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.
 

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:D

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.

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

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.

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!