Magneto Harness Top/bottom spark plugs feed

[Vlad]

I replaced an impulse coupling magneto with new the other day and have a question. What?s the reason behind feeding it to 2 bottom spark plugs and 2 top ones? Why not to connect just top spark plugs to impulse coupling magneto? Would it be more beneficial to start on cleaner top plugs instead? I am sure there is a manufacturer?s reason for the sequence but I can?t find it. Does anybody know?

[PilotjohnS]

I think this is tradition more than anything else. But with each mag firing 2 top and 2 bottom, I think the mag check would be more even if the bottom plugs are fouled. I would hate to have the bottom plugs fouled and then be 1 mag failure away from losing the ?good? mag.

[mr.sun]

Hmm, my impulse Mag sparks the 4 bottom plugs while the Lightspeed plasma II sparks the top plugs. Has worked great for over 1000 hours. Had to hand prop it once on a Sunday far from home. Worked fine.

[tim2542]

Its a plug wear scheme where the polarity gets reversed as you rotate the plugs, if you follow the Lycoming rotation pattern.
The polarity reversal causes the wear to occur on the opposite electrode so you get longer life out of the plug.
Ref: John Swarner?s magneto book
Tim Andres

[Canadian_JOY]

To build on Tim's response...

For those running one Electronic Ignition and one Mag, we often put the EI on the bottom plugs since we are running automotive plugs which are gapped much wider than aviation plugs. Since the bottom plugs are the ones most likely to foul in most situations, running the wider-gap automotive plugs on the bottom generally means fewer problems with fouled plugs.