I did an experiment with a magneto that I have that is driven by a variable speed motor. My question was: What is the widest gap between the magnets and field coil that will still operate the coil? The magnets on my test unit are well charged.
I set the gap to various widths and visually noted the spark at a .090" air gap, and measured the current through a fixed reactive load.
test 1. Gap as close as I could get it without rubbing:
Current - 2.2 amps - spark jumped the gap.
test 2. Gap .040"
Current - 1.9 amps - spark jumped the gap.
test 3. Gap .080"
Current 1.6 amps - spark jumped the gap.
test 4. Gap .160"
Current 1.2 amps - spark jumped the gap.
This is as wide as I went, as my machine was reaching its limit (pulley about to fall off of shaft) and I thought that the gap was ridiculously wide anyhow.
One interesting thing to me is that the current and gap seem to correspond in a linear fashion. I thought the reduction in current would be logarithmic.
That IS interesting and not what I would have expected. Thank you for sharing. Bill
What am I saying? The current rate change is indeed logarithmic. The gap increased by a factor of four, the current diminished by a factor of 2. Duh!
I don't know about the widest gap that you can use and still have an operational magneto but if you are searching for the proper gap, I gapped the mag ring on my newly re-babbited/rebuilt engine back in July of 2010 and ended up with .028 at the top and .032 at the bottom. I had my engine mounted vertically in an engine stand I built out of plywood which made it much easier to mount the transmision.
To attain this gap, I used the set of four brass teardrop shaped shims from Snyders. At the 9:00 and 3:00 poitions I used a complete shim at each position and at each of the 11:00 and 1:00 positions I used a shim minus 5 layers.
The magneto, which was rebuilt by Wally works GREAT! Jim Patrick
Jim, I am not implying that you should set your gap to anything other than Ford's recommended settings. I was just curious as to how wide you could go before the mag would cease to function. I didn't find out what that number was, so in a sense my experiment was a failure, but I still think the results are interesting.
.025-.040 is the gap recommended. Less than .025 run a risk of magnets hitting the frame. This would very likely break off the brass screws and send the keepers into the crankcase. Over .040 might work with a very strong magnets. There are several reasons I think for the gaps recommended. One is that wear on the thrust face of the rear main bearing could either decrease or increase the clearance to the point of either scraping, or too far for la good spark. The other is that over time the charge on the magnets can get weaker, and the coils can and do get out of adjustment for the strongest spark.
There could also be some variations caused by expansion and contraction as the engine heats and cools.
I have no doubt that your test would get a spark with well tuned coils and well charged magnets, but everything will last longer with fewer problems if the instructions are followed.
Last year I did some work on a 26 that had supposedly been rebuilt correctly, every thing was pretty good except the mag. gap. It checked at 40 to 48 thous. would run well after started on battery but would hardley ever start on mag. KB
Tom -- I think your results are interesting too. Sometimes I struggle with getting the gap within specs all the way around, and I never knew how much that would affect the mag's performance. Since the spark in your tests jumped a gap three times that of a Model T's spark plug while having more than four times the recommended gap at the magnets, I think I can rest a little easier. All the mags I have set up have worked fine; apparently I needn't have been too worried about them. Thanks for posting your results here.
Keith, that is interesting and something I hadn't thought of. Some time I would like to add that test to the experiment, namely, at what speed does the spark begin to jump?
What RPM were you using for the test? You said your setup used a variable speed motor. Did you try various RPM and to what effect?
Walt, I just spun it up until the amp gauge maxed out. The amperage will read steady once the mag has sufficient power. I would presume that to be around 600 rpm in the worst case scenario. As I mentioned above, I didn't take rpm measurements.
Tom, I my rusty memory serves me right, the varying magnetic field estsablish a voltage which through a load makes a current flow. You measurred the current, but what was the load? A Ford coil? Well thats a coil and you are dealing with a coil which are not a clean ohmic resistance as the coils resistance varies with the frequency.
You should meassure the voltage and that would probably vary more as expected.
Michael, the load was a coil, not a Ford coil though. I purposely chose this coil because I liked how it behaved with my amp meter. The "amp" value that I gave is somewhat meaningless, but was presented as a benchmark to compare between the different gap settings. The voltage at any given rpm would vary in the same ratio as the current, as the current is a function of the voltage and the impedance of the circuit. The voltage and impedance increase more or less together with the increase of speed. Now, my intent of this post was to show in a simple manner what happens when the gap of the mag is increased in a real world setting. For some reason some folks get angry when the discussion goes technical. I personally have no heartburn about tech talk, but some self styled arbiters have decided that this is not the purpose of this forum. In deference to them, I would be willing to take technical questions offline if you think it is inappropriate to discuss them here. By the same token, I personally think that this forum is for all aspects of Model T subjects and will talk about it online until everyone is satisfied, if they want. Also, please keep in mind that I have never been to school on this, and although I believe that I understand it very well, I am open to corrections of any mistakes that I may make.
Interesting stuff Tom. I've always been more worried about minimum gap than maximum gap. If it's .042" or .045" at the top and .025" at the bottom it still seems to work fine.
I can't recall the place I read or heard this, but it was my understanding that it is harder to jump a spark gap under "compression" than "out in the air". It doesn't seem right as I think about it now. I think what you visually observe with a spark at the plug is the air burning. If the air is more dense, as in compression, wouldn't it give an easier path?
I did have a friend that intentionally set the gap on the field coil to the magnets too close (less than 0.025") on a '13 T engine just to see what would happen. We didn't measure the output but you could just gently pull up on the crank and it started...no spinning required.
Tom, that is interesting and explains how they were able to adjust the gap so fast during engine assembly on the assemly line. Turns out it's not so important after all.
I assume that the Ford engineers determined the average number of shims that should be use to get the gap within a certain tolerance (say .030 on the top and .040 on the bottom) and issued orders that that is what should be used during assembly on all of the engines. That way an inordinate amount of time was not wasted getting each perfect so the assembly line would not need to be slowed at this point. Jim Patrick
Like Jim states above---Ford was producing 8000 cars per day in the early `20s. Certainly they were not spending too much time setting gaps, at that pace. I`ve always tryed to stay "within spec" ,020-.040 and have never had a weak mag problem on a rebuilt. I also ALWAYS use recharged magnets (out of car) as this also determines mag outout, as well. Paul
This reverse engineering stuff is great!
Trying to figure out why the Ford engineers did things a ceratin way helps us understand our vehicles.
Combining performance data like Tom reported and practical application such as Jim Patrick's input gives us hints as to why things were done certain ways.
Ford folks didn't always want to maximize the results - they were driven to do it "good enough" at the lowest cost/time.
WITH THE EMPHASIS ON LOW COST!
If the goal was to do it best they would have been making something like a Rolls Royce instead of the simple Model T that everyone could afford.
Just a though, we know that if there is not enough clearance, the mag and flywheel will touch. We know the mag coil will vibrate under normal usage. I wonder if the reason for the maximum gap limit is to keep "ringing" or "vibrating" to a minimum to prevent mag ring breakage. Again, just a thought.
Verne, do you remember the old spark plug cleaner/test machines? They had an air hookup so that you could pressurize and observe the spark after cleaning. I haven't seen one in years. KB
I remember seeing them but have not seen one in use nor do I know how they work. From what you are saying, this might support my posting above about it being harder to jump a gap in compression.
I think the Ford book says .035"