12 Volt Starter

[Smitty]

What changes are needed to the Model A starter when converting the system over to 12 volts?

Thanks, Steve

[John LaVoy]

None. A starter, if not operated for extended periods, will handle 12 volts quite well. My first conversion to 12 volt was back in the late 70s that car has never had a problem with the starter failing, burning up breaking in any manner, the car has around 50,000 miles on it now. You only hit the starter for a short burst to start the engine so it will simply spin a little faster with 12 volts.

[Purdy Swoft]

Save some money!!! I agree with John Lavoy. The original starter works good on 12 volt and cranks the engine real quick. I've run six volt starters on 12 volt in many applications in the past fifty years or more and never a problem. I can't imagine why a person would want to go backwards but if the starter is converted to 12 volt windings, it would no longer work on six volt if a person wanted to change back to six volt.

[jrelliott]

Suggest converting to a modern barrel drive for the bendix as the bolts can break off due to the extra torque. IMHO

[Smitty]

That was going to be my next question, I converted my 50 Ford to 12volt with the only problem being broken bendix.
This way in can remove my inverter and related wiring and wire directly to the CB etc.

[MikeK]

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It depends on how hard you want the starter to slam the drive. A 6V starter on 12V will deliver four times the torque, not just twice as much. Power =amperage squared times resistance. The amperage will double for a 6V starter on 12V. When you square that, you get 4X engagement slam force.

With the starter fields rewired from parallel to series, (making it a 12V starter) and then feeding that in series through the same armature, the average converted starter will still show almost 2X the power, but not so much as to severely abuse the drive.

As far as 'barrel' drives go, they are not without their own problems. The can and do fail and are extremely difficult to change on-the-road unless you carry a special tool to compress and expose the shrouded set screw. When a barrel drive fails and locks in the extended position because the much finer helix shatters from poor off-shore manufacturing heat treating, the rear often cannot be compressed to access the set screw. It becomes hack saw and abrasive cutoff wheel time.

People seem to forget an engine in good health and tune does not need a very fast crank to start. If it did, the hand crank would be useless.

I say change the starter fields to 12V configuration, it will still perform much stronger and crank way faster than a 6V starter on 6V. You will also experience much less voltage drop to the ignition circuit during cranking.

[johnbuckley]

Have put 12 v through my nominal 6v starter for over 10 years now. At one stage I fitted a so -called "modern" drive it lasted about 20 starts before disintegrating - standard old- style bendix has done the other 9 years and 51 weeks .

[Lawrie]

I also had the new drive explode on 12v, fitted the orig drive and changed the field coils,also fitted a resistor to the supply to the starter,this made a big difference ,but still not as nice as the 6v engagement.
Lawrie

[glenn in camino]

When I converted my car to 12 volts, I had the starter converted to 12 also. Works great, no problems. I also changed the bendix to the barrel type.

[Joe K]

And when we converted the 6v Ford 9n at the farm to 12v, all we did was buy a higher voltage regulator and a 12V battery. The guy at Auto Electric (starter/generator rebuilder) says that both components are WAY overdesigned compared to a modern unit.

The Model A/Delco 9n generator (same unit really) can provide up to 36 volts given sufficient field excitation.

Joe K

[steve s]

Originally Posted by MikeK

It depends on how hard you want the starter to slam the drive. A 6V starter on 12V will deliver four times the torque, not just twice as much. Power =amperage squared times resistance. The amperage will double for a 6V starter on 12V. When you square that, you get 4X engagement slam force.
.....

Mike,
That's just a rough calculation, right? Torque and power tend to correlate but are not necessarily directly proportional, at least as far as car motors go. Or, are they directly proportional with electric motors?
Steve

[Tom Wesenberg]

I did some amp, volt and RPM tests on my 1928 Phaeton by using my junk yard 6 volt battery, then testing again using a 12 volt battery. Due to the increased RPM's the amps don't double while cranking the engine, but increased by about 1.4 and the RPM's had quite an increase, but I don't know where my notes are to give the specs. Anyway the increase in torque and RPM was more than I like to do with my starter, and I'm fine with 6 volts.

Someone coverted my Allis WD to 12 volts and it took out the drive. The starter/generator shop in the small town gave me a heavy duty srive, which has worked fine, but someday I may go back to 6 volts on the tractor.

[johnbuckley]

Originally Posted by Lawrie

.....also fitted a resistor to the supply to the starter,this made a big difference ..................
Lawrie

What resistor did you fit to take that sort of amperage?

[MikeK]

Steve S & Tom W,
Yes, at increased RPM the amperage will drop, but I was talking about impact force on the engaging starter. For a brief moment, when the drive pinion hits the flywheel ring, inertia will hold the RPM at zero, then the flywheel accelerates to a higher RPM and less than 2X the amperage as noted by Tom.

At that initial engagement moment with the starter bogged down to zero RPM the force on the drive will be near 4X. If the amperage averages (reads) 1.4X while cranking the torque force on the gear will be 1.4 squared, or 1.96X as much.

So, you will have 4X the torque slam on initial engagement and then approximately 2X the applied torque while cranking. A bit too much for me, I still say convert the field configuration from parallel to series.

Steve S, Yes, the torque and power are not exactly the same, kind of like looking at both the HP and torque curves from a dyno printout. No mechanical system is perfect with zero friction, zero rotational inertia, and 100% linear conversion efficiency. Unlike internal combustion engines, DC electric motors produce maximum torque at stall speed.

The CEMF line on this graph represents Counter Electro Motive Force. This is why Tom Wesenberg's running (cranking) amperage reading is different than the initial, stalled engagement amperage: