Ballast resistor for 12V coil

[Bob703]

I've upgraded my 37 to 12V, negative ground. I also had the stock coil rebuilt to 12V neg ground as well. I was told to use a 2-ohm ballast resistor with the coil, but I haven't been able to locate one. Any suggestions? thanks

[TJ]

Chevrolet used ballast resistors for years on their ignitions. You might check to see what the ohm readings are for those resistors.

[Tony, NY]

Here's 1.8 for Chevy. https://www.summitracing.com/parts/s...MGL4lwMGmq19UI

[Gene1949]

Don't forget the starter solenoid that has the tap to bypass the resister. I stuffed a 406 BBF in my '56 T-bird. The 312 started fine. Stuffed in the 406 and could not start it until I discovered the solenoid was junk.

The '49 has GM electronics so no problem. When I ran the Chrysler electronics bypassing the resistor was a must.

[rotorwrench]

A 1.2 to 1.5 Ohm ballast resistor will work for a 1.5 Ohm primary type coil. A person can use a 3 Ohm primary type coil and not need a ballast for 12-volt conversion.

[tubman]

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After struggling with this for a long time, I think I have it finally figured out. You don't want to run more than 4 AMPS through the points to avoid burning them prematurely. A practical application of a variation of Ohm's Law will tell you what you need to have for total resistance through the coil (coil resistance plus ballast resistor resistance).

R=V/I

Where "V" is voltage, "R" is resistance (ohms), and "I "is current (amps).

12/4=3 ohms

6/4=1.5 ohms

Bypassing the ballast resistor for easier staring is not part of this equation.

[paul2748]

57 and up to the 80's some where Fords used a ballast resistor that is easy to get. I don't know the spec's on it.

[paul2748]

They do make a coil that has a built in resistor. Check with a parts store.

[Bob703]

thanks. I'm planning on staying with the original/restored coil. I'm going with the 1.8 ohm Chevy resistor. I found plenty of 1.5 Ohm resistors on Amazon. Skip recommended 2 ohms so hopefully the 1.8 is sufficient.

[rotorwrench]

The built in resistor is a misnomer that I see a lot. It just has more resistance in the primary coil. The higher resistance primary makes more heat but modern internal insulation is a specific epoxy compound that can take the heat better than the old tar that they used to fill them with.

[tubman]

Originally Posted by Bob703

thanks. I'm planning on staying with the original/restored coil. I'm going with the 1.8 ohm Chevy resistor. I found plenty of 1.5 Ohm resistors on Amazon. Skip recommended 2 ohms so hopefully the 1.8 is sufficient.

It should be pretty simple. Just find the resistance of your coil in (ohms) as well as the resistance of the actual resistor. Add them together and run them through a version of the formula I posted above (I=V/R) to get the amperage you are putting through your points. If it is much more than 4 amps, your points will wear (burn) faster; lower, and you will not be getting the maximum potential from the ignition system. Actually, anything close to 4 amps should work fine.

[JayChicago]

Originally Posted by Bob703

Skip recommended 2 ohms so hopefully the 1.8 is sufficient.

You will be fine. Theoretically your current will be 4.3 amps instead of 4. Close enough. None of this stuff is exact to the theoretical anyway.

[tubman]

I agree with Jay (above). In thinking this over today, I would think it would be more accurate to use the actual voltages typically present in properly functioning automotive electrical systems (6.3 volts and 12.6 volts). I don't think it makes a lot of difference.

[rotorwrench]

Most 12-volt systems are regulated to 13.5 to 14-volts when the alternator or generator is in function. A 6-volt system should run at around 7.1 to 7.2-Volts.

[JayChicago]

Originally Posted by rotorwrench

Most 12-volt systems are regulated to 13.5 to 14-volts when the alternator or generator is in function. A 6-volt system should run at around 7.1 to 7.2-Volts.

Yes. And that normal variation in the applied voltage has a significant effect on everything downstream. Bigger effect than a few tenths of an ohm in the resistor. But everything downstream is used to it, can handle it. No microprocessors that would get all upset by small variations.