Why are mags normally wired to 2 lower plugs and 2 upper plugs?

Why are mags normally wired to 2 lower plugs and 2 upper plugs? Does the reason apply also to electronic ignition?

The wire runs are a little easier if one mag runs all the lower plugs and the other all the upper. If one mag fails you would still have a set of working plugs. It might be the upper or lower, but it should all continue just fine.

Thoughts....or is it obvious except to me?

With my Lightspeed, the LS fires all of the top auto type plugs, and the impulse mag fires all of the lower aircraft plugs. And yes, the leads fit nice that way.

To minimize RPM drop between L & R Magnetos.

Steve Sampson said:

Why are mags normally wired to 2 lower plugs and 2 upper plugs? Does the reason apply also to electronic ignition?

The wire runs are a little easier if one mag runs all the lower plugs and the other all the upper. If one mag fails you would still have a set of working plugs. It might be the upper or lower, but it should all continue just fine.

Thoughts....or is it obvious except to me?

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Okay, this is how aircraft with magnetos wire their plugs. The reason is that they are trying to minimize the RPM drop between magnetos. Since the lower plugs foul first, if you had all top on one magneto and all bottom on another, you would amplify the RPM difference between them. This could lead you to rebuild a magneto prematurely.

On my dual pmag plane I wired one all the top plugs on one side, and all the bottom plugs to the other pmag (obviously). Since I don't expect the ignitions to wear, I want to use the RPM drop between top and bottom plugs as an indication of the bottom plug fouling.

Nucleus

nucleus said:

Okay, this is how aircraft with magnetos wire their plugs. The reason is that they are trying to minimize the RPM drop between magnetos. Since the lower plugs foul first, if you had all top on one magneto and all bottom on another, you would amplify the RPM difference between them. This could lead you to rebuild a magneto prematurely.

On my dual pmag plane I wired one all the top plugs on one side, and all the bottom plugs to the other pmag (obviously). Since I don't expect the ignitions to wear, I want to use the RPM drop between top and bottom plugs as an indication of the bottom plug fouling.

Nucleus

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So, can I jump to to the conclusion that with dual electronic ignition I can wire one to the top and the other to the bottom in a system with direct crank sensor, because one sensor drives boths ignition?

When you heat the center electrode on a spark plug, you get thermionic emission which is worth up to 1000V. A negative spark adds to that potential, and a positive spark has that emission subtracted from it. It's a good practice to have a negative spark on the bottom plugs since they tend to accumulate more grunge. I'm not sure but I think that magnetos alternate the spark polarity; distributor-less electronic ignitions do have a positive spark from one terminal and a negative spark from the other. On the LSE I and III, the spark itself goes through several positive-negative alternations, so it doesn't make a lot of difference. Also, the EIs have so much more output voltage that polarity doesn't make much difference. With the lower output from a mag at low rpm, spark polarity could have an effect on smooth running.

nucleus said:

Okay, this is how aircraft with magnetos wire their plugs. The reason is that they are trying to minimize the RPM drop between magnetos. Since the lower plugs foul first, if you had all top on one magneto and all bottom on another, you would amplify the RPM difference between them. This could lead you to rebuild a magneto prematurely.

On my dual pmag plane I wired one all the top plugs on one side, and all the bottom plugs to the other pmag (obviously). Since I don't expect the ignitions to wear, I want to use the RPM drop between top and bottom plugs as an indication of the bottom plug fouling.

Nucleus

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I always thought that a magneto charged two plugs at each polarity, so the standard set up puts the negatively charged plugs of one mag and the positively charged plugs of the other mag on the same side. It is good practice to swap plugs from +ve to -ve charge from time to time for maximum life, with the standard set up that can be achieved by swapping top and bottom plugs each side.
If you install an EI I don't think it matters which plugs are fired by which ignition, but it would probably pay to swap the mag fired plugs from side to side every 50 hours?

Pete

Piloto.Mendes said:

So, can I jump to to the conclusion that with dual electronic ignition I can wire one to the top and the other to the bottom in a system with direct crank sensor, because one sensor drives boths ignition?

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This is not actually true with the Lightspeed direct crank pickup. While it is true that there is one assembly mounted behind the starter ring/flywheel or whatever it is called, there are independent sensors used to drive each of the two ignition systems.

As a data point, I have dual Lightspeed systems, and I have one wired to the top plugs and the other to the bottom plugs. In the first 160 or so hours, I have not had even the slightest roughness on ignition checks. In fact, the only way to tell that one ignition is off is to listen very, very carefully when each is shut down, since no rpm drop is occurring.

It was very satisfying to simply chuck out the plugs at the last oil change. I don't miss cleaning those dang aircraft plugs every year! All eight auto plugs cost about what one aircraft plug costs, also.

Keeps you from loosing all your power in the event of one of the mags failing. If the mag that failed was the mag wired to the top plugs and it occurred that the lower plugs were fowled or became fowled you could loose all your power.

elippse said:

When you heat the center electrode on a spark plug, you get thermionic emission which is worth up to 1000V. A negative spark adds to that potential, and a positive spark has that emission subtracted from it.

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Paul: please explain more about thermionic emission as regards spark plugs. (It is true that a magneto alternately fires center electrode positive, center electrode negative because the magnet reverses polarity with each discharge.) Thanks.

See, if you're one of the late arrivals on the world scene, you've probably never seen a vacuum tube! It had a red glow in the center which was a cathode, or negative electron emitting device, heated by a wire filament. Some of them emitted directly from the filament. If you have one of the older TV sets with a CRT (Cathode Ray Tube), It has one of these back in the neck of the tube. It has focusing electrodes and magnetics that direct the cathode's emission, which, with the high voltage on the tube's surface, causes high velocity electrons to smack into the phosphor coating on the screen. If the electrons go too fast, proportional to the high voltage, when they strike metal they give off X-rays. That's why they always told you not to sit too close to color TVs! Sorry! I got off track! Basically, when you heat up metals, they emit electrons. Hot plugs for cold running engines have longer center electrode insulator length to increase the heat path to the colder shell and so the center electrode runs hotter. Cold plugs for high performance engines have shorter insulators to carry off the hotter combustion heat. You select the heat range to get that electrode to the best temperature range. Too cold, deposits build up; too hot and you might get pre-ignition. For a good discussion of the differences between pre-ignition (Very Bad) and detonation (Not Necessarily Bad), go the Contact! magazine's web site, www.contactmagazine.com, look up issue #54 in the back issues, and use the link to download the article. Then as a gesture of appreciation to Pat Panzera for making excellent info like this available, subscribe to his outstanding publication!

I'm still confused

Paul: Let me zero in here a little closer. I don't understand why a center negative firing spark plug has an advantage over a center positive firing plug. It seems that in either case blue hot electrons are streaming off of one electrode for another. It is simply a matter of whether the cathode is the center electrode or the ground electrodes. I don't understand the difference. Could it be that a center negative electrode is hotter and emits more electrons than a center positive electrode? If that is the case, why can't a ground electrode heat up more when it is the cathode? It seems unlikely to me that the instantaneous temperature of the metal could change that quickly, but I don't know. What am I missing?

BTW 1) thanks for the Contact magazine reference. I am going to subscribe, and 2) when are you going to design a propeller for RVs so we all can pick up another 20 kts?

The ground electrode doesn't heat up enough to produce much emission, although it is certainly more at 300F-400F than in a car at 220F. The emission from the hot center electrode acts in series with the applied spark voltage. So if the mag at low rpm is putting out -10,000V and the emission is -1000V, then there is 11,000 V available to jump the gap. But if the applied voltage is +10,000V then the resulting pressure across the gap drops to 9000V! At normal pressure, the required voltage to jump a gap with rounded electrodes is about 50,000V/inch. This increases somewhat linearly with increasing pressure, and decreases with sharp edges, which concentrate the voltage pressure over a smaller point. At the start, the cylinder pressure is at a maximum since the low flow rate into the cylinder while cranking does not produce much of a pressure drop across the throttle valve so the cylinder fills almost completely. That's why you'll sometimes hear pinging, detonation, while starting a car. So if your cranking pressure is about 150 psi, you'll need about 12,500V to jump the gap with used, normal, somewhat-rounded electrodes, even more if there are contaminents on either or both electrodes.
I designed blades for a three-blade CS or GA prop but there have been some manufacturing difficulties. Also, Jim Smith of Wichita is flying his recently-acquired three-blade, carbon-over-wood FP on his RV-6. Initial performance data from a quick test based on IAS is somewhat questionable, so he will be setting up to do a full-blown GPS test as soon as the weather quiets down. The TAS based on IAS was somewhat on the high side, but the rpm was somewhat on the low side. Three RVs I've had people do tests on have shown TAS from IAS to be 8mph to 11mph high as compared to GPS-derived speed. I hope I didn't goof and get the pitch a little too high. If that is the case, it should perform better with 160HP or 180HP rather than Jim's 150HP. He's going to a fly-in in NM where you can see the prop.