Minimum coil operating voltage

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Model T Ford Forum: Forum 2015: Minimum coil operating voltage
Top of pagePrevious messageNext messageBottom of page Link to this message  By Robert Anderson on Sunday, March 01, 2015 - 10:43 pm:

I am amazed at the not so obvious complexity of the Ford T ignition, in part necessitated by the requirement to be able to hand start the car by the average person in cold weather without a battery!

For those that want to start on mag, a low coil operating voltage would seem to be important. But the HCCT and the more modern testers don't measure this value, though it should be easy to do. I assume that reducing the spring tension would reduce the needed voltage to open the contacts, but would that produce a weak first spark? Does anyone know if Ford had a minimum voltage requirement for the coils?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bob Cascisa - Poulsbo, Washington on Sunday, March 01, 2015 - 10:55 pm:

One of the not so obvious complexities is that the coil is NOT a voltage operated device, it is a current operated device. The voltage is not important.

Be_Zero_Be


Top of pagePrevious messageNext messageBottom of page Link to this message  By Robert Anderson on Sunday, March 01, 2015 - 11:04 pm:

Good point. So is there a minimum coil operating current requirement? Or is that directly related to the average operating current?

BTW, any one have a carbide generator they would be willing to part with?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bob Cascisa - Poulsbo, Washington on Sunday, March 01, 2015 - 11:53 pm:

The coils will fire at 1.3 Amps which will occur during an increasing pulse (plus or minus) from the mag. The coil does not operate on the RMS (mathematical average) of the current.

Be-Zero_Be


Top of pagePrevious messageNext messageBottom of page Link to this message  By keith g barrier on Monday, March 02, 2015 - 03:36 am:

All I know is that you have got to crank like mad to get up to two or three volts on a HCCT, the amps come up almost as soon as you start turning so there is really no need to crank fast. I am in the process of motorizing my HCCT as I think having a steady rpm would be more helpful plus having both hands free to work on the coil. KGB


Top of pagePrevious messageNext messageBottom of page Link to this message  By Robert Anderson on Monday, March 02, 2015 - 07:25 am:

So as you slowly crank up the HCCT is there a consistent point where the coil starts firing? I'm just wondering if that rpm point varies a lot from coil to coil, and would it be worth it to find coils adjusted so that they fire at a low rpm to hand start on a mag only car.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Andre Valkenaers on Monday, March 02, 2015 - 08:12 am:

Hello All,
Interesting discussion that I found this morning.
It makes me thinking and I tested a few coils (34) on my home made HCCT. Between the coils were 4 new Fun Project coils and 5 metal top coils rebuild by Ron. The others I did myself, at the start with some help.
The HCCT give me an output, without a coil in the box, of 3.2Vac . With a coil in the box the tension varies from 1.66Vac and 1.69Vac with a current of 1.3A . This was for all the 34 coils.
After I went to my two cars and tried to start them up cold on magneto.
The 1922 has a master vibrator coil and run after priming and two pulls on magneto.
The 1926 did the same but needed six pulls to keep running. Both cars didn't run for 3 months and temperature is 7C.

Andre
Belgium


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Monday, March 02, 2015 - 02:07 pm:

A Model T ignition coil is more accurately described as a magnetically operated device. The magnetic field is generated by a current passing through the wire composing the coil's primary winding.

True, the magnetic field created by the primary winding is proportional to the magnitude of current flow in the primary winding and needs to reach a specific value, B, before it is sufficiently strong enough to pull the lower vibrator spring down towards the iron core and opening the contact points. However, the magnetic field will never reach the critical value B necessary to open the points if the applied voltage to primary coil winding is too low. Therefore, the applied voltage to the ignition coil is very much a factor in ignition coil operating performance. The higher the applied voltage, the higher the primary current and the faster the magnetic field is generated (ie. less delay aka retarded ignition timing) that occurs. The other primary variables that effect the value of B necessary to open the points is vibrator spring tension, cushion spring tension and point gap.

Here is a simulated comparison between voltage characteristics of an HCCT, Magneto and 12V battery. Note how the 12V battery (Red) pulses are not as strong as Vmag (blue)operating at 1000RPM but far more accurately represents normal magneto operation than does the HCCT (Green) does.

Vhcct vs Vmag vs Vbat


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Monday, March 02, 2015 - 05:42 pm:

I forgot to add some details about the graph depicting the HCCT output (Vhcct), the Vmag and Vbat.

The chart illustrates voltage amplitude on the vertical, y axis, 6V each. Time is displayed on the horizontal, x axis, 0.02s (20ms) each.

The HCCT output (shown in green), Vhcct, rises to a much slower with time and rises to a much smaller value vertically when hand cranked as compared with normal magneto operation at 1000RPM. At 1000RPM, the magneto voltage rises very rapidly to a much higher value; resulting in a much more rapid rise in coil current and rapidly rising magnetic field as compared with the HCCT driven coil. The difference has a profound affect on the opening and closing of coil points and thus ignition timing.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Robert Anderson on Monday, March 02, 2015 - 10:55 pm:

Thanks, Mike for your valuable contribution to this thread. Does it really matter how fast the current rises before the points open, other then the dwell time?
Looks like voltage is back in the picture. Maybe we should look at the power (V x I) delivered to the coils?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Tuesday, March 03, 2015 - 07:10 am:

Robert,

I've been studying Model T ignition coil operation and effectiveness over the past 3 years for the ECCT project. I believe the dynamic characteristics of the coil current/magnetic field behavior is a very important factor in coil ignition coil performance. One of the observations I noted was that a set of coils set up to fire precisely at a specific time operating on 6VDC (slowly rising current and corresponding magnetic field) could all fire at markedly different times operating at higher voltages (faster rising current and associated magnetic field), and hence, cylinder to cylinder timing variation. That is why I abandoned support for setting up coils on 6VDC.

Another thing I noted was a coil set to fire consistently the same time at idle (500RPM) could have considerable timing variation at a higher firing rates (higher engine RPM) where coil firing timing is 3 to 5 times as critical for optimal engine performance.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Hal Davis-SE Georgia on Tuesday, March 03, 2015 - 07:35 am:

Mike,

Please don't take this as a negative, for it is certainly not intended that way. I ask only to satisfy my curiosity. With today's high tech measuring equipment and folks like yourself that know how to use it, I'm sure that even very small differences can be very accurately measured. But when the coils are in the car, how noticeable is the variance? Is the difference in performance something that would be noticeable to the average T driver on a Sunday afternoon drive? Or would it show up more when climbing hills? Or would only a Montana 500 driver notice it when he compared his times to last years? I see some members here say they can tell no difference between 6 volt and magneto operation, which blows my mind, because when I swap from battery to mag, it feels like you switched it to turbo. On the other hand, from the driver's seat, I can tell little to no difference between 12v and magneto. So from the driver's seat, how much difference can one expect to notice between coils set up on an HCCT and an ECCT?

Again, no negativity intended. Just trying to gain a perspective.

Thanks,

Hal


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Moorehead on Tuesday, March 03, 2015 - 07:36 am:

Most of what you present above I understand. But the last post brings a question to mind. Let's say a model t gets in a drag race and let's say the flag drops for the car to take off. If the car is at 400 RPM at take off on magneto, is the car going to run the same at this RPM as it will as the engine RPM increases? Also, if a model t is on a distributor set up, either 6 or 12 volt, will the overall performance of the engine, no matter what the RPM, be better or more consistent than with the mag and coil set up? I am just trying to understand all of the characteristics of both set ups. And maybe my questions are not appropriate for this discussion at this time.
Thank you


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Thode Chehalis Washington on Tuesday, March 03, 2015 - 11:14 am:

Hal,
You asked, "How much difference can one expect to notice between coils set up on an HCCT and an ECCT?". From Mike's own test that are posted at:
http://www.modeltecct.com/uploads/TDAS_V4.pdf
it is reported that both sets of coils perform essentially then same.

I think the ECCT is a very neat tool and depending on the price may become the best and most efficient way to set up coils. However I have not seen anything that really shows that then timing variations from cylinder to cylinder makes a really big difference. When running on MAG the timing changes in 22.5 steps and that difference seems to be no problem. It would be interesting to a see real power dynamo test of coils with no timing variations and some more normal with some variation and if a normal driver would notice any difference. I have not seen any test yet other a theory that if there is some measurable timing variation then it must have an adverse effect on performance. Like I said above, Mike's own test show no real difference.

Jim


Top of pagePrevious messageNext messageBottom of page Link to this message  By Hal Davis-SE Georgia on Tuesday, March 03, 2015 - 12:20 pm:

Thanks Jim. That's just what I was looking for.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Tuesday, March 03, 2015 - 01:57 pm:

Jim beat me to the keyboard, the TDAS test data did indeed support the conclusion ECCT adjusted coils performed as good as HCCT coil point adjustment done by a knowledgeable and experienced professional.

Could ECCT adjusted coils make a car perform better then HCCT adjusted coils? Absolutely, if there were no other ignition timing variables limiting performance like Timer cylinder to cylinder timing variation or CAM/Crank variation or coil box/wire arcing or carburetor/fuel issues limiting performance. The goal in performance sensitive events is of course to minimize the variability of each component in the chain.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Eric Hylen- Central Minnesota on Tuesday, March 03, 2015 - 10:40 pm:

I like to describe the HCCT as a tool that helps adjust a coil for optimum performance in the worst case scenario of a car running at high RPM with a weak mag. We crank the HCCT at about 60 RPM, giving low voltage. But, we fire the coil 16 times per revolution, which is the same rate that it would have to fire if the engine were turning 1920 RPM.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Chris Barker, Somerset, England on Wednesday, March 04, 2015 - 05:28 am:

I rebuilt a set of coils for a friend with an 'unrestored' 1913 T. They were fairly bad, as supplied to me, so I was expecting good reports.
In fact, his car ran much worse.
The reason was that the flywheel magnets were very weak, so the mag output was low. The untouched coils coped because none of the cushion springs was working correctly, so the points opened as soon as they got some current, and the engine ran (with weak sparks). Properly set up coils require the current to be maintained while the points pull down until the top point is arrested by the rivet head. These will be harder to start on mag than coils without working cushion springs. It occurs to me that that explains how early Model T owners were able to manage with cars with no battery - the early coils didn't have cushion springs.
An in-car re-charge of the magnets with 3 x 12v batteries fixed things for my friend.


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