How to choose a ballast

The ballast serves as the pull-up for the ignition module. The MSD needs a trigger that goes high and low, ballast pulls high, ignition module pulls it low. Without it the module tries to pull the 12V to ground, a short circuit and the module blows! A ballast limits the current to a safe value. It would even work with 100 Ohm resistor, but that would not work with an ignition coil, for roadside fix, if MSD CDI fails. The ballast in the MSD CDI circuit, is not in series with the coil, it is in series with the ignition module supply, so no change in performance.

For a standard MOPAR ignition, the ballast limits the coil current to a safe value at low engine speeds. Most ignition coils are rated at 4 to 7A. With that said, few provide the current specification. Failure depends on operating conditions, average current, ambient temperature, coil construction.

If an ignition coil measures 1.2 Ohms, and a 1.4 Ohm ballast is used, the current is limited to I = V/R , 13.8/2.6 = 5.3A. A rule of thumb in the old days, was to match the ballast resistor to the coil resistor, so the coil would see 6V. Modern HEI controls the coil charge time, and has a current limit in the coil drive, to safely limit coil current, so a ballast is not required.