Coil and spark plug polarity

Paul LeClair

Paul LeClair

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Location
Calgary, Alberta, Canada
as I have been puzzling out the wiring on my 82 1200 TS Mirage project, and am currently wiring the new ignition, I started wondering about coil and spark plug polarity, and whether polarity matters.

after some internet searching, I came across this article  http://www.chicagolandmgclub.com/techtips/general/574.html  which suggests getting the polarity correct  (negative, apparently) at the spark plug center electrode is worth 15 % spark energy.

I don't know if the 15 % figure is correct, but the rest seems to make sense to me, although electrickery has never been my thing.

In my 82 1200 TS Mirage build I have been trying to stick as close to stock as I can, and I am going to try and re use the low mileage original coils. Those coils don't have any polarity markings I can see. The dual coil has one black wire lead, and one green wire lead. The single coil has one black lead and one orange lead.

To wire the coils from the ignition, the triggered output from the ignition is going to be individual, the one to the dual coil  is going to wire to green on the coil, the one to the single output coil is going to the orange lead. That wiring scheme would require the common power feed to go to the black wire on each coil? Will that give the correct polarity to the coil and plug?

anyone?

Paul LeClair

EDIT as my wiring harness is not yet fully completed and I can't yet power it, is there a test I can do on the coils with an ohmmeter to determine polarity of each of the black wire and the coloured wire on each coil?
 
Hi Paul
Just finished making a complete loom for my Series 2 and corrrect the Orange and green are the trigger wires ( your terminology but I like it ) and the black is the power.... Laverda used to love using black as the power... think about the horn wires which when you loose a horn end up getting melted and shorting on the exhaust and then you fuse blows in the middle off woop woop...
Still better than some of the French stuff I have to work on every wire is green... they use numbering on the wires to identify... guess someone who sold green wiring made a fortune.
Again good luck with it you have inspired me to get a little more motivated.... now just waiting for the last of my bits and bobs from Wolfgang to make it here down under...
even got my paint on so am up to the wet sanding part.... bugger not having a dust free booth but hey its all part of the fun I guess..
Cheers and good luck cant wait for the video of it up and running
Brett
 
Paul,

In a perfect world, you want the center electrode to be negative, the tip to be as sharp as a needle and red hot. This promotes ionization of the air fuel mixture and make it easier for the spark to jump to ground. It does not produce a bigger or more powerful spark which is a function of the ignition coil and the polarity, coil wires and plugs have no effect. This only really matters if you have a weak ignition system. Don't worry about the spark plug polarity on your 1200 TS. If we are talking about a 1914 BSA with an almost dead magneto, then it matters. There have been millions or cars, motorcycles, boats and lawnmowers produced over the years with the center electrode being positive and they run fine.

And finally, unless you intend on converting your 1200 TS charging system / battery to positive ground, it is impossible for the center electrode to be negative. If the the bike chassis (ground) is negative and you somehow made the center electrode negative then there would be no spark at the plug at all as they would be both the same polarity.

Bruce
 
The article you mentioned has a good write-up of checking coil polarity. I've had bikes in the shop with coils hooked up backwards and they overheat--some badly. One H/D got so hot it split wide open and made a mess. Some coils--especially Bosch have a #ing system instead of - and + so I always double check them. I've also noticed undue wear at the plug electrodes.
 
Rrosco, my issue was that I have not powered the wiring harness yet, so I will try the polarity test once I get power flowing. Then Bruce (1200ts) just warped my mind with his logic that the center electrode of the plug can never be negative in a negative ground electrical system.......  :o which seems to make perfect sense.

long story short, I now know that the black wire on each stock original coil is positive, that is the way I am wiring, then I will test resulting polarity at the plug later just for my own curiousity, knowing that I will have the coils wired correctly.

now on to those who like me "index" the spark plugs so the open gap aims directly at the intake valve, hopefully no one comes along and proves to me that is not worth doing.....  ;) ;) indexing the plugs makes my slightly OCD perfectionist personality happy.

Paul LeClair
 
Paul LeClair said:
now on to those who like me "index" the spark plugs so the open gap aims directly at the intake valve, hopefully no one comes along and proves to me that is not worth doing.....  ;) ;) indexing the plugs makes my slightly OCD perfectionist personality happy.

Paul LeClair
Click to expand...

I was running around my project with a set of allen-keys, ensuring all my S/H cap screws where positioned so the flats of the inside hex were horizontal ................. that satisfies my OCD, but indexing spark plugs?  ::)

BTW (for those that scoff), I figure that's why the allen-keys are designed the way they are: Turning them until either 3 or 9 o'clock means the flats are horizontal.

And, adding insult to my way of thinking, why oh why do they make open ended spanners with the jaws angled to the handle? Means you have to line the flats of the hex head to horizontal by eye.  :(

OCD is strange, it's only these two things that erks me, mostly I don't give a toss.
 
chrisk said:
And, adding insult to my way of thinking, why oh why do they make open ended spanners with the jaws angled to the handle? Means you have to line the flats of the hex head to horizontal by eye.  :(
Click to expand...

When the jaws are angled you can flip the spanner over to get an extra bite in tight spots - that's why they are that way.
I suppose we are all different. The angle of nut/bolt heads to me means nothing as long as they are done up correctly.
 
The whole spark polarity thing is moot anyway when it comes to twin lead coils. You guys do know don't you, that the spark on those goes : center electrode to plug body on one plug then plug body to center electrode on the second one...

Just out of curiosity I had a play with a single spark magneto from the Rudge i was rebuilding when I was thinking of making it twin plug. If you isolate the mag body completely, you can run the normal plug lead to one plug and a lead from the second plug to the mag body. Current path is then center electrode to plug body screwed into the head,second plug body to center electrode which has a lead to the mag body...Much to my surprise i got two very good sparks.
Never actually built it in but it wouldn't have been hard to isolate the mag as it was belt drive anyway.

Paul, correct orientation of the plugs helps in very large bores or where spark energy is poor. Don't think you have a problem with either.
 
Warning - long and boring post.
If you're not interested in having a deep and meaningful relationship with your spark plugs, you're better off ignoring it.

Paul LeClair said:
Then Bruce (1200ts) just warped my mind with his logic that the center electrode of the plug can never be negative in a negative ground electrical system.......  :o which seems to make perfect sense.
Click to expand...

There are some myths floating around here.

Yes, with a negative earth system, the engine is grounded, but that is only with respect to the 12V electrical system. The 40,000V (or theabouts) circuit from the coil's secondary (HV) windings is an entirely separate electrical system that has nothing to do with which battery terminal is connected to the frame. So you can put that myth to bed. If you have any doubts, think about a twin coil. As GregT said, the plugs are connected in series with the electrons flowing in one plug wire and out the other. So on one plug the centre electrode is positive, and on the other it is negative. The plug with the negative centre electrode (therefore positive body) doesn't short-circuit the electrics or blow fuses.

The second myth is that you will get a stronger spark if the electrons jump from the centre electrode. The only difference between centre and outer electrodes is that the centre one runs hotter. The fuel mixture is more easily ionsed at a higher temperature, so the resistance to initiating a spark is a bit lower than if it were going the other way. So the only advantage of "correct" polarity is that it makes it a little easier for the first electron to make the jump. Once that first electron has made the jump, the other 3.5 billion* will follow like penguins jumping off an iceberg. Once the spark has been initiated, it will be exactly the same regardless of which way the electrons are travelling.

Electronic ignitions with good coils provide plenty of voltage to overcome the initial resistance at the plug gap, regardless of polarity. So if your bike starts easily, idles nicely and runs well, don't lose any sleep about which way the electrons are jumping.

And reiterating what Greg said, if you're using a twin coil, forget about spark plug polarity. No matter which way you wire it, one spark plug will always be "wrong". If you want to get all plugs with the same polarity, you need 3 separate coils.

However (this is aimed at PLC) - knowing you to be a perfectionist Paul, you probably will lose sleep  ;)

So to put your mind at ease, you can perform a simple test to figure out which way the electrons are jumping. Find an old analogue (moving needle) voltmeter. Connect the negative lead to the plug terminal and the positive lead to the cylinder head. Set the meter on the highest volt range. Crank the engine over and you should see an upward movement of the voltmeter needle. If the needle moves down off the scale, your polarity is reversed. 

If you can't find an alalogue voltmeter, use a soft graphite pencil (the softer the better) sharpened to a fine point. Disconnect a spark plug lead and remove the resistor plug cap so you have the bare HT lead conductor exposed. Hold the end of the lead close enough to the cylinder head for a spark to jump. Put the tip of your graphite pencil in between the lead and the head. Now with your 3rd and 4th hands (you will need helper) switch off the workshop lights and crank the engine. A spark will jump from the lead to the engine via the graphite pencil. In the darkened room, you should be able to see a flare (of ionised graphite) from the pencil tip. If the flare is toward the cylinder it indicates correct polarity. A flare toward the HT lead indicates reverse polarity.


*3.5 billion electrons? Where the fuck did that come from?

For the nerds among us ...  :D

We only need two formulae:
Voltage across a capacitor V= Q/C, Q is charge, C is capacitance
Capacitance C = εA/d, A is area of the electrode, d is spark plug gap, ε is dielectric constant of the stuff in the gap.

A = 1.00E-06 m2 (1mm2 - an estimte of the area of the centre electrode)
ε = 8.85E-12 F/m (Farads per metre) I used the figure for air because I couldn't find a reference for air-fuel mixture.
d = 0.000635 m (25 thou in metres)
So ...
C = εA/d = 1.39E-14 Farads
V = 40000 V (a guestimate of coil HT voltage)
Then the electrical discharge in a spark is ...
Q = V*C = 5.58E-10 Coulomb
The charge for one electron is
e = 1.60E-19 Coulomb
So, the number of electrons is
Q/e = 3.49E+09 or 3,485,826,771.65

There, doesn't that give you a warm inner glow?  :D
 
Jeez Cam thanks for the expose, hopefully that will educate the electricitilliterates* on the board.

I have seen a car where it was possible to measure a negative voltage (on a -ve earth system) at one point, something to do with a high resistance connection).
Cheers,

bazzee

*I made that up myself. Not meant as derogatory.  :)
 
Oh, and while I'm at it, I may as well have a rant about indexing spark plugs.

I can't imagine any benefit from indexing a plug so that gap is aimed t the inlet valve. When the spark plug fires, both valves are closed. So from Mr S Plug's point of view, he doesn't care where the valves are, he's just lighting a mixture in an enclosed space.

I've heard it said that the incoming fuel mixture will help cool the plug if the gap is facing the inlet valve. FFS if the plug is overheating, then wouldn't it be simpler (and safer) to choose a plug with a cooler heat range?

There may be some marginal advantage to indexing a plug in a 2-valve head where the plug is (by necessity) offset in the combustion chamber. It is possible that the ground electrode could cause a "shadow" area in the expanding flame front immediately after ignition. Ideally, you want the flame front to travel over the piston crown to the other side of the combustion chamber at a predictable speed that is compatible with your ignition timing. So if you're OCD enough to index plugs, then you should index them so that the base of the ground electrode isn't pointing toward the centre of the combustion chamber.

However, I don't reckon a microsecond or two of difference in flame travel duration is going to be of any significance in our old clunkers, so I've never bothered to index plugs myself.

The above is my own opinion. If someone has an alternative theory, I'd be happy to be educated to the contrary.
 
You guys do know don't you, that the spark on those goes : center electrode to plug body on one plug then plug body to center electrode on the second one...
Click to expand...

Hi greg - I used to supply 2 double outlet coils with some ignition systems I sold for the triples - I made sure that the "wrong" HT lead was the one  that went clamped to earth! - regards - Phil
 
thanks Cam and all!

Phil, that sounds like a very practical solution, with the two double coils and the unused fourth lead earthed.

the other current (groan....)  solution would appear to be Red's triple coil set up for the triples, with the Ignitech programed to fire each of  the three coils individually.

Paul LeClair
 
Have just helped put Reds Ignitech on a 180 with twin plug head 3 twin lead coils 6 plugs. With the Ignitech set to fire 1 and 3 at the same time.
 
martymoose said:
Omg will, we forgot to index the plugs!  Our souls are doomed to eternal damnation.
Click to expand...

I recall reading something somehere about it being a good idea to stagger the ignition timing on twin plug heads so that one plug fires a couple of degrees before the other. I don't remember the reason for doing it that way though.
If you wanted to, you could experiment with staggered ignition by using each double coil to fire one plug in each outer cylinder rather than one double coil connected to both plugs in each cylinder. Using the double coils to fire one plug in each cylinder also gives you a back up. If one of the coils stops working, your engine will still run on all cylinders.
 
It makes no sense to me to stagger the firing points in a twin plug head. Using twin plugs, the optimum ignition timing is usually later than a single plug. With two ignition points, the combustion is more rapid and hopefully, more complete.
If you stagger the timing, the lead spark would have to be at single plug timing surely,unless the chamber is a very odd shape...
I've done a fair amount of fiddling with chamber shape and piston dome shapes over the years. The one i'm best pleased with is the road motor based Aermacchi singles. Very deep hemi head shape - much deeper than the "real racer". With our piston crown shape, 12.3:1 com and single ignition, best power is at 28 deg advance...The real racers use 36 deg or more.
 
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