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Lycoming IO-390 E-mag Split Top and Bottom Plugs

iamtheari

Well Known Member
A few of my pilot and A&P friends have asked, and my rule is that I won't fly my plane if they can ask me questions I can't answer, so here goes...

My Lycoming IO-390 with one E-mag and one magneto came with the automotive spark plugs on the top of cylinders 1 and 3 and the bottom of cylinders 2 and 4. Is there a reason for this top/bottom split?
 
A few of my pilot and A&P friends have asked, and my rule is that I won't fly my plane if they can ask me questions I can't answer, so here goes...

My Lycoming IO-390 with one E-mag and one magneto came with the automotive spark plugs on the top of cylinders 1 and 3 and the bottom of cylinders 2 and 4. Is there a reason for this top/bottom split?
I was told this is to balance the mag check RPM drop.
 
The simplest answer is "Because Lycoming said so."

Since basically forever, the two magneto system has been set up so that a mag will fire the top plugs on one side and the bottom plugs on the other.

As to why it's that way, I have to confess that I've never really thought about it, but off the top of my head, I'd say there are a couple of benefits.

Way back in the day, Lycoming issued a service letter for pretty much all their engines where they recommended rotating spark plugs every 25 hours due to lead fouling. In that document, they specifically stated that the top plugs scavenge better than the bottom ones. Presumably, if you had one mag fail, you wouldn't want it to only be driving the top plugs because those are the "good" ones, so they wire them half-zees left/right/top/bottom.

Fast forward to today, and the IO390 has you rotate spark plugs "on condition" at 50 hours and clean/gap/rotate at 100 hours. The rotation they call out is a cake walk where every plug moves one step fwd in the rotation and moves from top to bottom.

All this is in the maintainability manual, and is based on Mags. I have no idea if theres any practical benefit if you're throwing a P-mag into the mix, but it seems like there might be.
 
Old habits die hard. This was an approach set up to deal with unique characteristics of mags.

Mags alternate spark polarity on each 180* rotation (it is using a round magnet to generate the power for coil energizing and it is one solid magnet, so each side has a different polarity, think alternating current from traditional, magnet based AC generators - In AC, the polarity swaps 60 times per second. In DC, we use a rectifier to stabilize the polarity, but a Mag uses AC for power generation). The wiring recommended allows you to swap the plugs from top to bottom on a cylinder to get the opposite polarity. The polarity determines whether the wear will be on the center electrode or ground electrodes.

The recommended approach (specified wiring + top/bottom rotation) balances the long term wear between electrodes when using two mags. traditional auto style coils, including P mags, (coil energized by airship DC source, which has a consistent polarity) do not do this, so swapping adds no value.

That said, with one mag a rotation schedule could be arranged to swap plugs between different cylinders to reverse the polarity on mag attached plugs. 1st and 3rd plug to fire are one polarity and 2nd and 4th are the other.

Larry
 
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Does the polarity work out so my cylinder 1 and 3 bottom plugs and cylinder 2 and 4 top plugs will be opposite, and rotating them will give that benefit even though the automotive plugs and E-mag don’t get it? That would justify the split.
 
Does the polarity work out so my cylinder 1 and 3 bottom plugs and cylinder 2 and 4 top plugs will be opposite, and rotating them will give that benefit even though the automotive plugs and E-mag don’t get it? That would justify the split.

re-read my post, as I added info after you read it.

You need to look at the mag harness routing. 1st and 3rd plug to fire are one polarity and 2nd and 4th are the other. Each successive wire / plug in the rotational arrangement will be a different polarity.

Frankly, I don't rotate mine. I just adjust the gap every year and replace when the wear becomes objectionable or becomes difficult to get a consistent gap.
 
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Cheap "redundancy"

It is still in the Lyc manuals that I have. Per the old powerplant guys, conditions could exists (more often in the past) where top or bottom plugs would be more susceptible to fouling; lead, oil, other fuel properties. Cross connecting top/bottom across a set of leads reduces the chance of losing a cylinder if a mag drops out. Most of us are flying behind only four jugs. Is the lead configuration as important today? Probably not. Free reliability increment? Yes.
 
Mag/EI

I’m going to run 1 impulse Slick and 1 Electroair. I thought a decent plan would be to have the Slick run the top plugs and the EI the bottom. Is there a better way?

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
When joining the electronic ignition bandwagon, what do you all think of running all the bottom plugs off the electronic "non-mag" and all the top plugs off the tractor mag?

My thought is that the bottom plugs have been the ones more prone to fouling (Lyc IO-360) and therefore the fouling might be mitigated by the more vigorous spark produced by the electronic mag. Also, how does "wasted spark" factor into plug condition?

George
 
I’m going to run 1 impulse Slick and 1 Electroair. I thought a decent plan would be to have the Slick run the top plugs and the EI the bottom. Is there a better way?

Don Broussard
RV9 Rebuild in Progress
57 Pacer

When joining the electronic ignition bandwagon, what do you all think of running all the bottom plugs off the electronic "non-mag" and all the top plugs off the tractor mag?

My thought is that the bottom plugs have been the ones more prone to fouling (Lyc IO-360) and therefore the fouling might be mitigated by the more vigorous spark produced by the electronic mag. Also, how does "wasted spark" factor into plug condition?

George

IF I were running ONE Mag and ONE electronic ignition system, I would do as you say and have all the EI (automotive spark plugs) on the bottom and the Mag on the top. When I take the plugs out, I would replace the automotive plugs with NEW and rotate the aircraft plugs to next in firing order.
 
Does the polarity work out so my cylinder 1 and 3 bottom plugs and cylinder 2 and 4 top plugs will be opposite, and rotating them will give that benefit even though the automotive plugs and E-mag don’t get it? That would justify the split.

Lycoming firing order is 1-3-2-4 for four cylinder engines. Every other time the mag fires would be same polarity. Plug 1 and 2 have polarity opposite of plug 3 and 4.

With one EI (E-mag), you could have all EI plugs on the bottom and all mag plugs on the top if you want to do that.

When it comes time to rotate the plugs, rotate them to next in firing order. For what the automotive plugs cost, you may just want to install new ones.
 
When joining the electronic ignition bandwagon, what do you all think of running all the bottom plugs off the electronic "non-mag" and all the top plugs off the tractor mag?

My thought is that the bottom plugs have been the ones more prone to fouling (Lyc IO-360) and therefore the fouling might be mitigated by the more vigorous spark produced by the electronic mag. Also, how does "wasted spark" factor into plug condition?

George

Its all about the heat range of the plug, not the spark energy. I put the 37by's on the bottom and auto plugs on top. The 37by has a much larger cavity between the ceramic and shell (designed that way to reduce fouling), making it more resistant to lead ball collection than the auto plugs. They collect plenty of lead balls, but they are far away from shorting the center electrode to the shell (i.e. fouled)

wasted spark has no relevance to this. Lead collection has little to do with the spark plug heat range and more about combustion gas termperature.
 
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Polarity link to specific electrode erosion.

I am trying to get a little more detailed information on the polarity. And linked specifically to what erodes on the plug activated.

1st question - will Left mag and right mag have the same polarity on cylinder #1?

I'm assuming yes, but what polarity is it? Ground eroding or center eroding?


Why do I want to know? Well, not having not been religious about the rotation and with 230hrs now notice unbalanced erosion on the plugs. Maybe just looking at then would be conclusive, but want to do some homework to be sure it won't have to be done twice.
 
Mag & Emag plug wire routing

I admit that I am not a practicing mechanic. But I did read in previous posts that someone would find the "Automotive Spark Plug" wire laying in the bottom Cowl after a long flight. (not the exact quote) But it got me to thinking about how HOT those plugs get and the automotive couplers are rubber over metal clips and hanging upside down. The Aviation plugs have threaded couplers and very unlikely to fall off. So my solution is: EMAG with Automotive plugs ALL on TOP and Slick Mag with Aviation plugs ALL on Bottom. So Far So Good.
 
inline below

I am trying to get a little more detailed information on the polarity. And linked specifically to what erodes on the plug activated.

1st question - will Left mag and right mag have the same polarity on cylinder #1?

I think so. The mags are set up identically. The L and R used during mag assembly is related to mag rotation, not installation side. The mag uses an index to match the distributor gear to the armature gear, which is keyed to the armature shaft that has two magnets, so you will always get the same polarity for each of the 4 outputs. Though I must say that I don't know the science behind why. The gear is installed on the armature shaft with just a straight slot to index it, so it can go on two different ways (index mark 180* apart). There is no instruction to align it one way or the other when installing the gear. Yet, they always seem to create the opposite wear when installed with Lyc's recommendation.


I'm assuming yes, but what polarity is it? Ground eroding or center eroding?

Have no idea. Have never tested it and no one publishes that. Pretty easy to test. Pull the cover off and put a meter with blk to the case and red to the wire connected to the points and spin. When you can get the 3/32 drill bit all the way in the L hole, it is firing #1, so back it up 180 and spin 180. That would be the polarity for the #1 output. Normal polarity should flow from center electrode to ground and negative should flow from ground to center electrode. If I remember correctly, while we manage electricity as flow from + to -, in reality it flows the other way and I believe that the heat and thereforee wear is always greater on the pos side in a elec gap application. In arc welding, the pos side always gets more heat and modern Tig gear has the ability to favor one side of the wave or the other in AC to manipulate this effect. In DC mode, Tig uses Neg on the electode to keep the heat in the workpiece and not the un-consumable electrode.

Why do I want to know? Well, not having not been religious about the rotation and with 230hrs now notice unbalanced erosion on the plugs. Maybe just looking at then would be conclusive, but want to do some homework to be sure it won't have to be done twice.
 
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I love nothing better than ancestor worship! In such a rule-rich environment, it amazes me the things that are left unsaid, other than to keep doing it the way we always have. Even though we're doing it with entirely different components.

I'm wiring up an IO-375 with two E-Mags and we're looking at this exactly. If there is no "E-Mag versus old Mag" question, and everything is identical, what guidance is there?

I guess to keep the redundancy totally redundant, each E-Mag will be connected to all 4 cylinders, but then split between upper and lower on opposing sides of the crank.


Unless I hear discouraging words from the experts here.
 
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I guess to keep the redundancy totally redundant, each E-Mag will be connected to all 4 cylinders, but then split between upper and lower on opposing sides of the crank.


Unless I hear discouraging words from the experts here.

I've run mine both ways. Originally I was top on left pmag, bottom on right, then switched to mixed top/bottom on Brad's advice to better balance the mag drop. It didn't change the mag drop delta (about 90 RPM/120 RPM), by the way.
 
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