a true ballast resistor, as opposed to a fixed resistor, has minimal resistance when cold & increases resistance as it heats up. The purpose is to allow max current to the coil for start up & then reduce current while the engine as running.
It depends on the internal resistance of the coil you are using.
For example, a front coil (2N & 9N) had a very low internal resistance (.3 to 1.5 ohms) and requires a ballast resistor. An 8N w/ a round coil does not need a ballast resistor because the internal coil resistance is greater than 3.25 ohms.
The A model Ford used a round coil, no ballast resistor. AFAIK, the 4 cylinder B model Ford engine also used a round coil & probably does need a resistor. But, to be sure, just measure the internal coil resistance.
And BTW, a B model Ford engine is only similar to an N engine in that it's a 4 cylinder flat head.
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