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HEI does not have built in ballast resistor. It uses the coil drive transistor in the "active mode" to limit current. Yes, it does heat the transistor. The active mode limit happens at low engine speeds. At medium speeds and above, the coil charge time is precisely controlled to limit current. The transistor is fully "on", to charge the coil right before the ignition event. The heat generated is minimal in that mode. This is an analog predictive mechanism that is self adjusting. It is more energy efficient that a ballast resistor without "dwell control", used in Mopar box. Less coil heating in HEI ignitions, when operated above low engine speeds.

On the waveform the current starts at zero and builds to about 5.4A in 3.1 ms. The current then flat tops for about 2 ms, and the ignition occurs. At higher speeds the flat top part is minimized, saving energy, and reducing heat in the transistor and coil. That is done by starting the charge 2 ms later. At lower speeds the active current limit provides extra time for when the engine speed varies quickly, like when letting out a clutch when starting out. The predictive mechanism fails to work well with varying speeds, because the 3.1 ms is small compared to the time between ignition events at low speeds.