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Automotive vs Aviation Spark Plugs

Lizard Lips

Active Member
I'm sure this subject has been kicked around for a long time, but I would like to know what most of you guys do.
Building a RV-14A with a IO-390 engine hung in it. Coming from the old school (I was an Army aircraft mechanic in the late 1950's) I had not even considered using automotive spark plugs in a Lycoming engine ? mercy! However there seems to be a lot of discussion on VAF about plugs and it's hard to sort through all the posts.
My IO390 will have one P-mag and one standard magneto if that makes any difference. Auto plugs would certainly be much cheaper than aviation plugs but I don't want to paint myself into a corner.

Joe Reaves
Longview, Texas
2019 Dues Paid

RV-14A - Empennage complete, starting on QB fuselage and wings
Arion Lightning LS-1 Flying regularly
 
Auto plugs for the P-mag and aviation plugs for the mag.

No issues running auto plugs with P-mags.

Use adapters with the auto plugs. That way you can go to aviation plugs, if, for some reason, you wanted to switch.
 
+1 Bill

What he said. If you read the P-Mag manuals, they explain why they recommend against aircraft plugs with their units...
So think of it this way: you're doing exactly what you would have done back-when: following the mfr's instructions, Just like we were trained to do.
 
To add to Bill's post... go check out his recent post on P-Mag timing. Of critical importance in that thread is the need to properly time the P-Mag to your engine. The 390 times at 20BTDC while most of us out there with 320's and 360's get our heads stuck on 25BTDC. It would definitely not be good to set the timing at 25BTDC on a 390 engine!
 
I'm sure this subject has been kicked around for a long time, but I would like to know what most of you guys do.
Building a RV-14A with a IO-390 engine hung in it. Coming from the old school (I was an Army aircraft mechanic in the late 1950's) I had not even considered using automotive spark plugs in a Lycoming engine ? mercy! However there seems to be a lot of discussion on VAF about plugs and it's hard to sort through all the posts.
My IO390 will have one P-mag and one standard magneto if that makes any difference. Auto plugs would certainly be much cheaper than aviation plugs but I don't want to paint myself into a corner.

Joe Reaves
Longview, Texas
2019 Dues Paid

RV-14A - Empennage complete, starting on QB fuselage and wings
Arion Lightning LS-1 Flying regularly

Another question just occurred to me: I will be running one p-Mag and one magneto because Lycoming will not put 2 p Mags on their engines. Seems to me from what I've read that p Mags are the way to go and I probably should put them on both sides rather than have one standard magneto and one p Mag. Seems to me that reliability is not an issue. What do most of you do?

Joe Reaves
Longview, Texas
2019 Dues Paid

RV-14A - Empennage complete, starting on QB fuselage and wings
Arion Lightning LS-1 Flying regularly
 
SEARCH

Search is your friend here. Look at dual P mag threads. And since you are considering plugs.... look at the recent thread on counterfeit NGK plugs on the market. I throw mine out yearly. They have lots of life... but I don't have to clean a plug. Be sure to look for the ones with a locked ferrule. The threaded variety vibrate loose.
I fly a P mag and a lightspeed. That means I have an internal alternator in the P mag. A dead DC power system will not create a forced landing. A dual P mag will be roughly the same. P mag has simpler install details. And Brad, the owner at EMAG provides outstanding support. You will see that in the thread search as well.
 
Another question just occurred to me: I will be running one p-Mag and one magneto because Lycoming will not put 2 p Mags on their engines. Seems to me from what I've read that p Mags are the way to go and I probably should put them on both sides rather than have one standard magneto and one p Mag. Seems to me that reliability is not an issue. What do most of you do?

Joe Reaves
Longview, Texas
2019 Dues Paid

RV-14A - Empennage complete, starting on QB fuselage and wings
Arion Lightning LS-1 Flying regularly
Now you are talking!

Many people run mixed ignitions and many run two P-mags, as do I.

Look at installing an EICommander to monitor and control your P-mags It is like an engine monitor for your ignitions but will also allow you to tune your P-mags.
 
not so fast there..

You can change to MSD 8.5 wires and use auto plugs with BENDIX mags. I know this because I have done it. Finally, I can get the engine to idle at 500RPM with this setup - about 650 was the low limit previously. Mag drop at 1600RPM is 10rpm. Yes - that is TEN RPM. TCM IO-550-N.

Seriously - I thought my switches were broken when I 1st ran the engine.

EGTs and CHTs have not changed - that I can tell.

Get in touch with Tom at G3 Ignition - he has instructions to change to the MSD wires, and I think he has a method to modify Slick mags too. Generally, Bendix mags have a hotter spark...and are rebuild-able.

Get the high quality auto plug adapters from Ross.

After getting this all set up and running, I had a discussion with a well-known exhaust system builder - he's been doing that mod for 10 years (if I can recall what he said). He asked why had I not done this before? Uhhhh....

At any rate, if you want to go this route, I have 'a few' extra parts left over from my setup - doubtful I'll need them, so I can help you with your parts order if you choose this option.
 
I have a dual Lightspeed electronic ignition and Auto plugs are mandated for this application
 
I’ve used the G3i method on two slick mags and run cheap auto lite 18mm plugs off of them. No adapters, excellent spark, and cheap to replace. Caveat: I’ve also run the CPI on the top both times which really does most of the heavy lifting.
 
What do you need to run auto plugs with magnetoes?
Will they run with any mag or only one brand?
 
I?ve used the G3i method on two slick mags and run cheap auto lite 18mm plugs off of them. No adapters, excellent spark, and cheap to replace. Caveat: I?ve also run the CPI on the top both times which really does most of the heavy lifting.

CPI ? what does the acronym expand to in words ?
 
What plugs and what gap?

You can change to MSD 8.5 wires and use auto plugs with BENDIX mags. I know this because I have done it. Finally, I can get the engine to idle at 500RPM with this setup - about 650 was the low limit previously. Mag drop at 1600RPM is 10rpm. Yes - that is TEN RPM. TCM IO-550-N.

Seriously - I thought my switches were broken when I 1st ran the engine.

EGTs and CHTs have not changed - that I can tell.

Get in touch with Tom at G3 Ignition - he has instructions to change to the MSD wires, and I think he has a method to modify Slick mags too. Generally, Bendix mags have a hotter spark...and are rebuild-able.

Get the high quality auto plug adapters from Ross.

After getting this all set up and running, I had a discussion with a well-known exhaust system builder - he's been doing that mod for 10 years (if I can recall what he said). He asked why had I not done this before? Uhhhh....

At any rate, if you want to go this route, I have 'a few' extra parts left over from my setup - doubtful I'll need them, so I can help you with your parts order if you choose this option.

Boss,

What plugs do you run at what gap? What CR (Comp. ratio) is your 550?
 
No problem@

Boss,

What plugs do you run at what gap? What CR (Comp. ratio) is your 550?

I run 'em at the stock gap from the factory. I measure it at over .022 with a wire gauge. I am running fresh Bendix mags, and those are a bit stronger than the Slicks. Probably ought to close 'em to ~.020 if you are running Slicks.

Compression: apparently TCM fibs a bit - I have heard that the 550 has 9:1 (from one of the original designers), or maybe a bit more...8.5:1 in the published paperwork tho.
 
I?ve used the G3i method on two slick mags and run cheap auto lite 18mm plugs off of them. No adapters, excellent spark, and cheap to replace. Caveat: I?ve also run the CPI on the top both times which really does most of the heavy lifting.

Would be interested in learning what part number automotive plug you use without adapters.
 
CPI ? what does the acronym expand to in words ?
Coil Pack Ignition. It's made by Simple Digital Systems. I believe they're a sponsor on the site. Great programmable electronic ignition system. I believe the hotter spark from this system and the greater advance means that most of the time when the slick mag fires, the ignition process has already begun. I like the slick as a backup in case of battery failure though, and the engine will run fine on one.


Would be interested in learning what part number automotive plug you use without adapters.
Get in touch with G3i for the instructions and parts to convert your slick harness to auto plugs. As for the particular plugs I use, they're Autolite 386's. i only use them with the mag. For the electronic, I use the adapters and NGK plug type Ross sent with the system.
 
Coil Pack Ignition. It's made by Simple Digital Systems. I believe they're a sponsor on the site. Great programmable electronic ignition system. I believe the hotter spark from this system and the greater advance means that most of the time when the slick mag fires, the ignition process has already begun. I like the slick as a backup in case of battery failure though, and the engine will run fine on one.



Get in touch with G3i for the instructions and parts to convert your slick harness to auto plugs. As for the particular plugs I use, they're Autolite 386's. i only use them with the mag. For the electronic, I use the adapters and NGK plug type Ross sent with the system.

I'm confused. Your previous post implied that you are running with both G3i and CPI. Or did you have G3i, then replaced with the SDS solution? My assumption is that these are competitive products and not complimentary.
 
We have an RV7 with a Titan IO-360 running dual P-Mags and an MT 3 blade prop.

I have conversed with, discussed with, cussed, abused, agreed and hugged Brad Dement since the mid 2000's - he is a good man and produces exquisite ignitions.

We were the second dual P-Mag install in the UK and have had no issues whatsoever since install.

They went back for a firmware update once.

We re time them every annual because it takes moments.

The plugs cost $8 a piece from O'Reilly's - good people to deal with, especially on OBT in Orlando ;-)

Oh, and there is some old gadge in the Carolinas that does other stuff for P-Mags - diddles with the timing, tells you all sorts of stuff that is super cool and useful. Likes Dragons apparently ;)

Move up to the 21st Century and enjoy easy and simple ignition.

M

ps Bill - You can buy me a beer at Sun n Fun :D
 
I'm confused. Your previous post implied that you are running with both G3i and CPI. Or did you have G3i, then replaced with the SDS solution? My assumption is that these are competitive products and not complimentary.

He states that he runs the G3i "solution" on his Slick mag... And a CPI on the other. In other words, one mag, one CPI- all with auto plugs, just like the OP is considering.
 
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We have an RV7 with a Titan IO-360 running dual P-Mags and an MT 3 blade prop.

I have conversed with, discussed with, cussed, abused, agreed and hugged Brad Dement since the mid 2000's - he is a good man and produces exquisite ignitions.

We were the second dual P-Mag install in the UK and have had no issues whatsoever since install.

They went back for a firmware update once.

We re time them every annual because it takes moments.

The plugs cost $8 a piece from O'Reilly's - good people to deal with, especially on OBT in Orlando ;-)

Oh, and there is some old gadge in the Carolinas that does other stuff for P-Mags - diddles with the timing, tells you all sorts of stuff that is super cool and useful. Likes Dragons apparently ;)

Move up to the 21st Century and enjoy easy and simple ignition.

M

ps Bill - You can buy me a beer at Sun n Fun :D


How far off is the timing at that time?

I don't expect it to move at all due to the fact that there is nothing to wear out.
 
Probably not shifted at all, but it has just become part of the annual.

We had CHT issues some while ago and it was thought that the timing may have altered, we re timed the P-Mags and the CHT's came down.
 
How far off is the timing at that time?

I don't expect it to move at all due to the fact that there is nothing to wear out.

This thread stated out about spark plugs, and that has been answered. But since this has turned into another Pmag review let's throw some reality on the fire...

First off, per company guidance the Pmags need to be removed at very CI so they can be inspected for wear and damage, so even the company recognizes there are things to go wrong with the moving parts. Bearings can and do wear, magnets can and do shift, and control boards can and do corrode due to water intrusion. I have personally experienced ALL of these things with my Pmags over the years. Second, I do experience timing drift between inspections despite the fact that the ignitions "pass" the subsequent bench inspection.

This reality is a significant downside of stuffing the electronics, power generation, mechanical timing, and high voltage elements in one package. Sure, it's a nice marketing niche with good consumer appeal in theory, but in practice it's an engineering hurdle that the company has strugled with for its entire history - and has yet to overcome completely. Its also one of the reasons the auto manufacturers abandoned unitized ignition decades ago in favor of maintenance free distributed system.

And as for the well documented need to "spoof" or intentionally mis-time the Pmag so that it will function on an angle valve without damage - well, there's plenty of threads and flight test data on THAT subject too.
 
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Autolite 386 18mm plug

Would be interested in learning what part number automotive plug you use without adapters.

I have 200 or so hours on my first pmag w Autolite 386 plugs and when I added second pmag 50 hrs ago went right to that plug. Been flawless so far.
 
Autolite 386 plug question for Pmags

will the Autolite 386 plugs work on an IO390 with required long reach plugs? I would really not prefer to add adapters if possible.
 
Something to mention. Several engine builders including Lycon strongly recommend you not mix mags with electronic ignition. As mentioned it has to do with flame front inside the cylinder since the two plugs would often fire at different times.
I know it?s done all the time but it might impact overall lifespan on a engine.
 
Something to mention. Several engine builders including Lycon strongly recommend you not mix mags with electronic ignition. As mentioned it has to do with flame front inside the cylinder since the two plugs would often fire at different times.
I know it’s done all the time but it might impact overall lifespan on a engine.

Lycoming now ships their IO-390's to RV-14 builders with one mag and one P-mag installed, if desired.
My guess is they might have changed their mind. I really don't know as I have not spoken to anyone at Lycoming about mixing ignition types.
 
Hi Bill, just to clarify I was referring to Lycon not Lycoming. EMAG also has the same recommendation to not mix ignitions. They consider it acceptable but recommend you keep them alike.
G
 
The Lycon guy was however pretty adamant that the have seen issues. You would be changing the pressure distribution across the cylinder when firing from one side first. On the other hand most of their engines are hopped up to some extent.
G
 
The Lycon guy was however pretty adamant that the have seen issues. You would be changing the pressure distribution across the cylinder when firing from one side first.
G

Be specific please. What issues, evidence, and explanation?
 
He was giving you a good marketing ploy, without any evidence that is worthless.
 
It was several years ago that I talked to Lycon. I was in negotiations to purchase a aircraft with one of their engines running dual electronic ignitions. I wanted to replace one with a mag for the usual reason. I did not want to be electrically dependent. He strongly advised against that and we had a 5 minute discussion that went mostly over my head about internal cylinder pressures and flamefront propagation. They certainly had no financial interest with that recommendation.
G
 
It was several years ago that I talked to Lycon.

Ok, thanks.

It's actually an old subject. The original discussion dealt with full power operation on one ignition, which if you think about it, is the ultimate extension of a "one plug fires late" argument. The concern was extended burn time (due to the need for the flame front to travel all the way across the cylinder, rather than just halfway to meet an opposing flame front) might allow sufficient heating and pressure rise in the pre-flame front gases to trigger detonation. It was George Braly who said "Nope", as he had run single ignition detonation tests on the GAMI dyno, using the trusty 540K. Running on one delayed peak pressure enough to make detonation a non-issue.

The twist before us now is the heavily advanced timing locked into some of the current EI's, which would move peak pressure back toward TDC. It's been a long time, and I don't recall if George did any of his single ignition detonation runs with the mag twisted to the 38 BTDC ballpark, as some folks are doing these days....thus my renewed curiosity. Thank you for taking the question.
 
Automotive Vs Aviation Spark Plugs

Autolite 386 plugs are not recommended for Lycoming IO390. They will leave the last few threads open to the combustion chamber as the plug does not reach the full length.

I could not find any 18mm spark plugs of any type (and I bought many) that had the correct heat range that would fit correctly.

NGK spark plugs (B9Exx) are also too short even with the adapter.

The only spark plugs the are the correct length and correct heat range are
Denso IKH27 (recommended) or Denso IKH24 (one heat range hotter). These are 14mm and require an adapter.

Rocky
 
As for the particular plugs I use, they're Autolite 386's.

Be careful with those 386's. Many moons ago I ran the Autolite 386 plugs with a ElectroAir EI. On one particular flight I was leaned for cruise and saw a friend in his 7A; Bogey at 11 I called while I smashed the throttle forward.

The proverbial stuff hit the fan as soon as I stuffed the throttle forward; the engine ran rough and nothing I could do would smooth it out. Power was dropping and I called my wing man to find me a place to land which he did, a farm strip and it was a welcome sight.

The landing was uneventful and the engine seemed to purr on rollout. I did a mag check which confirmed the EI was the culprit. I switched the EI off and headed for home, a mere 5 minutes away.

Post flight inspection showed that #2 cylinder's Autolite 386 had turned into a glow plug. Nothing I did in flight would stop that glow plug from glowing. The piston and rings showed extreme detonation failure. An expensive lesson for sure.

I converted the ElectroAir to 37BY plugs after the cylinder change and never looked back.

Be careful of the plugs you choose!
 
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Autolite 386

For those using the Autolite 386 plugs, do you use the installed gasket or remove it and use an aviation spark plug gasket with it instead?
Tim Popp
RV-7 IO-360
 
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