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#1 |
Junior Member
Join Date: Apr 2024
Posts: 5
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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
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#2 |
Senior Member
Join Date: May 2010
Location: Napa,California
Posts: 6,097
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Chevrolet used ballast resistors for years on their ignitions. You might check to see what the ohm readings are for those resistors.
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#3 |
Senior Member
Join Date: May 2010
Location: Capital of Corruption , NY
Posts: 817
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Here's 1.8 for Chevy. https://www.summitracing.com/parts/s...MGL4lwMGmq19UI
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#4 |
Senior Member
Join Date: Jun 2016
Location: Pahrump, NV
Posts: 394
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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. |
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#5 |
Senior Member
Join Date: Jun 2010
Location: San Antonio, Texas
Posts: 16,564
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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.
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#6 |
Senior Member
Join Date: Dec 2013
Location: Minnesota, Florida Keys
Posts: 10,450
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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. |
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#7 |
Senior Member
Join Date: May 2010
Location: Midland Park, NJ
Posts: 4,018
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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.
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48 Ford Conv 56 Tbird 54 Ford Victoria |
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#8 |
Senior Member
Join Date: May 2010
Location: Midland Park, NJ
Posts: 4,018
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They do make a coil that has a built in resistor. Check with a parts store.
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48 Ford Conv 56 Tbird 54 Ford Victoria |
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#9 |
Junior Member
Join Date: Apr 2024
Posts: 5
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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.
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#10 |
Senior Member
Join Date: Jun 2010
Location: San Antonio, Texas
Posts: 16,564
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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.
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#11 |
Senior Member
Join Date: Dec 2013
Location: Minnesota, Florida Keys
Posts: 10,450
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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.
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#12 |
Senior Member
Join Date: Jan 2019
Location: Chicago
Posts: 767
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#13 |
Senior Member
Join Date: Dec 2013
Location: Minnesota, Florida Keys
Posts: 10,450
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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.
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#14 |
Senior Member
Join Date: Jun 2010
Location: San Antonio, Texas
Posts: 16,564
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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.
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#15 |
Senior Member
Join Date: Jan 2019
Location: Chicago
Posts: 767
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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.
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