Effect of harness misrouting to top vs. bottom cylinder

[DXB]

I'm aware that the left mag on my lycoming O-360-A1D is supposed to fire the top plug on #1 and #3 and the bottom plug on #2 and #4.  What is the purpose of splitting it up top vs. bottom plug for each mag this way?

I'm asking because during install of a Surefly mag, which requires changing out my Bendix harness for a Slick harness, the A&P noted that my harnesses were misrouted - i.e. the left mag was serving all the bottom plugs and the right mag serves all the top plugs. So now he has to mess with the right mag and reroute the right harness as well .  I bet it's been this way since the engine was overhauled in 2000, and no one ever noticed.  What adverse effect could this have had?

[smwash02]

With the correct harness this isn't possible or would be difficult. They're cut to a certain length and labeled like 1T 2B etc on the plug end. Would take quite the oversight to happen.

I'm not an AP but I've heard of some engines being like that by design (all top and all bottom), but all of my planes have been split as you describe. The bottom plugs can be hit harder since gravity will pull fuel downward so splitting that load is ideal. 

Will be interested in hearing how your Surefly performs, I'd like to get one.

[Andy95W]

Dev- it sounds like you decided to wait for the Surefly. Who did you get to do the work?  Is it still at CRE (IIRC)?

Good luck!

[N201MKTurbo]

I can't think of a good reason why it matters.

Maybe its one of those "That's the way we have always done it" things.

[N201MKTurbo]

Maybe, if you have a nasty old engine that fouls the bottom plugs, and you lose a mag, you will still have two good cylinders to get you home.

[kortopates]

57 minutes ago, DXB said:

I'm aware that the left mag on my lycoming O-360-A1D is supposed to fire the top plug on #1 and #3 and the bottom plug on #2 and #4.  What is the purpose of splitting it up top vs. bottom plug for each mag this way?

I'm asking because during install of a Surefly mag, which requires changing out my Bendix harness for a Slick harness, the A&P noted that my harnesses were misrouted - i.e. the left mag was serving all the bottom plugs and the right mag serves all the top plugs. So now he has to mess with the right mag and reroute the right harness as well .  I bet it's been this way since the engine was overhauled in 2000, and no one ever noticed.  What adverse effect could this have had?

That's not a mis-wiring but a different ignition harness than what the factory originally used or spec'd (i.e. as shown in their Overhaul and maintenance manuals). The Lyc io-360 is an oddball in that Lycoming's original ignition harness was opposite to most of the rest of the industry by having the each mag fire the bottom plugs on that side rather than the top. Very few engines did it that way. So it should be no surprise that some/many of the ignition harnesses approved for the io-360 actually use the conventional routing where each Mag fires the Top plugs on its side and bottom's on the opposite. It really makes no difference and as long as you know what you have then you can properly interpret your engine monitor. 

[DXB]

46 minutes ago, Andy95W said:

Dev- it sounds like you decided to wait for the Surefly. Who did you get to do the work?  Is it still at CRE (IIRC)?

Good luck!

Thanks - yes it's still in CRE getting worked on by a local shop traveling out to the field. I took your advice and did all I could to see if they could replace the left with a Surefly. Because I already had a mag on backorder with the company, they were nice enough to go into their reserve AOG stash of SIM4Ns to help me out. Hopefully I can retrieve the plane this weekend.  (BTW Surefly's customer service folks have been superb!)

Now this issue with the harness is adding hours and leading to messing with the right mag, which comes off next week for IRAN anyway.

[DXB]

18 minutes ago, kortopates said:

That's not a mis-wiring but a different ignition harness than what the factory originally used or spec'd (i.e. as shown in their Overhaul and maintenance manuals). The Lyc io-360 is an oddball in that Lycoming's original ignition harness was opposite to most of the rest of the industry by having the each mag fire the bottom plugs on that side rather than the top. Very few engines did it that way. So it should be no surprise that some/many of the ignition harnesses approved for the io-360 actually use the conventional routing where each Mag fires the Top plugs on its side and bottom's on the opposite. It really makes no difference and as long as you know what you have then you can properly interpret your engine monitor. 

Ugh.  Since the Surefly requires changing my left mag harness from Bendix to Slick, the A&P naturally used the markings for top vs. bottom on the new harness I ordered but found them to conflict with the existing harness on my right Bendix mag.  So now a bunch of time is getting spent messing with the right mag and rerouting its harness. I wish I had known about this issue so I could have prevented the unnecessary rearranging - can't really blame the installer. I guess the only benefit here is finding out I've been misinterpreting my engine monitor all this time.

 

[kortopates]

5 minutes ago, DXB said:

Ugh.  Since the Surefly requires changing my left mag harness from Bendix to Slick, the A&P naturally used the markings for top vs. bottom on the new harness I ordered but found them to conflict with the existing harness on my right Bendix mag.  So now a bunch of time is getting spent messing with the right mag and rerouting its harness. I wish I had known about this issue so I could have prevented this unnecessary rearranging - can't really blame the installer. I guess the only benefit here is finding out I've been misinterpreting my engine monitor all this time.

 

I can certainly see that happening. Did you check with Surefly for their advice since they must be familiar with this issue by now? Just curious if they had a specific recommendation. I'd think a good % of io-360's were set up just like yours - not unusual at all. 

[DXB]

45 minutes ago, kortopates said:

 So it should be no surprise that some/many of the ignition harnesses approved for the io-360 actually use the conventional routing where each Mag fires the Top plugs on its side and bottom's on the opposite. 

One clarification here - what I have is a little different - Left mag was driving ALL the bottom plugs, not just the ones its side, and right mag was driving ALL the top plugs - per my reading very very few engines had this original setup, so it may have been an aberration setup at time of last overhaul? Is this still no big deal? 

[cferr59]

I heard that it is wired this way because the lower plugs tend to get dirtier and this evens out the RPM drop between the mags.  Otherwise, you would get more drop on the magneto that fires the (dirtier) bottom plugs.  Seems to make sense I suppose, but this I don't remember the source of this tidbit.

[kortopates]

1 hour ago, DXB said:

One clarification here - what I have is a little different - Left mag was driving ALL the bottom plugs, not just the ones its side, and right mag was driving ALL the top plugs - per my reading very very few engines had this original setup, so it may have been an aberration setup at time of last overhaul? Is this still no big deal? 

I have never seen that on a O-360. But it is common with electronic ignition but just on experimental engines to my knowledge. I would be curious to where the approval for that comes from since that's news to me. Let me guess that your Mag RPM drop has never been even?   But you can really only see that with a engine monitor or digital tach,  yet I would expect the left mag isolated will have a bigger drop than the Right mag. 

With Electronic ignition it makes sense to do that,  by having the electronic ignition fire the bottoms since the higher voltage of the EI can still fire a dirtier bottom plug better than a conventional mag. But with 2 conventional mags it doesn't seem like a good idea when you lose the right mag and can only fire your bottom plugs that are not likely to be in the best of shape - the odds of finding yourself running on less than 4 cylinders is much higher than with a conventional arrangement. 

Here is a diagram of what the O-360's ignition harness is as they came from the factory, as mentioned the conventional routing  on most engines is opposite of this.

Edited December 12, 2019 by kortopates

[carusoam]

Summary...

1) in an ideal, clean running engine, in new condition... running all top plugs from one mag would be fine and dandy... fuel distribution is pretty even all over once it gets to the cylinder... gravity doesn’t effect fuel vapor...

2) The challenge is... aged, dirty, oil soaked, lead balled, plugs, are usually all bottom plug problems... gravity really matters with solids and liquids...

3) When the mag dies on one side, it is better to have its output split 1/2 top and 1/2 bottom...

4) It would suck to be only able to run bottom plugs only when a mag goes on vacation...

5) really pay attention to the run-ups...  burning crud off a spark plug often?

6) At annual... getting a lot of lead shot?

 

7) Having a single electronic mag... they are balancing out the spark strength differently... because they can.

8) Pay attention to how the plugs are going to wear... a stronger spark, may cause a different wear rate..(?)

9) Sounds like some of the old rules are still going to be needed...

10) Sounds like some new rules are going to be added...

11) either way... expect champion plugs are still going to suck.  And tempest fine wires may lose their center electrode...

12) it’s good to know what plugs are where on the monitor...

PP thoughts only, not a mechanic...

Best regards,

-a-

[DMM]

The Lycoming overhaul manual shows an optional wire routing. Figures 4.7 and  4.8. In both cases each mag fires two top and two bottom plugs on each side. 

[carusoam]

Interesting OH manual, DMM.

The date at the bottom is a tad aged.  There may be a newer one...

Revised Oct 1974...?

Best regards,

-a-

[N201MKTurbo]

3 minutes ago, carusoam said:

Interesting OH manual, DMM.

The date at the bottom is a tad aged.  There may be a newer one...

Revised Oct 1974...?

Best regards,

-a-

I was 17 in 74. It was a great year, I had a lot of fun!

[MikeOH]

2 hours ago, kortopates said:

I have never seen that on a O-360. But it is common with electronic ignition but just on experimental engines to my knowledge. I would be curious to where the approval for that comes from since that's news to me. Let me guess that your Mag RPM drop has never been even?   But you can really only see that with a engine monitor or digital tach,  yet I would expect the left mag isolated will have a bigger drop than the Right mag. 

With Electronic ignition it makes sense to do that,  by having the electronic ignition fire the bottoms since the higher voltage of the EI can still fire a dirtier bottom plug better than a conventional mag. But with 2 conventional mags it doesn't seem like a good idea when you lose the right mag and can only fire your bottom plugs that are not likely to be in the best of shape - the odds of finding yourself running on less than 4 cylinders is much higher than with a conventional arrangement. 

Here is a diagram of what the O-360's ignition harness is as they came from the factory, as mentioned the conventional routing  on most engines is opposite of this.

Hmm,  I'm probably missing something, but that diagram doesn't seem correct to me.  Assume it's clockwise rotation, so firing order is 1-3-2-4. Looking at the left mag, turning counter-clockwise, we see that firing order.  However, if you look at the right mag, rotating clockwise, I get 1-3-4-2??

Show me the error of my ways...

[DXB]

11 hours ago, kortopates said:

I have never seen that on a O-360. But it is common with electronic ignition but just on experimental engines to my knowledge. I would be curious to where the approval for that comes from since that's news to me. Let me guess that your Mag RPM drop has never been even?   But you can really only see that with a engine monitor or digital tach,  yet I would expect the left mag isolated will have a bigger drop than the Right mag. 

With Electronic ignition it makes sense to do that,  by having the electronic ignition fire the bottoms since the higher voltage of the EI can still fire a dirtier bottom plug better than a conventional mag. But with 2 conventional mags it doesn't seem like a good idea when you lose the right mag and can only fire your bottom plugs that are not likely to be in the best of shape - the odds of finding yourself running on less than 4 cylinders is much higher than with a conventional arrangement. 

Here is a diagram of what the O-360's ignition harness is as they came from the factory, as mentioned the conventional routing  on most engines is opposite of this.

Oddly, my mag drop has always been quite symmetric on a digital tach.  Looking at the record of my mag check at runup done minutes before my left mag died in flight, the drop was precisely 140 on both sides.  By the way, I really appreciate your sharing your seasoned perspective on this issue as well as other ones in the past.  

[DMM]

12 hours ago, carusoam said:

Interesting OH manual, DMM.

The date at the bottom is a tad aged.  There may be a newer one...

Revised Oct 1974...?

Best regards,

Hey, it was revised 11 years after my engine came out of the factory.   A 1974 revision is recent for us vintage mooney owners:)

I don't rebuild 'em.  I just fly em.

[DMM]

11 hours ago, MikeOH said:

Hmm,  I'm probably missing something, but that diagram doesn't seem correct to me.  Assume it's clockwise rotation, so firing order is 1-3-2-4. Looking at the left mag, turning counter-clockwise, we see that firing order.  However, if you look at the right mag, rotating clockwise, I get 1-3-4-2??

Show me the error of my ways...

Interesting.  Looks wrong to me too. Compared to the old manual, Cyl 1 and 3 are switched on the right mag. 

[kortopates]

12 hours ago, MikeOH said:

Hmm,  I'm probably missing something, but that diagram doesn't seem correct to me.  Assume it's clockwise rotation, so firing order is 1-3-2-4. Looking at the left mag, turning counter-clockwise, we see that firing order.  However, if you look at the right mag, rotating clockwise, I get 1-3-4-2??

Show me the error of my ways...

Assume the artist took some liberty with where the wires attach to the mags. Its emphasis is only in showing what cylinders plugs the mag fires - but not precise location on mag. 

[MikeOH]

3 hours ago, kortopates said:

Assume the artist took some liberty with where the wires attach to the mags. Its emphasis is only in showing what cylinders plugs the mag fires - but not precise location on mag. 

Well, if that's the case, then the Figure 5 title: Ignition WIRING Diagram is pretty misleading.  I would think a reasonable person would figure (tee hee hee) that he could rely upon that diagram to hook up the mags!

YIKES!!

[kortopates]

Well, if that's the case, then the Figure 5 title: Ignition WIRING Diagram is pretty misleading.  I would think a reasonable person would figure (tee hee hee) that he could rely upon that diagram to hook up the mags!
YIKES!!

You'd think so but It's really not an issue because you can't buy the mag harness without the ignition leads already in the cap. The cap comes with wires already cut to proper length and built into the cap. It's takes some pretty expensive tooling to replace a single ignition wire but I've done it. So all you really need is the diagram and firing order to confirm. The hardest part is actually determing where cyl #1 is on the cap and mag to ensure proper orientation of the cap to mag - which isn't even on the diagram.


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[MikeOH]

1 hour ago, kortopates said:

You'd think so but It's really not an issue because you can't buy the mag harness without the ignition leads already in the cap. The cap comes with wires already cut to proper length and built into the cap. It's takes some pretty expensive tooling to replace a single ignition wire but I've done it. So all you really need is the diagram and firing order to confirm. The hardest part is actually determing where cyl #1 is on the cap and mag to ensure proper orientation of the cap to mag - which isn't even on the diagram.


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Ah, did not know the cap comes with pre-cut leads pre-installed.

 Thanks for the education!

[carusoam]

Nice Catch, Mike!

With all the cad drawing software around, today...

That drawing would have been fixed before going to print...

It would also have been a scaled copy of a master drawing being used elsewhere...

Just One of the oddities of hand drawings, and hand lettering from the old days...

The engineer that knew the firing order, didn’t check with the tech draftsman who was making the drawing... it looked good to both people...     lost in the shuffle...

Best regards,

-a-