[Flathead Fever]

The type of coil determines if you need a resister or not. Some coils have built in internal resistors and some use external resistors. Sometimes the coil are stamped "internal resistor". If you run a coil designed for a ballast resistor on straight 12 volts it will burn up the wiring inside the coil in a short time, probably within a couple of days. The internal windings in the coil are made from really thin wire. They can handle 12 volts for only a short period of time, like when cranking the engine when the ballast resistor is by-passed for a hotter spark.