Which Mag Fires Which Cylinder?

[WrightBro1]

I saw on 2007 posts, "High EGT, One Cylinder only, Full power Climb", discussion of the subject. I'm going to install a new harness and want to get it right so given 16 years since that post I felt it okay to ask for affirmation. My SL6N-21 Bendix fired C145-2 in my 1948 C170 is wired as follows: Lft Mag fires cylinders 1, 3, 5 Upper plugs & cylinders 2,4,6 Lower plugs; Rt Mag fires cylinders 2, 4, 6 Upper plugs & cylinders 1, 3, 5 Lower plugs. Of course I've seen documents stating Lft mag fires all Lower and Rt mag fires all Upper plugs. I want to be proper and consistent. Is there any reason to not change to the documented arrangement (Lft mag fires all Lower and Rt mag fires all Upper plugs)? Lft mag is timed 28Deg BTC and Rt mag 26Deg BTC; several log book references indicate past timing was to the same values.. The log book shows the harness was changed in 1987; the existing harness appears to be that unit installed in 1987.
THANKS

[Bruce Fenstermacher]

Here is all the answers you seek directly from Continentals Overhaul Manual X130013. I don't know what documentation you are looking at but you need to look no further than this document. It is important to follow this exactly as the split timing is intentional to get better fuel burn and presumably meet horsepower expectations.

C-145 Mag info.png

Your current set up, with one mag firing half the lower and half the upper and the other mag doing the same, is the proper wiring for later larger Continentals which use the same timing for both mags

Your installation is currently wrong and that is why you want to change it to the correct method outlined in X30013.

[n2582d]

On first glance, looking at figure 12-14 of the '62 and Prior Cessna Service Manual, it appears that Bruce is incorrect. But read the fine print in the top left. The O-300 was used on the C-172 through 1967.

Screen Shot 2024-07-15 at 7.13.16 PM.png

The ignition wire routing is also displayed in figure 17 of the O-300 IPC. Finally, Section 9 of the O-300 Overhaul Manual shows three diagrams of where each ignition lead should go depending on what model of magneto you're using.

IMG_1038.jpg

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What's curious is that fig. 10 and fig. 11 are both for the Bendix S6LN-21 while fig. 12 is for the Slick 664 magneto. Look at the ignition harness lead pattern on figure 10. Weird. The terminals aren't numbered in a counter-clockwise manner but jump around; #2 is opposite of #1, etc. The J.I. Case Model 67 magneto is on the TCDS for the O-300. I've never seen one and, with a cursory Google search, don't find a picture of one on the internet. Do you think that fig. 10 should have been labeled as the wiring diagram for a Case magneto?

On studying this a little more I think fig. 10 should be labeled "Bendix SF6LN-12 Magnetos". I had the 4 cylinder version of this on a 1941 Taylorcraft years ago. Bulletproof but heavy.

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[johneeb]

What gives me pause is why the engineers staggered the timing? Are the wires to the upper plugs longer Is the flame front faster from the lower plugs? Inquiring, although feeble, minds want to know.

[WrightBro1]

Many THANKS! I expect climb performance will increase greatly! Well, a little perhaps?

Anyway, one more thing. My engine baffles bear down really hard on several of the ignition leads exiting the tops of spark plugs; the 37 year old ignition harness is chafed. I have RHM40E Champions and want to change to Tempest UREM40E expecting they'll give me a little clearance; they're about 3/8" shorter. McFarlane has a sale through July so I'm getting new plugs and a new harness. I now can't find the reference but somewhere I had seen that P/N KA12130 is the proper harness; I have asked the distributor to verify but after two queries I've heard nothing. Perhaps they don't want to take on any responsibility. Any direction from you all on the harness?

Thanks

[GAHorn]

What gives me pause is why the engineers staggered the timing? Are the wires to the upper plugs longer Is the flame front faster from the lower plugs? Inquiring, although feeble, minds want to know.

It’s been passed around anecdotally the lower plugs are more likely to foul than uppers and that firing them first keeps them cleaner.

[mit]

Isn't this kind of stuff you should know and be able to look up if you are working on any aircraft?

[WrightBro1]

Agree on knowledge reference. I'm working on gaining that. I did find the reference to the P/N and moving forward.
Thank you to the TIC170A community!

[voorheesh]

One more piece of advice after reading your posts. You don’t tell us your experience in airplane maintenance other than as an owner, and whether or not you are getting assistance from qualified technicians. Maybe you have these qualifications, but just seek info on an older model. In any case, A&P mechanics can identify approved parts and wiring data. Installation of ignition leads should probably be at least under supervision of a technician to ensure airworthiness. Most could take a look at baffles and verify they are correct parts and installed properly. If a baffle is chaffing an ignition lead that can cause engine problems and the condition should be investigated and corrected. Hopefully it is only a minor adjustment, but probably not considered “Preventive Maintenance”. I realize many pilots undertake projects like this on their own and do so with competence, so please don’t take this advice the wrong way. It is offered in the interest of safety. Just in case. Good luck and hope you get your airplane back in good condition soon.

[Bruce Fenstermacher]

What gives me pause is why the engineers staggered the timing? Are the wires to the upper plugs longer Is the flame front faster from the lower plugs? Inquiring, although feeble, minds want to know.

You are correct John, though I could not state my source, I've been told and for one reason or another have always remembered engineers discovered the flame front faster from the lower plugs and the staggered timing gives a more complete burn. The timing is also staggered for the C-75 and C-85 at 28 and 30 but the C-90 is 26 and 28. The timing for the A-50 is 25 and 28, A-65 30 and 30, A-75 and A-80 is, 29 and 32. I don't have the timing for the A-40 dual mag handy, half of those engines were single mag installations. As you can see with the exception of the A-65, all small Continentals have a staggered timing.

[cessnut]

Except for the most common, the O-200.

[cessna170bdriver]

As far as which mag fires which cylinders, in order for any dual mag setup to provide the redundancy for which it is intended, each mag must fire ALL cylinders. The only option is how the top and bottom plugs get divvied up between the two mags.

[Bruce Fenstermacher]

As far as which mag fires which cylinders, in order for any dual mag setup to provide the redundancy for which it is intended, each mag must fire ALL cylinders. The only option is how the top and bottom plugs get divvied up between the two mags.

Miles, I don't understand your point. The engine will run being fired on only the top plugs or only the bottom plugs and it will run at 26 or 28 degrees BTDC. So there is mag redundancy.

[cessna170bdriver]

As far as which mag fires which cylinders, in order for any dual mag setup to provide the redundancy for which it is intended, each mag must fire ALL cylinders. The only option is how the top and bottom plugs get divvied up between the two mags.

Miles, I don't understand your point. The engine will run being fired on only the top plugs or only the bottom plugs and it will run at 26 or 28 degrees BTDC. So there is mag redundancy.

I wasn’t talking about the C-145/O-300 in particular, just any dual magneto piston engine in general. Some engine models, for whatever reason, have one magneto fire half the top plugs and half the bottom plugs, and the other mag fires the other half, but each mag fires one plug in all of the cylinders. Others, like ours, have one mag fire all the top plugs, the other fire all the bottom plug. Either way, each individual mag fires all the cylinders. From the way the OP worded his question, I got the impression he thought one mag fires some cylinders, and the other mag fires the rest. If that’s not the case I apologize.

[WrightBro1]

Thought I posted a reply; I may have stepped away before the final execution of necessary steps. Anyway, '70's trained American Honda & BMW of America (motorcycles); working as a mechanic decided to go back to school and received Mechanical Engineering degree. Grew up w/ Dad fixing everything; I do the same but I won't mess w/ automatic transmission or chassis work that requires a lift and a big hammer. 38 yrs away from flying; returning and hoping to have a good 10+ yrs, ASEL. All work is done under the direction of an A&P. I won't go to him w/o being pretty sure I know the answer. Anyone can make a misstep and I want to know what we're doing is accurate. I was surprised at the few things we've found that aren't accurate even though it has been through annual inspection many times; ignition wiring being one example. I'm sure every previous inspection and maintenance work was done w/ the same conviction; again, anyone can make a misstep.