Electronic ignition systems were just coming online as the original muscle car era was fading out. The electronic triggering replaced traditional breaker points, eliminating a maintenance item while providing both more consistency and the ability to run a more powerful spark.
General Motors had offered “transistorized” ignitions on some models in the ’60s, but those systems were notoriously troublesome. However, during the 1974 model year, a new electronic ignition debuted on select GM models called High Energy Ignition, which would quickly come to be known as simply “HEI.” The new system became standard on all GM passenger car and light truck V-8 engines for 1975, and soon proved to require very little maintenance. The coil was located inside, under a cover at the top of the distributor cap, and the control module was mounted in the base so that the entire ignition system was contained in the distributor itself —only a single power feed wire was required.
Thanks to the new HEI’s high-powered spark and reliable performance, in addition to its plug-and-play arrangement, some enthusiasts began swapping the units into older models. Another benefit of the HEI was expanding dwell, a feature built into the control module. This means that as rpm increases, the duration of the spark also increases. The feature helps to yield more complete combustion for cleaner emissions, but it can also provide a boost to power output.
One downside of the HEI was that, since it came into being during the depths of the “smog” era, it was common for the factory advance curve rates to be very conservative, and not necessarily conducive to performance. Fortunately, these distributors can be recurved just like earlier breaker-point units. As the HEI made its way further into performance applications, another shortcoming revealed itself: The HEI’s output often begins to trail off after around 5,500 rpm. That’s the point where most factory engines of the time were reaching their maximum rpm, but obviously well short of the levels racing engines typically see.