Electrical: What OHMs is for ballast resistor?

[57 Silver Hawk]

Went to Autozone to get ballast resistor and I was told they have some different OHMs to be chosen. Which one should I get it for my '57 Hawk with stock ignition coil?

Thanks, Steven

[gordr]

That's a good question. And it's not real easy to answer. "Ballast" resistors are intended to get hot (or at least quite warm) in operation, and their resistance changes with temperature, so the nominal resistance value is kind of meaningless. Ballast resistors are supposed to limit the voltage "seen" by the coil to about 9 volts. Using an ohmmeter accurate at low values, measure the resistance of the coil primary winding. Divide that number by 3, and select that as the nominal value of your ballast resistor. Use whatever available value is closest.

If your nominal value is, say 3 ohms, and available resistors are 1 ohm and 5 ohms, I'd use the larger of the two. If the engine runs well on it, great. If not, try the alternate. The lower the value of the ballast resistor, the more current that will flow through the coil, and the hotter it will get. So in the interest of preserving a relatively expensive part, the coil, use the largest value of resistance that the engine will tolerate. If the ballast resistance is too high, I would expect to see missing at high speed, or missing under load as a symptom.

Let us know what works for you.

[Bud]

A stock ignition with the original Delco coil needs a resistor that is around 1.5 ohms. You can go down as low as 1.2 or 1.3 ohms and be in good shape. Bud