ballast resistor limited the voltage to the coil extending the life of the coil and points. Once the car got warm, the ballast would heat up and reduce the coil feed, making the points trigger a lower voltage, extending their life. in an HEI, the transistor switches the HEI coil ground. no more arcing points and thus no ballast required and no burned coil since the HEI coil is designed to work on full voltage.
"..The term also referred to a (now obsolete) automobile engine component that lowered the supply voltage to the ignition system after the engine had been started. Because cranking the engine causes a very heavy load on the battery, the system voltage can drop quite low during cranking. To allow the engine to start, the ignition system was designed to operate on this lower voltage. But once cranking is finished, the normal operating voltage would overload the ignition system. To avoid this problem, a ballast resistor was inserted in series with the ignition system. Occasionally, this ballast resistor would fail and the classic symptom of this failure was that the engine ran while being cranked (while the resistor was bypassed) but stalled immediately when cranking ceased (and the resistor was re-connected in the circuit)..."
