MSD coil, Mallory Unilite, using ballast resistor, Some coils and EI have or don't have very much internal resistance thus may or may not need external resistance. The MSD coil is .7 ohm not much internal resistance and needing external resistance to bring the ohm value up to 1.5 ohms or it will run hot and burn up same thing with points not much resistance there. All depends on how the manufacturer of components built the coil/ ei unit. Mallory wants a resistor to be used because it's designed not to use full batt v. The stock ballast resistor is usually .8 ohm and stock coil are usually 1.5 ohm range so basic math could figure it out. However it depends on HOW the wiring is run. Is there ign on wires split to coil and ei or both running to coil/ resistor first has a lot to do with properly wiring.
It all boils down to how the manufacture designed the coil and electronic ignition, and how the engine is currently wired up.
If you want a bullet proof ignition system, stick with stock coil and points and keep points in good shape or I have a prestolite distributor with Mallory E-Spark EI that I swapped out for a unilite dizzy (another story, lol) either way the ballast resistor is needed along with stock ohm value coils. Msd coil .7 + ballast .8 = 1.5 ohm = stock coil rating, but mallory unit needs to be resisted by .8.
Basically it all comes down to combinations as to whats required for the ignition to work properly.
