Spark Plug wiring pattern ?

Some are wired with 2 cyls top and 2 cyls bottom from one mag and the opposite on the other mag. I have 2 Pmags, with one on the top plugs and the other on the bottom plugs. Turning off either mag in flight is barely detectable in engine tone or feel. I suspect mixing top and bottom on an EI would make ignition check vague .

I have dual CPI's for my -4 / IO 360 project. What is the most logical plug wiring pattern for this high powered EI ?

I think they wire magnetos like that for polarity reasons, it allows a plug rotation scheme to maximize wear by reversing the polarity seen by each plug.
It doesn't matter on EI systems. I suppose it could but most of us just toss the auto plugs we run.
Tim Andres

I cannot speak from experience with the CPI or even much experience with the P-Mag. In talking with Bill Repucci concerning the P-Mag, his advice was clear: if using only one P-Mag, have it fire the bottom (automotive) plugs. This logic would seem to dictate an architecture where the tops are fired by one ignition and the bottoms fired by the other ignition.

In the old days of mags and massive electrode plugs, the "half top / half bottom and rotate the plugs frequently" concept was used to even out spark plug erosion. With the price of auto plugs, I don't think that's a concern any longer - just replace them.

I have wired mine with one P-MAG wiring two on top and two on bottom on the other side. The reason for this method is that the bottom plugs are more prone to fouling and with this each mag will have same set up and hopefully shows the same RPM drop if there was some spark plug that was fouled.

Bavafa said:

I have wired mine with one P-MAG wiring two on top and two on bottom on the other side. The reason for this method is that the bottom plugs are more prone to fouling and with this each mag will have same set up and hopefully shows the same RPM drop if there was some spark plug that was fouled.

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The flip side of this point is that the automotive plugs, with their much wider spark gap, are much less prone to producing power loss as a result of fouling. That was the explanation provided by Bill R, and one that seems to make logical sense.

I have seen no effect or disadvantage by having one CPI (or EM-5) firing the top plugs & the other module firing the bottom ones. Its much easier & neater routing the wires this way, especially when mounting one coil pack on top of the engine case & the second coil pack either on a mag cover bracket or firewall.

I've done the same with Lightspeed & Electroair units in a he past, EI firing the tops & Mag doing bottoms.

Canadian_JOY said:

The flip side of this point is that the automotive plugs, with their much wider spark gap, are much less prone to producing power loss as a result of fouling. That was the explanation provided by Bill R, and one that seems to make logical sense.

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Less prone but it still could happen, and what is the down side to doing it this way? BTW, I had spoken to Brad at PMAG and he believed this is the best way to wire them.

I have my SDS EFI (CPI IGN) wired with top pack on top plugs, bottom pack on bottom plugs. I notice a 100 rpm "mag drop" one one, which still runs smoothly, and very small, almost unnoticeable drop on the other. An A&P around my field mentioned that mags used to be configured this way, but then were changed to make the mags drop equally. Could be an old wives tale, but sounds good to me!

Caleb