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newmexTR3
09-29-2009, 11:13 AM
After reading an article on converting from positive to negative ground from a Sprite site ( Positive to Negative Ground Conversion.pdf (https://www.spritespot.com/Downloads/Positive%20to%20Negative%20Ground%20Conversion.pdf ) ), I'm confused. The article states that you are supposed to reverse the wires on the coil: <div class="ubbcode-block"><div class="ubbcode-header">Quote:]4. Reverse the wires on the ignition coil (this is an important step, as itll preserve
the energy in the coil). One side is usually marked SW (for Switch) and the
other marked CB (for Contact Breaker). Other common markings are -
(instead of SW) and + (instead of CB). [/QUOTE]

My car was converted to negative ground years ago and I have the hot lead going to the + post on my coil. The coil is a Lucas Sport. Should I switch it, or is this article in reference to a coil that was made for positive ground cars?

I did a forum search and found Randall's reference to the lead pencil test to determine the coil's polarity, which I will try too: https://www.chicagolandmgclub.com/techtips/general/574.html

Andrew Mace
09-29-2009, 11:47 AM
If I'm reading correctly, your coil wires are correct as they are (hot lead to + post on coil)!

TR3driver
09-29-2009, 11:53 AM
:iagree:
Whoever made the conversion already made the switch as well.

My experience has been that coil polarity makes almost no practical difference anyway. No doubt it gives you more margin between firing and not firing, but most engines don't run on the edge anyway. Kind of like the difference between lighting a bonfire with a kitchen match instead of a book match.

dklawson
09-29-2009, 11:53 AM
Ditto that. It sounds like your coil connections are correct.

Brosky
09-29-2009, 11:54 AM
Think of it this way. The hot lead comes from the ignition switch to power the coil, so it is called "hot".

So ignition switch is "hot" on (+) and distributor is "not" on (-). Not exactly fact, but an easy way to remember.

This of course applies to a negative ground system. The opposite would be true for a positive ground.

martx-5
09-29-2009, 11:55 AM
Yeah, Gavin has the coil wired correctly for a neg. ground car. Coils will work either way, but when "backwards" they lose some of the high voltage.

Edit: The way I always think of it is that the points are the grounding circuit for the coil, so whatever the polarity of the ground, that's where the points hook up to.

TR3driver
09-29-2009, 12:03 PM
What I find easier to remember is that the points ground the coil; so they get wired to the coil terminal that should be grounded. On a negative ground car, that is "-". On a positive ground car, "+".

The theory as I've heard it is that the plugs work better with a particular spark polarity: it's easier to pry electrons out of the hot carbon center terminal than the relatively cool iron side terminal. This is similar to the way a vacuum tube diode works, over a certain voltage range it will only pass current one way because electrons will easily leave the heated cathode, but not the relatively cool anode.

TR4
09-29-2009, 12:13 PM
The previous owner had my coil hooked up backwards and it ran fine on the points but when I switched to Pertronix, the car would not run until I switched the coil wires back to being correct.

newmexTR3
09-29-2009, 01:11 PM
Thanks for the sanity check. I thought it was fine, but in reading those articles I started to second guess the setup. Much appreciated.

Geo Hahn
09-30-2009, 06:24 PM
...The theory as I've heard it is that the plugs work better with a particular spark polarity: it's easier to pry electrons out of the hot carbon center terminal than the relatively cool iron side terminal...

So which is which -- i.e. which ground (based on that assumption) gives the better spark?

angelfj1
10-01-2009, 08:36 AM
I found this in some obscure application notes written by someone at LODGE Sparking Plug Co. It's a little esoteric but sounds plausible. Personally, I know that there have been times when by coil was connected backwards, but I have never noticed a difference in performance or spark plug life.

"<span style="font-weight: bold"><span style="font-style: italic">There is a good reason spark coils are wound the way they are. Positive and negative ground coils have their windings thrown (wound) in different directions in relationship to each other. Spark coils are always wound in such a manner so that the high tension connection on the coil goes negative when the distributor points open. This means that the rising potential at the spark plug is also negative. Why would one want an operating condition where the rising pulse from the spark coil is negative?

The center electrode in a spark plug runs hotter than its grounded electrode. Since electrons can be striped off of a hot electrode more readily than off of a cold electrode, the center electrode is always made the negative electrode. The result of this choice is that the arc across the spark plug electrodes can be initiated at a slightly lower potential when the center electrode is the negative electrode. (In this manner, thermionic emission aids in the establishment of the initial discharge.) As Pete observed, his vehicle will run with either a negative output or a positive output from the coil. I have operated my TD with the incorrect high tension polarity from the spark coil for over 20 years. So it requires a slightly higher potential to strike the arc - big deal. This polarity reversal occurred when I converted to negative ground so I could more conveniently install contemporary electronic devices. I never bother to fit the 'correct' negative ground spark coil.

After the arc (plasma) has been initiated, the circuity oscillates and the thermionic emission consideration becomes meaningless.

Conclusion -- The coil will work and your MG will run no matter what you do. If you want to be sure that you have the slightly more desirable initial negative output then pay close attention to the polarity of the primary connection. The primary terminal marked (-) must connect to the (-) terminal on the battery.</span> </span>

TR3driver
10-01-2009, 11:54 AM
So which is which -- i.e. which ground (based on that assumption) gives the better spark? It's not really a question of "ground" polarity (ie which way the battery is connected). The coil can generate either polarity, depending on how it is connected.

But as the article Frank quoted said, it's better if the plug center conductor goes negative relative to the side electrode.

Twosheds
10-02-2009, 09:00 AM
But as the article Frank quoted said, it's better if the plug center conductor goes negative relative to the side electrode.

I revealed my true identity as a Pompous Know-it-all on the TR List, I think, a while back when I pooh-poohed this fact.

I was thinking about aircraft engines, in which the center electrode/ground electrode polarity alternates from plug to plug down the firing order, and never heard that either had an advantage.

I stand before you humbled and corrected.

TR3driver
10-02-2009, 10:12 AM
I was thinking about aircraft engines, in which the center electrode/ground electrode polarity alternates from plug to plug down the firing order, And that is true of many automotive and motorcycle engines as well. It's called a "waste spark" system, and is common in distributor-less ignitions because it halves the number of coils required (each coil fires two plugs).

As long as the voltage goes high enough to fire the plug under all conditions, it doesn't matter which way is which. Having the spark voltage the 'right' way only increases the margin by a bit, so if you never need the extra margin, it makes no difference at all. And it's easy to design in a bit of extra margin if it's needed.

That's basically all a "Lucas Sports" coil does, is to give the ignition system a little extra margin.

angelfj1
10-02-2009, 10:20 AM
But as the article Frank quoted said, it's better if the plug center conductor goes negative relative to the side electrode.

I stand before you humbled and corrected.

John: No need for humble contrition! I used to think that <span style="font-weight: bold">I</span> knew a little about LBC's until I started to read post from the experts <span style="font-weight: bold">(like you)</span> who frequent this forum! :yesnod: