MTFCA Forum

Its been a while since the MTFCA Webinar on Model T coil testing and tuning and with over 400 views; was wondering if there were any questions or interest in a follow up webinar. Anyone come across a problematic coil they cannot figure out? Need further details on diagnosing performance issues and how to solve them? Coil testing/tuning is no longer the obscure art it once was, there are hundreds of skilled experts today and that number continues to grow. This would be a Good opportunity for them to share their expertise with others just getting started out that also want to enjoy the benefit of optimal engine performance.

Webinar Link: https://youtu.be/RYcGD-8Ol3s?t=364

Sometimes you're lucky! Rich C.

I love it when that happens...

Yes indeed, excellent firing consistancy is very important for performance.

Some coils are easier to attain excellent firing consistancy than others, largely due to excessive cushion spring travel. Finding the sweet spot for cushion spring tension can be a real challange when there is more than 0.010" of cushion spring travel. Sometimes you have to change the point gap (more or less than the nominal 0.030" gap) to achieve excellent firing consistency. If that does t work, it may require crimping the limit rivet to shorten the cushion spring travel. That has to be done very carefully to avoid over crimping which would render the points useless.

I have an original 1913 coil that I cannot figure out. Any thoughts or help would be greatly appreciated.

Windings measure OK: 3.62 k Ohm
New capacitor: just put in
Electrical connections measure: OK

When tested
No spark
TTF = 0.35ms
Imax = 22A

Any thoughts on the problem, what to try or how to fix it?

By the way since tuning my coils to TTF my cars have never run better - smoother and more power.

Best Regards, John

John, can you post a screen shot showing the single spark test result and the multi-spark test result.

Run both tests then press the following 3 keys simultaneously: Ctrl + Alt + PrtSc That will take a screen capture of the test screen that you can paste the screen shot into Word or Paint then save as a .JPG file that can be attached to your post by clicking on the Attachments then Add files button. You can also paste the screen capture into an email and send it to me ( mictel@comcast.net ) for review and comment.

I play around with coils and am wondering if you could answer questions that I have had for some time. I understand that the point dwell should be at least 2 milliseconds. What would the upper acceptable limit be? I fully understand that the Dwell time of each coil should match each other.

Does the dwell time change if the primary voltage changes?

Art, the coil dwell time to fire spark is inversely proportional to the operating voltage. The dwell time to fire operating on 6V is approximately twice as long as operating on 12V (2ms). So where does this magic 2ms dwell time come from? Physics.

The relationship between coil voltage (VL), coil inductance (L, which depends on the number of turns of wire on the primary), coil current (IL) and dwell time to fire spark (t) is given by the following equation: t = L x IL/VL where L=0.0033H, VL=12Volts, and Coil current is the saturation current (ie. the maximum level of current that will flow in the coil primary). Plugging in the numbers: t = 0.0033x7.3A/12V = 0.002s (2ms). In reality, the relationship is not linear due to coil and wiring resistance. Coil current typically reaches 6A in 2ms operating on 12V as the coil current approaches saturation. The reason why the coil saturation current is important is because even if you could double or triple dwell time to fire spark, the spark energy (which depends upon the coils current at the time of firing spark) will not increase significantly (ie the spark will not be any hotter by increasing the coil point dwell time) because the current level has saturated.

An interesting benefit of operating on Magneto is the magneto voltage increases as engine RPM increases so the coil dwell time to fire spark decreases (Advances spark) as the engine RPM increases.

Mike, Thanks for the quick and detailed explanation. I was not aware of the saturation time equation. It is easy to see why cars with 6 volt battery powered coils will outrun the coil speed at road speeds above 35 mph.
4 ms. Results in approximately 33 crankshaft degrees.

Art, a few clarifications; The equation I presented does not predict current saturation. The equations specifies the how fast the current rises with time given the values of coil inductance, coil voltage and length of time the current is flowing in the coil primary starting from rest. If there was no coil resistance or magnetic saturation of the coil core in the circuit, the current would rise linearly from 0A and rise steadily. In the real world, there is coil resistance and the coil core does saturate (magnetic field reaches a maximum despite further increasing coil current) so the coil current becomes non-linear (deviates from the equation) reaching a maximum saturated current value regardless of how much longer the current flows.

It is true the longer coil dwell time operating on 6V requires more timer advance to compensate as engine speed increases. The operator runs out of timer adjustment range at a lower RPM than when operating on 12V battery so that is a disadvantage of operating on 6V battery. There are two way to maximize the timer adjustment range. The first is to replace all worn spark rod linkages to remove any excess play between the spark lever and timer housing movement. The second is to reduce the safety margin for the fully retarded (starting) spark lever position. Ford recommended the timer be positioned 15 degrees After Top Dead Center (After TDC). I some folks reduce that margin to gain more timer adjustment range when operating on 6V battery but this practice is not advisable from a safety standpoint, especially if there is play due to worn linkages.

Mike, thanks for the reply. Attached is the screen shot. I am using an Arduino based DIY TTF tester, based on the forum postings. Currently the multi spark is not working as I need to add some EMI shielding. I have refurbished the 3 other coils that came out of a relatively untouched 1913 coil box without issue. This one is being stubborn. Comments and suggestions appreciated.

Hi Art, I'm not an expert but am tuning coils to TTF. I typically adjust between 2 and 3 ms and usually within 0.1ms. I had one set that started about 4 ms and I tuned them down to 2.4 ms.

John

John, thanks for the screen shots of your coil test results. I'm not too familiar with the DIY TTF. If the measurement is correct, 22.6A in 0.35ms indicates a shorted (low resistance) primary coil (inductance well below the 0.0033H) which would result in high DC current that is reached very fast (0.35ms) OR bogus result to EMI/Noise. Can your power supply even supply 22.6A? The saturated DC coil current is approximately 7A to 8A as I recall and takes about 2.5ms to 4ms to reach if the coil primary winding is good and the secondary winding is not shorted. The primary winding resistance measures about 0.3 Ohms and the secondary (high voltage winding) measures around 3200 Ohms.

MKossor wrote: ↑

Wed Nov 18, 2020 11:04 pm

An interesting benefit of operating on Magneto is the magneto voltage increases as engine RPM increases so the coil dwell time to fire spark decreases (Advances spark) as the engine RPM increases.

...and I wonder if Henry and Edison was aware of that positive side effect?

Its good to see that people are thinking,and others are learning the basics of electricity and its circuits.
As I have said before,
Done Knock The Ability of a HCCT to correctly adjust a ModelT trembler coil for correct firing of ignition at varying voltage and engine speed of a Model T magneto.....N..

...and I wonder if Henry and Edison was aware of that positive side effect?

Yes, I believe the design and operation of the Model T magneto ignition system was very well understood. The fact the magnets on the flywheel are advanced with respect to the piston position to account for the coil charge delay (phase shift of coil current with respect to magneto voltage) at higher engine RPM also supports this.

Quiz Question: How well are trembler coils adjusted for correct firing when they are all precisely adjusted for 1.300A average coil current if the coils were collected over time from different manufacturing lots and the coil inductance (L) varies by +/- 20% ? Assume they operate on 12V battery (VL=12) and points pull open to fire spark when the peak coil current (IL) reaches 7A?

Hint: Dwell time to fire spark (t) = L x IL/VL

Offline reply got it right, No math required. VL/IL is a constant so Dwell time to fire spark (t) varies directly with coil inductance (L) variation when all coils are adjusted for Exactly identical average coil current. If Coil inductance (L) varies by +/-20%, coil dwell time to fire spark varies +/-20%. That is up to 40% max to min (t) ignition timing variation coil to coil (cylinder to cylinder)! Hardly "correct" firing of ignition.

This example illustrates the importance of matched coils if tuning Coils using average current as an indirect approximation for dwell time to fire spark.

What is point arcing? Why is it worse than "Double Sparking" and how is it eliminated?