MTFCA Forum

Will a cheap digital tachometer where you wrap a wire around a spark plug wire work on a Model T due to the magneto being near by? I assume not too well as my digital multimeter acts crazy. I don’t want one permanently installed. Just curious as to my normal driving rpm’s. I think I already know the answer but wonder about anyone’s else success.

You can calculate speed/RPM pretty accurately. Digital instruments don't seem to get along with Model T ignition systems. My car has a nice elecrtic analog tach, but I don't know anything about it. There is no ID on the dial face It gives plausible readings, but I don't know how accurate it is. I think it is hooked up to the ignition lead under the dash. It works on Bat or Mag. The underdash area is not easily accessible due to the gas tank.

I’ve seen the chart in one of the Model T books for road speed/rpm’s/gear if I can find it but will have to check the accuracy of my speedometer first with a phone app as the previous owner installed a S.S.White unit that may or may not be correct.

Both my Grandfather and Father were Dentists and I grew up spending thousands of hours in their offices around S.S. White dental equipment. I had no idea they manufactured products in other categories…..

My timing light has a digital tachometer. It will read the engine rpm.
I used to use it to check coil dwell time by checking the change in timing at low and high rpm. Must run the engine on battery to do this. Double sparks after the prime spark causes trouble. I haven't used it in about 4 years.
Art Mirtes

Hi Michael,
I'm pretty sure Art is correct that you will have to run on battery to use the Tach. I have one in my speedster with single point ignition. Found it on Amazon for under $25.00 it can be programed for many different cylinder configureations. I found the number of wraps around the plug wire was very different that the instructions stated. Used a light reflective Tach. on the crank pulley to verify the engine speed and was able to get it accurate to about 3%. They don't react very fast ( have a 1-2 second lag ) I installed it to check the accuacy of my boat Tach I have used for years. Found it to be more accuate than I thought it was ( but vibration makes the boat Tach. hard to read over 2000 rpm )
Craig.

I doubt this will work.

Modern cars have one spark only, while the T gives an entire firework of sparks until the timer moved over to the next buzzer coil.

No idea if a digital tachometer is smart enough or not...

I would go for connecting to one of the timer connections first. there you have a relatively stable ground level while the buzzer coil is active.

Use a GPS speedo off a bicycle?

TeveS-Nor Cal wrote: ↑

Wed Dec 01, 2021 1:19 pm

Use a GPS speedo off a bicycle?

That will give me engine rpm’s?

I'd like to hear Luke's analysis of the feasibility of monitoring the primary frequency of the AC Ford Magneto voltage output and using that to compute engine rpm? I suspect it's a pretty noisy waveform. Counting the 'pulse rate' of two timer contacts eg, #1 and #4 would give rpm as well. FWIW, jb

And no, GPS will not give engine rpm.

I put a digital tach on a mower engine and ran it for years. I didn't care about RPM but the unit had an hour meter that I used to track oil changes. The tach was next to the engine's magneto and it worked reliably.

If you simply want to know the engine speed while you are driving. A GPS will give you road speed. Then convert this to engine rpm by multiplying mph times 1000 divided 25.

For high gear
Rpm = road speed ×1000/25.
Based on 3.63 axle and 3.5x30 tires.

For low gear
Rpm = 2.75 × road speed × 1000/25

For reverse gear. ????

This is very close and is probably accurate enough to suit your needs.

Art Mirtes

Right, and when parked?

jab35 wrote: ↑

Wed Dec 01, 2021 5:36 pm

I'd like to hear Luke's analysis of the feasibility of monitoring the primary frequency of the AC Ford Magneto voltage output and using that to compute engine rpm? I suspect it's a pretty noisy waveform. Counting the 'pulse rate' of two timer contacts eg, #1 and #4 would give rpm as well. FWIW, jb

You called?

I'd not responded to Michael's initial question as I've no experience of these one-wire tachometers. My feeling is that Rainer may be correct, given the standard T ignition system, however it will depend to some extent upon the methodology used to determine the pulses and any filtering that has (or has not!) been implemented in the unit. If they're not expensive I guess I'd give one a go and then report the results...

... as for the magneto, yes it's entirely feasible. I did something along these lines around the time I was developing the FACT, and would have carried that on further except the motor died in my T (and replacements are not that plentiful here in NZ). At the time I found that while there is a fair bit of RFI around it was possible to implement a crude analogue LPF which, followed by some software filtering, would work reasonably well to determine RPM.

Out of interest it wasn't the only possible method. Measuring LT current pulses (eg the coil box supply lead, or timer lead[s]), using an accelerometer to determine vibration frequency, and/or using an optical sensor on the front crank pulley were also valid options. None of them are particularly difficult to implement with the inexpensive digital tech we have today.

Finally, my preferred option to report the results was a servo controlled analogue gauge. This would give an 'old-school' look, and it's easy to make a suitable scale that would be appropriate for the vehicle. Here's a view of such a gauge I made years ago for a FTD (instrument flight trainer) I designed:


Alternatively you could output to a stepper motor that would then drive an original RPM gauge. In effect it would become an electronic drive cable that might otherwise be mechanically driven from the motor.

Anyway, this is digressing rather a lot from the original enquiry!