[Carbroke]

I think there is some confusion as to resistors when used in the coil circuit.

A ballast resistor is usually bypassed when cranking, but in series with the coil when not cranking. This allows a little hotter spark for starting, but saves the coil from getting hot for long runs. If not switched in/out by ignition switch, the ballast resistor has a thermal capability in that it has a lower resistance when cold and gradually gains resistance (to point) as it warms up due to current flow.

When a 6 v coil is retained after a changeover to 12 v, you need to put a series resistor that equals the 6v coil dc resistance to drop 1/2 of the voltage so that the 6v coil doesn't see too much voltage. Otherwise twice the normal current will be drawn through the coil and it will eventually fail. This would also necessitate a value change for any existing ballast resistor as well. The dropping resistor would be in series with the coil in both phases of cranking or running but the ballast resistor would still be switched in for run mode
and bypassed for cranking mode.

If your car was changed to 12v AND the coil was also changed to a 12v version, you don't need a dropping resistor in the circuit. However it would still be prudent to have a ballast resistor. It's been a while since I messed around with it, but I think the ballast resistor only drops a couple of volts in run mode, so that the coil only sees about 10.5 volts or so.

Keep in mind that in run mode and properly charging, a battery will see around 14.2 volts.