MTFCA Forum

A curiosity question from me

Why does a T run better on mag than on battery? Is this actually the case or is it a myth? Is it a stronger spark? If so, once combustion occurs, does the size of spark even make a difference to the explosion?

I'm not a T owner, but in several years I would like to get my own well-running stock one, and I'm trying to pick up useful knowledge here and there.

The magneto produces about three times the voltage while cruising. Then, once it goes through the coil, it get boosted even higher.

My understanding the greater the voltage the hotter the spark. Also note the spark plug gap affects the spark too. A hot spark produces a quality ignition of gas.

My T pulls a hill will easy on magneto much better than on battery.

I would guess that at speed the magneto is creating up to 18 volts and making a hotter spark.

Well, a bunch of words I tried to post just dissappeared...let's try again:

It is all about ramp time of the coils. Ramp time at 6VDC is much longer relatively speaking, than 20+VAC on magneto. You have a finite amount of advance that you can input to the timer, so at 6VDC you will ultimately end up running quite retarded whan at cruising speed.

There are tricks you can do to timing and coil amp draw which negates much of this, though it comes at a safety cost if hand cranking and your timer and timer rod is the least bit worn or arbitrary in it's setting. All of this trouble goes away if you go to a distributor and modern coil with a 6V battery. But, if you want the "genuine T experience" find a car with a working magneto.

No myth, Ryan. They do run better on mag. Mine noticeably picks up rpm on starting as soon as I switch from battery to mag. I think there's something else beyond the boost in voltage but I don't have a head for electrical stuff. Others here are very knowledgeable. I hope they will join in.

Ron Patterson wrote a very good paper on this subject which goes into far more depth than I did above: http://www.funprojects.com/pdf/More%20o ... Timing.pdf

Scott has it right. Folks without a functional magneto can still enjoy about the same engine performance as operating on magneto by operating the ignition coils from 12V battery. That is because the coil dwell time to fire spark on 12V is faster than 6V battery and about the same as operating on magneto. The faster dwell time means more spark timing advance with respect to piston position for the same physical timer housing rotational movement (which provides mechanical timing advance) as Scott eluded to.

Regardless of source of ignition power, Model T engine performance also heavily depends upon how well the coil points are adjusted. All 4 coils should be adjusted for equal and consistent dwell time to fire spark for best engine performance. Doing so minimizes coil to coil (cylinder to cylinder) ignition timing variation that produces a smoother running engine with more power.

Part of the issue here is that we have been living in the age of breaker-less ignition so long, few people remember how a standard ignition coil even works. Concepts like "dwell" and "coil saturation" are pretty much lost on amateur mechanics. Even some of the newer ones seem to have only a cursory understanding of it. So, it's always great to hear from the experienced hands on this forum reminding us of this stuff. One thing I have not heard mentioned is the speed by which the primary coil is broken down on mag vs. points. The faster the primary field is broken down, the higher the voltage induced in the secondary. If you have ever watched the firing process on an oscilloscope you can see how efficient that process is on magneto vs. points. No wonder it run better on magneto. Not only is the inducted voltage greater, but the speed of the breakdown of the primary is so fast when that sine wave reverses, that you get really outstanding spark output from the secondary.

The 'problem' is that on 6V a coil takes 3.5ms to fire, whereas on 12V it takes 2ms. There's also the question of timer quality - its contacts must be good for a longer period of rotational time, than need be with 12V or magneto. With a good mechanical timer, and the initial timing advanced to just after TDC, and of course coils all set for equal firing time (such as by using an ECCT), it is possible to get quite good performance with 6V and a mechanical timer; good enough to do 75km/h down the freeway. I have driven a few magneto powered cars for comparison and cannot see enough difference to justify rebuilding the magneto. That's definitely the case since installing an E-Timer. I would recommend this over a distributor and modern coil. It is far more authentic with original coils and wiring.

Kevin, I have looked at the firing process on an oscilloscope and have done so rather extensively. Not sure what you mean by efficiency changing between "magneto vs points". The magneto is a source of power and the points are the means by which the magnetic field collapses. The speed in which the coil fires spark (i.e. dwell time to fire spark) will be faster if operated from magneto versus 6V battery. The faster the coil fires spark, the less rotational travel of the engine crank shaft and hence the sooner the spark fires relative to piston position; that is the definition of ignition timing advance. That is what is responsible for engine performance improvement between the ignition operated on magneto versus 6V battery. The speed in which the coil fires spark between magneto and 12V battery is essentially the same so there is no significant improvement in engine performance based on my test data (see chart below comparing engine RPM and vehicle speed vs source of ignition power).
Regardless of the source of coil power, the coil points open at essentially the same level of magnetic core saturation, the magnetic field collapses with the same delay following the contact points opening based on the value of internal capacitor (condenser), the speed in which the field collapses and coil voltage rings (sine wave oscillation caused by alternating exchanges of energy between primary and secondary windings as the ignition arc ignites and burns across the spark plug as energy stored in the coil is released) is also essentially the same regardless of the source of power because that source of power is disconnected from the ignition coil during this process because the contact points are still open during this process; physically disconnecting the coil primary from the source of power.

JohnH makes a good point. By adjusting the range of timer movement relative to piston position (initial timing with spark lever fully retarded) and ensuring the maximum range of timer rotational travel (by replacing all worn linkages) it is possible to advance the spark more. That will result in better engine performance with the longer dwell time to fire spark when operating on 6V battery. A word of caution is in order; setting the initial timing to less than 15 degrees BTDC in effort to extend the range of ignition timing advance reduces the safety margin between safe cranking (piston is on the way down on compression stroke when spark fires) and kick back (piston is on the way up when spark fires). This is another reason why it is very important to eliminate worn linkages and "slop" in the spark lever adjustment of the timer rotational travel. To ensure the accuracy and consistency of timer position with spark lever fully retarded.

Unfortunately, MTFCA forum policy now prohibits me from discussing ECCT or E-Timer operational principals which explains JohnH's observations with coils or the E-Timer because I developed those products. Folks interested to know why must now contact me individually via PM or Email if they want to learn more about principals of operation. My apologies if I am unable to respond to all inquiries.

Mike,

Did you mean 15 degrees ATDC?

Mike, Too cold out in the garage to play right now, but during the summer months I have been playing with my Sun machine hooked up to the Model T. I haven't had time to finish building all the wires harnesses to hook up the cylinders in a group and get a raster pattern, so I can only do one cylinder at a time. By the way, all my coils have been adjusted by one of your ECCT's and run very well. My system is 6 volt. It's been almost 50 years since I was trained on one of these, so I'm still re-learning all the stuff I used to be familiar with about waveform diagnosis way back when. But, one thing I did notice is that spark duration seems to be shorter on magneto, than on battery. I've seen some other curious stuff too that I need to look closer at. I'll have to do more work with the machine when there's time this summer and I'm not freezing my tail off in the garage then get back to the forum.

Hal, Yes, just noticed that too. I did mean 15 degrees ATDC (AFTER TDC) in my previous post. I guess you can't edit posts to correct errors like that in this new format either except for a narrow time window following the post.

Kevin, I would not expect to see much of a difference of spark characteristics between Magneto power and battery power for the reasons I cited earlier. Will look forward to reviewing your Sun Scope screen shots of Model T ignition coil operation this Summer.

This is just a practical experience without any technical details. I drove a 26 Roadster on 6 volts and coils with a new day timer for about 10 years. The car ran just fine, but was one of the slower cars on tours. However it did run all the way from Kanab Ut to Brice Canyon on Ford high gear. Then when I restored another T, I rewound several coil rings and I installed one in the Roadster which I had been driving all those years. Same coils and timer, but now I had one of the fastest cars on the tours. It seems that as the car goes faster the timing automatically advances and at full advance on the timer, it will still advance a bit more with magneto. The principal difference I noticed was that on battery, as you increase speed you need to pull down the spark lever farther and as you decrease speed you need to retard it a bit. On magneto, however, there seem to be nodes on the spark lever. You will pull it down a bit with now change and then the next notch it will advance. That is due to the point where the next pole of the timer magnet crosses the mag ring. It runs best about of the middle of the "node" when all coils have a chance to fire at the same position of the magnets. As you advance the spark lever there are at least 3, in some cases 4 different "nodes" So you just advance the lever to the place it runs best for the speed of the car and leave it there.
Norm

Thanks everyone. I enjoyed reading all of your responses!

So, does the voltage on the mag go up with rpm's? I have a non running motor that turns on the starter and I can get to pop on starting fluid. I'm pullit and will go through it. I get between 2 and 3 volts on AC coming out of the mag while running the starter on 6v battery. I'm presuming I need to at least zap the magnets. But is the low votage at the starting speed normal?
Or should I be getting 6v out of the mag on the starter or hand crank?

Q1: yes
Q2: no