B

Bevan

Well Known Member
Is there any convention as to which mags fire which plugs? ie Does the left mag typically fire the upper or lower plugs? Both are P-mags in my case, O-360.

Bevan
 
What I have usually seen is one mag fires top plugs on one side, bottom on the other side.
 
Google is your friend. I found this with a quick search.

wires.jpg
 
So what is the purpose of running the tops on one side and the bottoms on the other, with each mag? Why not left mag for bottom plugs, right mag for tops, or vice versa?

Is there a safety or mechanical advantage to splitting them?
 
Just a guess here that the conventional arrangement eliminates differences in RPM drop when doing mag checks. I.e. each mag is firing some tops and some bottoms. Just a guess.

Bevan
 
The idea is the shortest plug leads go to the bottom plugs.
Right mag to right bottom, left mag to left bottom.
Long leads mean weaker spark. Bottom plugs foul the easiest, so should have the hottest spark available, so, the shortest plug wire.
 
Good to know. Each P-mag came a package of 4 plug wires. One long, two medium and one short. I assume that means each one for the appropriate distance to comfortably get to the respective plug.

Bevan
 
SHIPCHIEF said:
The idea is the shortest plug leads go to the bottom plugs.
Right mag to right bottom, left mag to left bottom.
Long leads mean weaker spark. Bottom plugs foul the easiest, so should have the hottest spark available, so, the shortest plug wire.
Click to expand...

Is the difference in spark "strength" really degraded significantly over less than 2 additional feet of plug wire? It seems the Lycoming graphic in one of the above posts actually has the longer leads from both mags going to the bottom plugs. On the "optional" arrangement the shortest leads go to the bottom.

I'm just trying to figure out if there's any sort of safety or operational reason not to fire all the bottoms with one mag and all the tops with the other. I know that a lot of folks running mixed ignitions (one mag, one electronic ignition) are doing just that. Is this a bad idea, and why?
 
I have one mag and a Lightspeed plasma ignition system. The mag fires all six lower aviation type plugs and the Lightspeed fires all the top automotive type fine wire plugs.

Best,
 
Polarity and capacitance

It has to do with polarity on the plugs, the plug rotation procedure maximizes electrode wear for greater longevity. Reversing polarity moves the wear from one electrode to the other.
So when top plugs move to bottom the polarity is reversed, and the lead length is changed as well. The plug lead length does in fact make a large difference in the current and therefore the wear at the plug electrodes due to the capacitance of the leads. The longer the lead, the more capacitance, the collapsing field in the leads makes a big difference in the current the plug actually sees.
Tim
 
Really not trying to be difficult here. I just don't understand how rotating a plug from top to bottom in the same cylinder reverses the polarity. Wouldn't both magnetos be timed the same internally, such that the lead to the #1 cylinder would always be the same intenal polarity from the coil?

The only way I could see switching polarity would be to swap plugs in #1 and #2 with the plugs in #3 and #4 (in a four-cylinder, 1-3-2-4 timed engine such as a Lycoming).

I get that there are different levels of fouling potential between the top and bottom plugs in the same cylinder, but would this dictate they should be swapped with one another from time to time?
 
Opposite polarity

Timing has nothing to do with it, the magneto spark is positive on one lead, negative on the next all the way around the mag. One polarity causes most the wear, I don't recall which, but moving the plug to a position of opposite polarity will equalize the wear among the plugs, by virtue of the polarity and the lead length. There is a correct pattern to to rotate them, I'm not at all sure it is simply top to bottom.
Tim
 
The threaded portion of the plug is screwed into the head, which is ALWAYS negative. The engine case is grounded to the negative side of the battery.

The lead is positive and so it arcs/sparks. If the polarity to the plug is reversed to also being negative, then how could it fire. You can't make the case positive so this theory is going nowhere.

Best,
 
Ground and Mags

pierre smith said:
The threaded portion of the plug is screwed into the head, which is ALWAYS negative. The engine case is grounded to the negative side of the battery.

The lead is positive and so it arcs/sparks. If the polarity to the plug is reversed to also being negative, then how could it fire. You can't make the case positive so this theory is going nowhere.

Best,
Click to expand...

Nope, incorrect when talking about mags. What you said is true when your measuring electrical potential in relation the battery, but not the mags. The mags output is alternating (think AC) with reference to ground (due to which pole of the magnet is inducing current into the coil), thus one plug might be fired with +20,000 Volts (with reference to the engine case) followed by the next one at -20,000 Volts...hence the polarity change referred to by previous posters. Note that this is similar to measuring peak voltage of the "hot" side of an AC outlet with reference to the neutral or ground...the polarity alternates.

Here's an article on the subject:

http://www.aviationpros.com/article/10387932/why-rotate-spark-plugs




Skylor
RV-8
 
Last edited:
tim2542 said:
Timing has nothing to do with it, the magneto spark is positive on one lead, negative on the next all the way around the mag. One polarity causes most the wear, I don't recall which, but moving the plug to a position of opposite polarity will equalize the wear among the plugs, by virtue of the polarity and the lead length. There is a correct pattern to to rotate them, I'm not at all sure it is simply top to bottom.
Tim
Click to expand...

Yes, however a previous post indicated that simply swapping a plug from top to bottom in the same cylinder would "reverse" the polarity. This doesn't make sense to me. If the left mag is firing the top plug in #1 cylinder, and it is making +20kV (hypothetical), how would the right mag firing the bottom plug in #1 cylinder make -20kV? Mags (at least Slick 4300's) are internaly timed so that the rotor is always at the same spot (not 180 degrees off) when firing the #1 cylinder.

So the top and bottom plugs in #1 would always BOTH be either +20kV or -20kV. Not one at +20kV and one at -20kV. Swapping top to bottom in the same cylinder will not reverse polarity. Unless there's something I'm missing?

There must be some other rotation pattern if we're rotating for polarity. It makes sense to do so. Is there a document somewhere (maybe AC43.13, I'll look today) that outlines recommended plug rotation procedures?

(edit - the article by Schwaner linked above does exactly this. Cool, thanks Skylor!)
 
Last edited:
Since you're running p-mags I don't think it really matters if you fire all top, all bottom or top/bottom for each unit.

I run electronic on the right and mag on the left. I kept with the conventional top/bottom arrangement as I was to lazy to run the plug wires so the mag fired the top and the electronic fired the bottom. I left the mag firing the same plugs top/bot plugs as always and the electronic fires the opposing plugs in the cylinders. I also use standard aviation spark plugs for both mag and electronic, but that's another long story.

On a 6 I built using Klaus's system one unit fires the top, the other the bottom. I could see no reason for splitting the system as is done with mags.
 
You must log in or register to reply here.