Spark Plug Polarity When Running on Magneto

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Model T Ford Forum: Forum 2015: Spark Plug Polarity When Running on Magneto
Top of pagePrevious messageNext messageBottom of page Link to this message  By Mark Strange - Hillsboro, MO on Sunday, April 12, 2015 - 11:41 am:

My understanding is that when running on magneto, the coils are triggered by the peaks and troughs of the magneto output waveform.

Does this mean that some times the spark jumps from the center electrode to the ground electrode, and other times from the ground electrode to the center electrode?

Just curious... :-)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Steve Jelf, Parkerfield KS on Sunday, April 12, 2015 - 01:15 pm:

There are sixteen pairs of magnet poles. Does this mean the alternating current alternates sixteen times per revolution? If so, when you're cruising down the road at 1000 RPM it's alternating 16,000 times per minute, or 267 times per second.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Richard Eagle Ida Fls on Sunday, April 12, 2015 - 01:53 pm:

I'm not sure my spark plugs are smart enough to keep up with that. :0)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Sunday, April 12, 2015 - 02:39 pm:

Mark, You have a 50/50 chance that your spark will jump (predominately) one way or the other, depending on how the magnets are assembled to the flywheel, when driving with the timer set in the "sweet spot".


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Sunday, April 12, 2015 - 02:41 pm:

I should add, the coils can be wired with the secondary either in phase or out of phase with the primary, so some cylinders may jump one way and some another if there are a mix of coil polarities.


Top of pagePrevious messageNext messageBottom of page Link to this message  By James A. Golden on Sunday, April 12, 2015 - 05:28 pm:

Then too, look at the spark coil, which can not pass a DC voltage at all and relies on those points to make a false AC voltage that is somewhat rounded in to a current that will pass through at a higher voltage to the spark plug.

So the spark plug is getting a high frequency AC current and voltage whether the power source is a battery or the Magneto.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Fred Dimock, Newfields NH, USA on Sunday, April 12, 2015 - 07:44 pm:

Who cares - DC AC length of spark as long as the T runs OK?

In the words of someone in the past - much to do about nothing.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tim Wrenn-Monroeville OH on Sunday, April 12, 2015 - 07:59 pm:

Richard, I don't think I'M smart enough to keep up with that!! :-)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Charlie B actually in Toms River N.J. on Sunday, April 12, 2015 - 08:06 pm:

I'm kind of with Fred on this. I wonder why, besides HF's "if it ain't broke" attitude, that no actual improvements were made to the basic coil design. If any were possible that is. It seems we, in the present age, are able to find out things that just didn't concern the engineer's/designers of that era and some of us fret over them.


Top of pagePrevious messageNext messageBottom of page Link to this message  By arnie johansen on Sunday, April 12, 2015 - 09:02 pm:

Tom C.:

How does one determine if the coils are wired in phase or out of phase with the primary?

And is one way better than the other, or is this a secret that helps make your vehicles so fast!

Arnie


Top of pagePrevious messageNext messageBottom of page Link to this message  By Norman T. Kling on Sunday, April 12, 2015 - 09:36 pm:

You might find that the spark jumps one direction and then another direction depending on the position of the spark lever. No problem.
Norm


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Sunday, April 12, 2015 - 09:59 pm:

I applaud people like Mark that want to know things, for whatever reason. Some people are happy to have others do their thinking for them. That is fine too. I think there is room on this forum for folks from one end of the spectrum to the other.

Arnie, there are various ways to determine a coil's polarity. I use an oscilloscope. You can also do the "pencil eminence test".

here is link explaining it.

http://mgaguru.com/mgtech/ignition/ig104.htm


Top of pagePrevious messageNext messageBottom of page Link to this message  By George John Drobnock on Monday, April 13, 2015 - 08:13 am:

The odds of a spark plug sparking positive or negative with a magneto at a rate of 50% + or 50%- sounds about correct. About the same average starting the car for the first time of the day. So does it matter which occurs first when starting the motor? + or -?


Top of pagePrevious messageNext messageBottom of page Link to this message  By arnie johansen on Monday, April 13, 2015 - 02:49 pm:

Tom C.

I understand about the pencil test. My question is in regards to the Model T ignition coil itself. I thought they all were wired one way. Your statement "the coils can be wired with the secondary either in phase or out of phase with the primary" resulted in my comment.

I thought that all coils were wired such that the primary coil was energized at the bottom contact and that the battery current went through the primary winding , through the lower point, through the upper point to the upper side contact on the coil and then through the timer to ground.

For the secondary I thought the current flowed from the lower coil side contact to the spark plug to ground of the engine head and timer and back to the primary wire on the top side of the ignition coil. Are you saying that secondary current can flow in a different manner? Is the secondary path wired differently on some coils??


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Monday, April 13, 2015 - 05:51 pm:

Arnie, here is a really crappy drawing of coils that might help to illustrate. I haven't drawn the points and condenser because they have nothing to do with what we are talking about.

v

The top coil would be out of phase with the bottom coil. I have found Ford coils wired both ways. Of the ones I have tested, the "in phase" kind out number the "out of phase" kind by a ratio of about 20:1. Most aftermarket coils are "out of phase", by perhaps the same ratio.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mark Strange - Hillsboro, MO on Monday, April 13, 2015 - 06:16 pm:

Here's an interesting discussion I found on the subject. The article implies that it takes less voltage for the spark to jump from the spark plug's center electrode to the side electrode than vice versa.

http://mgaguru.com/mgtech/ignition/ig104.htm

(Message edited by cudaman on April 13, 2015)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mark Gregush Portland Oregon on Monday, April 13, 2015 - 06:33 pm:

Somewhere I remember reading years ago that that because of the AC voltage you had even wear on the points and when using a battery(for ignition only) it was a good idea to swap the plus and minus every so often to keep the points wearing even.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Monday, April 13, 2015 - 08:17 pm:

Mark S., that looks remarkably similar to the link I posted 4 posts above. :-)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mark Strange - Hillsboro, MO on Tuesday, April 14, 2015 - 10:11 am:

Oops, my bad, sorry Tom! :-)


Top of pagePrevious messageNext messageBottom of page Link to this message  By G.R.Cheshire on Tuesday, April 14, 2015 - 10:32 am:

What does it matter if the engine runs without missing?


Top of pagePrevious messageNext messageBottom of page Link to this message  By arnie johansen on Tuesday, April 14, 2015 - 10:51 am:

Tom C.

Thanks for the drawing. It is a lot better then I could draw!

Of course now that you helped with that excellent drawing I have another question! Remember Tom, no good deed goes unpunished! My next question is how can I tell which way the coil is wired internally without taking it apart by perhaps using a multimeter?

I am at a loss as to how to determine which coil is which without taking it apart. Of course one could use the "pencil test" to check for "in phase". However this is not practical when buying coils at a parts swap!


Top of pagePrevious messageNext messageBottom of page Link to this message  By Hal Davis-SE Georgia on Tuesday, April 14, 2015 - 12:20 pm:

What does it matter? To the guy who knows nothing about his car, and just wants it to start and drive and look like an antique, it matters about as much as whether or not he had a distributor or water pump. He'll never notice a difference or care if there is one. For guys who love the T for its idiosyncrasies and are fascinated by it's antique systems and want to learn more about them, it is interesting discussion.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Tuesday, April 14, 2015 - 01:16 pm:

Arnie, I know of no practical way to tell just by looking at them. Even if you take them apart it is hard to tell.

I say buy 'em all and sort them when you get home (if you really want to know).


Top of pagePrevious messageNext messageBottom of page Link to this message  By Glen Chaffin on Tuesday, April 14, 2015 - 03:01 pm:

You guys are concerned about nothing. Ford designed the system to work on AC or Dc. On DC you have fixed polarity to operate the coils. On Ac you have a power source that varies in polarity and voltage amplitude. The spark can fire anywhere on the sine wave curve except at the crossover point at the middle. The voltage is zero at that point and the coil will not fire. The point on the curve at which the coil fires is dependent on the setting of the spark lever and the RPM of the engine. These too are both variables. So the spark occurs all over the curve. Therefore, the polarity at the spark plug can vary all over the place and you will never feel the difference so why worry about it, Ford didn't, otherwise he would have rectified the mag voltage and gave you DC..


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Tuesday, April 14, 2015 - 03:28 pm:

It is true that the magneto puts out AC, but in operation on the Model T, even when running on magneto, the coil is operating on DC. This is because the timer is essentially acting as a mechanical rectifier. A mechanical rectifier is also known as a "commutator", which is what Ford called the timer. If you are driving with your spark set in the "sweet spot" (closing on the zero point of the waveform that gives the best operation), the coil will predominately, and for all intents and purposes be running on a single polarity.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Glen Chaffin on Tuesday, April 14, 2015 - 04:09 pm:

Tom. I have never heard of a commutator being called a mechanical rectifier. A sine wave has both a positive and a negative half. a coil can fire anywhere and everywhere on either half of the sinewave depending on where the timer lever is set and the RPM of the engine. It will not fire at the zero point because the voltage is zero at that point. The commutator does not rectify the voltage it simply makes the contact so that the coil will receive a voltage source, either AC or DC.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Tuesday, April 14, 2015 - 09:01 pm:

Glen said:

"I have never heard of a commutator being called a mechanical rectifier."

All rotating electrical devices (with the exception of a homopolar generator) actually initially produce AC. That includes a Model T generator. The reason it puts out DC is because the AC is commutated (essentially mechanically rectified) by the commutator.

Glen said: "A sine wave has both a positive and a negative half."

Agreed.

Glen said: "a coil can fire anywhere and everywhere on either half of the sinewave depending on where the timer lever is set and the RPM of the engine."

Not really. The coil fires about 5 degrees past the peak of the waveform (which is not a sine wave, by the way) plus a few degrees (always retarded), depending what part of the waveform the timer closes on).

www.antiqueautoranch.com/montana500/10N02/Volume%2010%20no%202.pdf#page=5

Here is an article from the Montana 500 newsletter where I try to explain.

The RPM of the motor has almost nothing to do with where the coil will fire in respect to the waveform.

Glen said: "The commutator does not rectify the voltage it simply makes the contact so that the coil will receive a voltage source, either AC or DC."

If the timer is closing at the "sweet spot", the coil will only see a rising current of one polarity, in other words, DC, so essentially it has been rectified.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mark Strange - Hillsboro, MO on Tuesday, April 14, 2015 - 09:20 pm:

Good stuff, Tom! I hadn't thought before about your insight that each coil only sees a piece of the magneto waveform, whatever piece is present during the time when the timer has closed the primary circuit for that coil.

Can you post links to parts 1 and 2?

I've been hoping that John Regan and/or Ron Patterson would contribute. I can think of three reasons why they haven't yet:

1) They haven't seen the thread yet.

2) They've seen it and decided it isn't worth their time to respond.

3) They've seen the thread and are carefully pondering the subject and formulating an eloquent and informative response.

I'm hoping for #3. :-)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Glen Chaffin on Tuesday, April 14, 2015 - 09:38 pm:

After thinking about it more, you are right, Rpm has little effect on where the coil fires because the timer position is fixed relative to the magneto and turns at the same RPM. However, the waveform is a sine wave and the point at which the coil fires on the sine wave will have a fixed polarity but will vary slightly when the RPM changes and the width of the sine wave increases or decreases as a function of RPM. I think what you are trying to say is that if the coil fires on the same half of the sine wave then it has the same polarity each time it fires. This may be true but no one would call it DC. The most important thing is to set the timer arm so that the sine wave peak , either upper or lower is near its peak so that you get the highest voltage. This is the sweet spot


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Robison on Tuesday, April 14, 2015 - 10:58 pm:

http://www.antiqueautoranch.com/montana500/newslettindex.html

Mark here is the index, you can find part 1 and 2.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Thode Chehalis Washington on Wednesday, April 15, 2015 - 12:47 am:

Mark,
Maybe the reason some have not commented on this is because it has been discussed before:
http://www.mtfca.com/discus/messages/411944/435971.html?1397706332

My thought on it unless you are running the MT 500 and want to get that extra 0.01 MPH at top speed, it really makes no difference.

Jim


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Wednesday, April 15, 2015 - 12:53 am:

Glen, we seem to agree in principle so I probably shouldn't split hairs, but if you chop a section of the waveform out and it is of one polarity, whether it is chopped mechanically or with a diode, it is definitely DC.

Mark, in my experience Ron and John like to stay out of these threads for various reasons, and I don't blame them. These threads do tend to go on and on for one thing.

I have spent hours and hours and studied thousands of T ignition oscilloscope waveforms to learn exactly what goes on. Nothing I am saying is weird or radical. If people don't understand what I'm trying to say I think one of two situations exist: Either they have closed their mind, or more likely, I am not explaining myself well.

I will continue to try to explain if people will keep an open mind.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Wednesday, April 15, 2015 - 12:57 am:

I think what Jim says is right. It really makes little difference. To me it is just fun to think about it and try to understand it. For me that is the big pay off.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mark Strange - Hillsboro, MO on Wednesday, April 15, 2015 - 08:51 am:

I had forgotten that we had discussed this before, thanks for the link Jim.

Tom, I took time last night to read and re-read your Part 3 article. If you have the time and inclination, I'd like to walk through it with you to make sure I understand it all. Please PM me with your phone number and a good time to call and I'll give you a ring and we can talk on my nickel. :-)

Mark


Top of pagePrevious messageNext messageBottom of page Link to this message  By Andy Clary on Friday, April 17, 2015 - 07:36 pm:

Quick question. If the coil can fire off either polarity how come you only get 16 sparks on your hcct and not 32 or even an odd number

Andy


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Carnegie Spokane, WA on Friday, April 17, 2015 - 07:51 pm:

Eight pluses, eight minuses per revolution.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Andy Clary on Saturday, April 18, 2015 - 09:40 am:

Thanks Tom, it's been a while since I had read any of the magneto stuff.

Andy


Top of pagePrevious messageNext messageBottom of page Link to this message  By Chuck Hoffman - Mokelumne Hill, Calif. on Saturday, April 18, 2015 - 04:33 pm:

When the piston is near TDC and the mixture is compressed and ready to expolde, does it really care which direction the spark is travelling?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Glen Chaffin on Saturday, April 18, 2015 - 06:09 pm:

I stand corrected again. After thinking about it some more. The crankshaft and magneto turn at twice the speed of the camshaft and timer. Therefore the timer and firing position of the spark plugs will vary in time to the position of the magneto sine wave. This means that the point at which the park plug fires on the magneto sine wave will vary as a function of RPM.

However, this is all academic. The only important factor is that you set your spark lever so that each of the four coils will fire somewhere on the magneto sine wave near the top of the sine wave and not at the crossover point. It matters not if it is the positive half or negative half of the sine wave. All that matters is that the coil get enough voltage to fire. Toms statement about DC is technically correct but means nothing to operation.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mark Strange - Hillsboro, MO on Saturday, April 18, 2015 - 06:25 pm:

Every time somebody comments that this thread is worthless, it gets bumped to the top again.... :-)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Fred Dimock, Newfields NH, USA on Saturday, April 18, 2015 - 07:39 pm:

Like I said "Who cares as long as my T runs good?"
AC DC what difference does it makes?

I start my T on DC and switch to AC when I drive it.
All I know is it runs and I have fun.

Today it got to 35 mph when I took my son to get coffee.
Later I taught him how to drive a T, Just like my dad did 50 years ago.
I refuse to get anal about the details when the important to thing is the experience and passing the torch to the younger generation


Top of pagePrevious messageNext messageBottom of page Link to this message  By Hal Davis-SE Georgia on Saturday, April 18, 2015 - 07:56 pm:

Nobody is mad. Nobody is arguing. Nobody is calling each other names. People are just having an in depth technical discussion and some people can't seem to handle that?:-(


Top of pagePrevious messageNext messageBottom of page Link to this message  By Glen Chaffin on Saturday, April 18, 2015 - 08:27 pm:

Toms comment about AC being DC is an interesting one. I never thought of it that way and I am an electrical engineer. It is only DC at the point in time when the coil fires but this is a technicality it is still AC. Your right Fred, who cares as long as the car runs good. What's more important is having the coils adjusted properly. When I adjust my coils on my hand crank coil tester they will fire on 2 volts AC. That means that they will still fire if asked to do so near the crossover point of the sine wave. You can't ask for better than that. Ok I'll shut up. Glen


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