I receive Hemmings Classic Car magazine and really enjoy most of the articles. The May 2014 issue has arrived, #116. There is an article in this issue discussing how if the polarity of the primary ignition coil is inadvertently switched, this change will adversely effect the firing of the plugs.
While this probably has zero effect on the model t ignition for the firing of the plugs, if a distributor is in use, this reversal would be effected by this switch.
Unless I am reading this wrong, has anyone ever heard of this?
That's probably a similar article to the link below that several years old. According to this article, the polarity reversal affects the spark at the spark plug. Something about electrons moving between the gap easier when the positive is hotter than the ground.
I personally haven't confirmed this phenomenon but on first thought, it sounds like an April 1st trick.
Thanks for the info. I might try this on one of the T's with a distributor that does not seem to run as well as the others with a distributor. Might show me something. Who knows.
The way it was explained to me is that the spark is supposed to jump from the center electrode of the spark plug to the side electrode. It takes less voltage to spark that way because the center electrode runs hotter than the side electrode.
If the polarity is switched, the spark has to jump from the colder side electrode and it takes more voltage for it to do so.
It appears several folks have heard of the concept but don't have nay experience with it.
Guess I will have to give it a shot to see what happens if anything.
Seems to me that if the car is negative ground and the coil is reversed as described in the article above, some problems might come up. But I guess the coil is isolated and it should be OK?
So...With a Model T magneto system putting out AC current, the coil ramps up and fires when connected to ground by the timer. This may be on a positive impulse or a negative impulse, depending on the position of the spark lever. I assume, since there are 16 magnets and 4 cylinders, that at one position all 4 cylinders fire on a positive impulse and as the spark is advanced to the next spot, all 4 fire on the negative impulse. Is this correct?
The Model T coil stores DC voltage in the capacitor. The capacitor is charged by AC voltage. When the coil fires, it discharges DC voltage.
I think you are correct, in one mag timing position the first spark that counts would be one polarity on all cylinders and with a little change in timing, they would all change polarity. This would provide a ideal way to check if the idea is valid and base on millions if cars and drivers, I don't think the idea holds water. There is no detectable difference in the polarity of the spark in actual practice.
Read what I wrote. Noel is mistaken.
Noel is correct. The timer ideally commutates (you may have heard it called a commutator) the AC of the magneto into positive or negative pulses. Which one that occurs is a function of the setting of the timer lever. The dwell of the timer will allow alternating polarity pulses after the initial pulse, but the first one is the important one. The rest are largely useless. The capacitor is not charged by AC voltage. The capacitor is used to suppress arcing at the points and to cause the field to collapse quickly. It is charged by DC.
You are correct that the coil stores DC but that "DC" switches polarity with each node of AC from the MAG power.
If a Model T is operating on "BAT" the capacitor is charged by DC voltage.
If the Model T is operating on "MAG" the capacitor is charged by AC voltage.
In either example a source of the other type of electricity is not present.
Royce said: "If the Model T is operating on "MAG" the capacitor is charged by AC voltage."
That is just wrong. First off, as I said earlier, the voltage is commutated, so even when running on mag the coil is effectively being operated on DC. Secondly, capacitors pass AC. You cannot charge a capacitor with AC.
"The capacitor is charged by AC voltage."
With the battery removed from the car, there is only one source of voltage. It is AC voltage from the magneto. Really.
as an electrician a capacitor can be charged by ac or dc. capacitors are used in both ac and dc motors also used for power factor corrections in ac circuits
For what it's worth, here is a link to a site about Model A ignition system. Scroll down to near the bottom of the page for an illustration showing how to check the polarity at the spark plug.
Sometimes a picture helps. This clearly show that MAG power switches polarity every 22.5°.
Royce said: "It is AC voltage from the magneto."
True, but the coil builds up current of one polarity or the other. When the points open, DC is dumped into the capacitor.
Paul said: "capacitors are used in both ac and dc motors"
True, but they are not storing voltage in an AC motor. They are used to alter the phase of the windings for more torque or to increase efficiency. They are passing AC, not holding a charge. OK, technically they are being charged and discharged 120 times per second (at 60hz).
When the points open on a T coil, regardless of whether operating on mag or bat, DC is dumped into the capacitor. It is then shunted and discharged when the points close.
Tom, going back to the original question, now you know why ignition coil's (distributor type) primary terminals are marked + and -.
The T engine has such low compression that the polarity of the spark is not a factor, but in a high compression engine it might as when the compression goes up, the spark has more difficulty jumping the gap.
Back to the original question: I've heard that it takes about 15% more voltage to push the electricity from the plug to the electrode. In a high compression fast turning motor, this probably matters. In a T motor, not so much.
When International Harvester bought the Hough Payloader company in the 50’s,
they sent out a service bulletin advising us to check the coil polarity on the 6 cylinder
gas engines as some had been wired incorrectly at the factory. I distinctly remember
the pencil test was shown in the bulletin. The wiring harness was correct, but some
coils were positioned half a turn out in the mounting clamp making it an easy mistake.
According to the article above, it explains that you can switch those leads and change the direction of the the way the plug fires. The article that I first referenced says the same thing. I have not had a chance to verify it yet, but sure thought it was an interesting concept. Who knows, it probably does not make a hill of beans either way. The discussion of the mag has been interesting though!
Simultaneous postings. Sorry to repeat what A. Gustaf said.
T's can have north or south polarized flywheels*. I don't know if Ford purposely charged them one way or the other, but in the wild they seem to about 50/50, which would suggest ramdomness. In addition, some coils are wound backwards from others which switches the polarity of the spark, all else being the same. The ratio of reverse to normal wound coils is about 1:10 from my observation.
*a "north" flywheel, by my definition has the north seeking poles in line with the dowel pins.
TINKERINTIPS vol one page 79 may explain
mag V bat preformanc.
Model T should run the same pos or neg ground.
Some Ford & gm models ran a NEG ground electrical
system into the 50s.
Have a good day.
Spark plug polarity does affect the voltage in which spark occurs but the greater concern is spark plug wear in my view as described in this article.
The Model T ignition system will produce spark necessary for ignition (1st spark) of either polarity depending upon how the magneto and coils are wired. Subsequent sparks will be produced of both polarities while operating on magneto which function to wear both spark plug electrodes. That means for maximum spark plug life, dual precious metal (Platinum, Iridium) spark plugs should be used.
Regarding the function of the coil "Condenser" a.k.a Capacitor, its purpose is to prevent arcing across the point contacts as Tom points out. The Capacitor does this by initially SLOWING the collapse of the magnetic field when the points open. This does two things: 1. Limits the voltage potential developed across the point contacts which reduces electromotive force available to propel electronics across the contact gap. 2. The delay in magnetic field collapse allows time for the point contacts to physically separate in space making it more difficult for the electrons to jump the point contact gap.
The capacitor initially slows the collapse of the magnetic field because it looks like a dead short the instant the points open since the capacitor has been shorted out by the points up until the moment the points open and has zero initial electrical charge. The capacitor voltage charges up exponentially to the potential of the Battery (or magneto of either voltage polarity)then abruptly stops current flow - the process takes about 2us (0.000002 seconds) which is determined by the value of the capacitor (0.47uF) and the resistance in the circuit (coil primary winding, wiring, timer contact, etc.). Once the coil current abruptly stops flowing, the magnetic field collapses very rapidly without arcing (because the points are too far apart for the electronics to jump) and a huge voltage is induced into the secondary winding of the coil which shares the common magnetic field. The secondary voltage continues to build until sufficient potential develops across the spark plug gap to begin the process of ionization; spark.
When spark occurs, there is residual energy transfer back to the primary winding which in turn transfers energy back to the secondary winding. The exchange of energy transfer continues but at a decaying pace. This process of oscillating energy transfer functions to prolong the spark duration to ensure complete combustion. Here is another dazzling oscilloscope photo of a coil primary current (y-Axis; 1A/div)versus time (x-axis, 0.5ms/div) when spark occurs; note the ringing that occurs when primary current abruptly stops and the magnetic field collapses to generate spark. This ringing is caused by the oscillating residual energy transfer between secondary and primary windings.
The frequency of energy oscillation depends, in part, on the value of capacitor connected across the points. There really is a lot going on in this seemingly simple function of spark generation and is why it is so interesting.
A spark plug doesn't know what the polarity is! It is simply a method for the spark to go to ground.
Larry said: "A spark plug doesn't know what the polarity is!"
Oh, but it does. The center electrode is hotter than the other electrode (typically) and therefore it is slightly easier for the electrons to jump off of it (due to thermionic emission).
Tom,It's early morning but are you and some of the others thinking the spark is jumping from the ground electrode to the spark plug center electrode?
In my thinking, the high tension lead is carrying the spark current regardless of polarity. So the spark still jumps from the center electrode to ground.
You should be able to prove my theory by gabbing on to a plug wire without touching the vehicle and have someone crank over the engine. If you get shocked both ways (Neg and then Pos ground) then I'm correct.
Let me know how it turns out.
Kenny, you're the one who wants to know. If you think that "gabbing on to a plug wire" will prove something, try it yourself.
Kenny, if that was directed at me, I appreciate the humor.
But there was an article that talks about hooking up an analog volt meter between the plug wire and ground. If the meter goes negative when the engine is momentarily turned over, the coil is reversed. If it goes positive, it is wired correctly. Can't imagine what difference it makes but that's what the article said. It all seems crazy to me. That's why I asked to begin with.
Do you set up and/or try to drive your MT 500 car so that the polarity of the spark is "correct" when the timing is just right for maximum preformance?
If so, how much difference in top speed can be realized if the spark polarity is "correct"?
Tom C. Your the one that said "I've heard that it takes about 15% more voltage to push the electricity from the plug to the electrode".
If your statement is correct, then do you think you'll get shocked? I think you will and I think the spark is jumping from the center electrode to the plug electrode regardless of polarity.
Please explain your rational otherwise.
I knew I had limited knowledge of electrical systems but after reading some of the above posting I my be smarter than a lot of others.
There is a lot of difference in electron theory and working on real systems. NEED I SAY ANY MORE
Your body tingles from the flow of electricity through it when you get shocked. It does not matter which way that electricity is flowing so you will get shocked when you grab a plug wire and it matters not which way the electrons are flowing through you but the fact remains that you can in fact see which way the spark is jumping across a spark plug gap that you set up and you can indeed notice that it changes direction when your swap the polarity connections to the coil primary. The spark jumps from center to side on the spark plug for one polarity and from side to center with the other. Either way the unsmart person touching the spark plug wire at the moment of spark will not be able to tell the difference in polarity but will be able to feel the sudden wetness in their underwear.
In addition to heat, it's about erosion. Metal migrates from the negative electrode to the positive. That's why the center electrode is beefier than the ground electrode in most sparkplugs. The voltage to the plug should be negative.
If it alternates, there should be less erosion.
Jim, I do everything that I can think of to try to improve performance. One thing I have done, which may have dubious results is to "index" my spark plugs. I have no numbers on what level (if any) of performance enhancement that results from this.
The issue of spark plug erosion is minimized with precious metal spark plug electrodes. I wonder if you can buy plugs with platinum or iridium electrodes that will fit a T??
I've been told that on modern cars w/the "waste spark" system, the plugs on one side of the coils erode faster than the ones on the other side.
Several years ago, my Model A brake light had stuck in the 'on' position and ran down my battery. I tried to start it with the hand crank, but the battery seemed too dead to fire the plugs. No problem, I'll just hook up the battery charger and then I'll have enough juice to hand crank it. Being in a hurry, I didn't remove the floor boards to get to the battery. I just hooked the charger up to the starter terminal and a good ground. BUT......I forgot it was positive ground! I hand cranked the engine. It started, but ran like CRAP! I looked at the ammeter to see if it was charging. It was discharging. The higher the RPM, the higher the rate of discharge. Then it dawned on me that I had hooked the battery charger up backward which reversed the polarity of my generator. So I flashed the generator back to the proper polarity and restarted the car. It ran MUCH better. So.......yeah, I believe that coil polarity can affect how well an engine runs. Model T on mag? Hmmmmmmm. I'd have to think about that.
"I wonder if you can buy plugs with platinum or iridium electrodes that will fit a T??"
They'll fit w/an adaptor.
Beginning with the Model A, Ford changed the polarity of the ignition system from negative ground to positive ground because of spark plug problems with the Model T. However, experience proved him wrong so beginning in 1956 Ford went back to negative ground. The current plan is to change future systems to 24 volts to minimize weight and increase capacity of the starter. It remains to be seen as to which polarity will be grounded.
Platinum or Iridium plugs need to operate at a higher voltage to avoid fouling. They should not be used in a low energy ignition system such as that in a Model T.
Erosion of electrodes is not a problem for the typical Model T or Model A owner. The best plugs you can get are Champion X plugs made 65 - 80 years ago. You can find them for $5 apiece or less if you are patient and watch eBay.
The voltage developed depends on compression and mixture and gap. The voltage ramps up in the secondary of the coil until arc over (spark) occurs. Want more voltage? Open the gap or run more lean.
Modern plugs are more likely to foul in a T because the typical T runs too cold/rich for the colder modern plugs. Put in a thermostat as I have, and you can run modern plugs without fouling, and get better fuel economy.
To agree with what many have said, spark polarity does make a difference. It is easier for a spark to jump from the hot center electrode to the colder outer one. It does take more voltage to jump the opposite way. The electrons do go from negative to positive, not positive to negative. The center electrode is negative and the block is positive. This is true whether the car chassis it positive OR negative- the secondary ignition system is always positive ground. Yes, I have seen the difference polarity can make on an engine. Having said this, this is true of conventional ignitions, I do not know about Model T's on AC in this respect.