Spark plug rotation conundrum

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Rich
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Spark plug rotation conundrum

Post by Rich »

... Nothing to see here for our Diesel Drivers 8-)

One of the traditional maintenance items for our mag-driven engines is to rotate plugs during things like annuals. It's to equalize plug wear and deals with:

1. Reversed polarity between half the plugs and the other half.
2. Cycle plugs between the top and bottom of the cylinders.

The IO-360-M1A is configured pretty standard. It has the firing order 1-3-2-4. Plugs on the top left side (2,4) and bottom right side (1,3) are fired by the left mag. Plugs on top right side (1,3) and bottom left (2,4) are fired by the right mag. Therefore a rotation like this fulfills both:

1T <-> 4B
3T <-> 2B
2T <-> 3B
4T <-> 1B

Enter Electronic Ignition. Now what? :scratch:

I did some analysis on how this might be affected by replacing the left mag with an EI system. It's complicated, and various factors pop up, depending on:

1. Is the EI a wasted-spark system? (Electroair is, Surefly does not seem to be.)
2. Does the EI induce a reverse-polarity effect? (likely true with wasted-spark)
3. Do you choose to have the EI power the lower plugs, the mag the uppers or keep the existing interlaced setup
4. Will you need to use different plugs (or gaps) for the EI and the mag?

Without spitting out the gory details, It appears if you choose to keep the existing distribution of both feeding upper and lower, the above rotation pattern will continue to be fine. If you use the EI for lower and mag for the upper plugs, then the rotation pattern needs to be changed and what it needs to be depends on whether the EI has a reverse-polarity effect.
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Re: Spark plug rotation conundrum

Post by Davestation »

At the risk of proving my ignorance, I have a few questions about your questions.

1. Why does it matter whether the EI is a wasted spark?
2. Why would make a wasted spark version mean that it reverses polarity?

I have never contemplated whether an EI had reversing polarity because that is just the nature of a magneto and the designers would have to very deliberately make the EI mimic it, but why would they? So the main reason to rotate plugs top to bottom becomes moot with the EI and thus you can just as well wire the EI to the bottoms and the mag to the tops.

Nevertheless, both the Electroair and the Surefly will use different plugs than the mag (Surefly says it is “designed” to use oem plugs but goes on to say they “recommend” plugs gapped to .036) - you won’t be rotating between them no matter what. So even if you keep the distribution the same, you’ll have to change how you rotate plugs.

Suffice it to say I’ve always put the EI on the left, wired to the bottoms, and rotated in firing order but not top to bottom, without giving it much additional thought.
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Re: Spark plug rotation conundrum

Post by Rich »

Daveatation wrote: Fri May 03, 2019 2:18 am At the risk of proving my ignorance, I have a few questions about your questions.

1. Why does it matter whether the EI is a wasted spark?
2. Why would make a wasted spark version mean that it reverses polarity?

I have never contemplated whether an EI had reversing polarity because that is just the nature of a magneto and the designers would have to very deliberately make the EI mimic it, but why would they? So the main reason to rotate plugs top to bottom becomes moot with the EI and thus you can just as well wire the EI to the bottoms and the mag to the tops.

Nevertheless, both the Electroair and the Surefly will use different plugs than the mag (Surefly says it is “designed” to use oem plugs but goes on to say they “recommend” plugs gapped to .036) - you won’t be rotating between them no matter what. So even if you keep the distribution the same, you’ll have to change how you rotate plugs.

Suffice it to say I’ve always put the EI on the left, wired to the bottoms, and rotated in firing order but not top to bottom, without giving it much additional thought.
I left out details because the analysis was getting super-wordy. Here are some:

Wasted spark is done by using two plugs on a single coil. Normally this is done by connecting each of the plugs to the opposite ends of the
secondary winding. This means the current flows into the coil from one plug and out the other end, which requires that it flow from the center electrode to ground on one plug and the reverse on the other. (The effect is very common in the motorcycle world and shows up if you're casual about plug-change intervals and care to examine said plugs.) It is possible to eliminate the imbalance in a "smart" ignition by alternating the direction of current flow in the primary coil between firings, but I don't know if any systems do this.

I guess I wasn't clear but you are correct about Surefly, specifically. (Whether it's true for all possible EI's remains to be seen.) The different gaps alone dictate that they are dedicated to the specific unit. If you put the left on the bottom and the existing on the top, then your plug rotation strategy changes, of course not switching top to bottom. It would look like this:

1T <-> 4T
2T <-> 3T
1B <-> 2B
3B <-> 4B

If you have a reverse-polarity effect in the lower mag specifically due to wasted spark (or don't know if you do), this would cover it. If not, it does no harm.

I did some reanalysis of the "keeping the plug distribution the same" and realized that if your EI does have a reverse-polarity effect (because of is wasted spark or something else), then things get way more complicated than I stated originally. This alone may be a good reason to use the "all bottoms are EI" strategy for all cases and just move on.

NOTE: This is, as much as anything, a thought experiment based on other known ignition systems. There is admittedly some conjecture in this. It would be worth someone checking with ElectroAir (the most applicable to us) to check into their take on their specific system. Keep in mind that what applies to EA may not apply to other wasted-spark systems.
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Re: Spark plug rotation conundrum

Post by RMarkSampson »

Hummmmm, I wonder if whomever came up with a plug rotation mandate was simply trying to see if they could make everyone go crazy following some meaningless ritual of symmetry. Caveat - I'm just blogging, i.e. I don't have years and years of experience or even an A&P certification but here is my plug rotation logic.

1. Keep the plug organized so you can see what type of wear is happening in each cylinder. Between inspecting the plugs and doing a borescope inspection you want to be able to spot issues during maintenance so you need to know what plug came from where.

2. Plugs that have worn tips, oblong shapes, etc should be rotated to the trash. That does not make every plug bad - just the ones that cannot be cleaned and gapped nicely. Reusing plugs is fine but why do they need to go back into the engine based on where they came out of the engine?

3. Bottom plugs are always nastier than top plugs - that means I rotate them to the trash more frequently than top plugs.

4. Quite frankly, many times I rotate them all to the trash and start with a new fresh set of plugs. That allows me to monitor the quality of spark more directly by not having some plugs that lived in different cylinders for various lengths of time during their lifespan. It is not like they are your family pet that you take to the vet no matter the cost... It also make the log entry easier.

5. I always have some "reasonable plugs" in my toolbox as ready spares. I've never used them but I do have them if one day I experience a fouled plug (cold CHT/EGT during a mag check) that can't be fixed by some aggressive leaning.
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Re: Spark plug rotation conundrum

Post by Don »

I have never heard of plug rotation before. I have noticed that during my annuals, when they pull the plugs, they are very careful to reinstall the plugs back in the same cylinder postion as before.
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Re: Spark plug rotation conundrum

Post by Davestation »

Rich wrote: Fri May 03, 2019 3:30 pm
I did some reanalysis of the "keeping the plug distribution the same" and realized that if your EI does have a reverse-polarity effect (because of is wasted spark or something else), then things get way more complicated than I stated originally. This alone may be a good reason to use the "all bottoms are EI" strategy for all cases and just move on.
I guess I'm still not getting it - I'm supposing you're saying that it matters if the EI is a wasted spark system because that implies that it is also a reverse polarity system? I'm not sure why that makes rotation significantly more complicated though because wouldn't the polarity still reverse between say even and odd cylinders? So if your EI plugs are all on the bottom, you just keep them on the bottom but continue to rotate between cylinders.


RMarkSampson wrote: Fri May 10, 2019 1:57 am Hummmmm, I wonder if whomever came up with a plug rotation mandate was simply trying to see if they could make everyone go crazy following some meaningless ritual of symmetry. Caveat - I'm just blogging, i.e. I don't have years and years of experience or even an A&P certification but here is my plug rotation logic.

1. Keep the plug organized so you can see what type of wear is happening in each cylinder. Between inspecting the plugs and doing a borescope inspection you want to be able to spot issues during maintenance so you need to know what plug came from where.

2. Plugs that have worn tips, oblong shapes, etc should be rotated to the trash. That does not make every plug bad - just the ones that cannot be cleaned and gapped nicely. Reusing plugs is fine but why do they need to go back into the engine based on where they came out of the engine?

3. Bottom plugs are always nastier than top plugs - that means I rotate them to the trash more frequently than top plugs.

4. Quite frankly, many times I rotate them all to the trash and start with a new fresh set of plugs. That allows me to monitor the quality of spark more directly by not having some plugs that lived in different cylinders for various lengths of time during their lifespan. It is not like they are your family pet that you take to the vet no matter the cost... It also make the log entry easier.

5. I always have some "reasonable plugs" in my toolbox as ready spares. I've never used them but I do have them if one day I experience a fouled plug (cold CHT/EGT during a mag check) that can't be fixed by some aggressive leaning.
Here's hoping that they came up with that pattern to even out wear considering all factors (ie. you could just rotate between top and bottom or just between cylinders but neither plan covers all the bases.

1. Plug wear over extended periods doesn't tell you much more than just that, plug wear. You can see what has happened since the last annual and still rotate the plugs after that.

2. Plugs must be trashed when they wear into the shape of a football, I don't think any sooner is of much benefit. If they are clean and properly gapped, they should still fire like new.

3. This is why rotating them is needed. If your bottom plugs are more worn than the tops, and they will continue to wear faster than the tops, then you will encounter ignition problems sooner, perhaps prior to your next annual. (Unless of course you're just throwing them away each year regardless).

4. If I give you my address, can you send me all your "bad plugs"? :D But seriously, rotating them as the OP suggests will make them stay perfectly healthy longer - it's not like taking your dog to the vet so much as just taking your vitamins.
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Re: Spark plug rotation conundrum

Post by Davestation »

Don wrote: Sun May 26, 2019 6:03 pm I have never heard of plug rotation before. I have noticed that during my annuals, when they pull the plugs, they are very careful to reinstall the plugs back in the same cylinder postion as before.
:shock:
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