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And now, for the CORRECT answer. Ballasts change resistance with current as they heat and cool, and thus provide a sort of buffer for the coil. Coil current will be different at higher and lower RPM.

But the REAL nitty gritty reason is "easier starting."

A normal system runs close to 14V, and Mopar coils used to run anywhere from 8-10V AT THE COIL at the bottom end of the ballast.

When you crank the engine for start, the switch bypasses the resistor, so the coil STILL GETS about 10V--so a nice hot spark for starting.

This is something the ol' 6V cars never had.



Thats the best answer. The ballast reduces the current flow thru the coil at low speeds and keeps the coil from overheating. As was said the coil can handle more amperage but not for long periods of time. So at idle and low eng speeds the points have more time to be closed and thus the current heats the ballast up and causes it to have more resistance and drop current flow at lower eng speeds when full coil output is not needed. When you go faster and speed the eng up the points have less time to be closed so the ballast cools down some and the resistance drops causes more current to flow thru the coil and letting the coil output higher when it is needed at higher eng speeds. And when cranking the ign system needs full output to help start the eng so the ign switch bypases the ballast and gives full battery volts to the coil while cranking. Ron