The Champion RD15Y stock #526 is an industrial plug, I buy them thru Summitracing for $3.99 ea. Two flights so far, seem to run perfect, but will need some time-in-service to know. In flight mag checks really show the Emag to out perform my Bendix 200, which is within it's 500 hour period. (new plugs on both)
Oh, O-360 parallel valve, 8.5:1, Airflow Performance Fuel injection, Vetterman Y type crossover exhaust, around a Superior cold flow sump and Catto 3 blade.
First flight flown over the airfield at full power for half an hour, the second flight flown locally at very low power, 1800-2100 RPM.
18mm resistor plugs are limited in selection. I've used Autolite 386 on my T-18 with 7:1 O-290, where they run OK but seem a bit on the hot side.
I'm hoping the RD15Y is a cooler plug. with resistor and extended tip, it could be an ideal plug for those who dislike plug adapters.
Time will tell.
One unusual variable: my O-360 came from a Brantley Helicopter. It has external oil cooling lines from the crankcase oil gallery to each cylinder where it cools the outside of each exhaust valve guide then spills into the valve chest. Yes, my heads run cooler.
 
4 Iridium for $20 is a great buy, where do you buy yours. My last Iridium (long reach) was at $11 the cheapest.

Amazon.com most of the time. I just checked and I guess I fibbed on the price. Last set I bought for $25 but the price seems to have gone up to $27.

https://www.amazon.com/gp/product/B004A72048/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

I also have an amazon credit card with points to use as cash on Amazon.com so sometimes my airplanes parts are FREE :)
 
As for performance:
I recently installed a newly overhauled Carb (amazing difference for me) and 1 P-Mag around the same time. With new plugs, at altitude (10-13k), I can easily run smooth LOP up to almost 100 degrees LOP. Maybe 85-90 degrees, its too rough. After about 20-30 hours, I can barely get LOP without it acting rough. As I type this, it occurs to me that the plugs are probably just dirty.

An interesting report. Perhaps clean a set and see if LOP capability returns? Or leave them dirty and reduce the gap a little?

E-Mag purposely limits spark energy, and debates the need for more. Best I know, the only way to do it is to limit dwell time and/or primary amperage. Might be interesting to scope the coil primary. https://emagair.com/home/design-principles/

Typical example from a set of IHK27's after 213 hours in the 390. Sorry, cell phone photo is a little fuzzy; I'll do better next time. Some edge erosion of the ground electrode, which increases required voltage rise. Small gap increase too.

Plug%20IKH27%20213%20Hrs%202.jpg


Plug%20IKH27%20213%20Hrs%203.jpg
 
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