6 volts ignition coil question
 

Hi guys,
I'm looking to purchase a 6v coil for a French Ford Flathead and I'm wondering what will be the best choice for an original 6v positive ground with the original contact points system.
The French made RB distributor have 2 contacts points, but I guess it doesn't make any difference.

Many coils are available but no other technical specifications except the primary resistance (between 1.1 and 1.5 Ohm).
Some coils are sold as "high performance" (and high price too) coils with no other explanation.

My main concern is to preserve the duration of the contact points.
I found this information for this 6v coil and I would like to know your opinion.
Thanks very much for your help!

https://fotoforum.fr/photos/2024/07/22.1.jpg

https://fotoforum.fr/photos/2024/07/22.4.jpg

Picture of the RB distributor

https://fotoforum.fr/photos/2024/07/22.3.jpg

Re: 6 volts ignition coil question
 

That coil should work fine. A "high-performance" coil usually just has more secondary windings to produce higher secondary voltage that is really not needed on a low compression engine, like a flathead. As a matter of fact, it may be a problem by "leaking" this excess voltage and causing crossfires and misfires. Protecting the contact points is actually the function of the condenser (capacitor). With each cycle of the points, a minute amount of material is transferred from one contact surface to the other. Which way it goes depends on the capacitance of the condenser. Theoretically, one could vary the capacitance of the condenser being used to minimize this transfer, but on a practical basis it would be extremely difficult to do as automotive condensers are not available in specific values and are not all that close anyway. Most currently available condensers are in the low .20 micro-farad range which has always been "close enough". My suggestion would be to purchase the highest quality condenser available. (I have good things about Vertex magneto condensers, though they are kinda expensive.)

One thing you could do is to run a coil with a little higher primary resistance to decrease the number of amps going through the points to reduce material transference, but that would also reduce the power going to the plugs which could have other adverse effects. In the whole scheme of things, flatheads don't require as much in the way of spark as a modern high-compression engine, so you could probably get away with it. Everything is a compromise.

Re: 6 volts ignition coil question
 

Good advice from Tubman, as usual.

The original on-the-distributor coils were about 1.1 ohm, and they were fed thru a 0.4 ohm resistor, for a total resistance of 1.5 ohm. Ohm’s Law says 6 volts then pushed 4 amps through the points. To preserve your points, you would want to stay at about 4 amp.

The 1.1 ohm coil should use an original Ford resistor. The 1.5 ohm coil can be run direct, without a resistor.

Re: 6 volts ignition coil question
 

Thank you both for your responses.
My knowledge is poor regarding the theory in electricity so if my understanding is good, this 1.1 Ohm resistance coil can be used without a resistor, but it would be better to add a 0.4 Ohm resistor to reach 1.5 Ohm right?

I agree that a high-quality condenser is necessary to preserve the contact points, but I can't find Vertex condensers here in France, as suggested by Tubman.
Do you have any idea where I can find the right specifications for such condenser?
I always used a Valeo Condenser 0.27 micro-farad as it is the most common to find here and I don't know if it is the right option.

Thanks again for your help.

Re: 6 volts ignition coil question
 

.27 microfarads will be fine. I have done a lot of work with condensers and have found that your vehicle will run fine over a wide variety of capacitances. The important thing is to make sure you get a quality product.

As to an additional resistor on the coil? It might be a good idea, or maybe not. It's the sort of thing that might be worth experimenting with, but probably not really worth the time and effort. It might be fun, though.

Re: 6 volts ignition coil question
 

Correct.

Re: 6 volts ignition coil question
 

Thank you both for your confirmation.
The original coil was this M52 RB coil without resistor.
I used it for a while and it seemed it was working well, but I was told that using an old coil like that would cause poor performance and increase fuel consumption.

https://fotoforum.fr/photos/2024/07/24.0.jpg

Re: 6 volts ignition coil question
 

I never heard that. I always thought coils either work or they don’t, they don’t get old and weaker with age. And I think the big can coils like that one will typically last for decades.

Suggest you pull a spark plug wire with insulated pliers and hold 1/4”-1/2” from earth. If you are getting a nice blue spark, coil is good.

If it ain’t broke …..