The ignition system has always been the weakest link in the combustion engine cycle. Belgian-born engineer, Jean Joseph tienne Lenoir, invented the stationary gasoline engine back in 1860 and he used a spark plug fired by a battery-powered coil device to provide ignition. In the 1880s Karl Benz came along and used a battery vibrator ignition on the first automobile. Not long after another German inventor, Werner Von Siemens invented the twin-T magneto. Then Gottlieb Daimler, another German, came up with an interesting ignition idea consisting of a heated ceramic tube that passed into the combustion chambers. Robert Bosch, yet another German came along with his oscillating low-voltage magneto, followed by the high-voltage rotary magneto. Bosch's high-voltage design became the standard magneto design. While, all of these ignition systems provided spark to the combustion chamber, none of them were very reliable, nor did they satisfy the range of the combustion engine ignition needs. Some ignition systems worked well after the engine was running, but were hard to start. Remember you were using a hand crank at the time. Some systems were good at low speeds, while other systems satisfied the engines high speed needs.
In 1906, good old American ingenuity stepped in. Charles Franklin Kettering, working out of a barn in Ohio (what is now Kettering, Ohio), invented the Kettering ignition. The Kettering ignition system was used from 1909 until the present and is what is referred to as the points ignition system.
In the points ignition system, the points were designed to create a spark as voltage arched across the opening point contacts. This constant arching caused the points to wear, thus causing the point gap to widen. This increased the current beyond that needed by the coil, causing the coil to heat up and reduce its output. While this was going on, the spark plug, because of the constant arching across the electrode, would wear and its gap would increase requiring more voltage to ignite the fuel/air mixture at a time when the ignition was sending reduced voltage. Setting and/or replacing the points and spark plugs were a scheduled practice. Every 5 - 10 thousand miles a "tune up" was required.
In 1974 General Motors, through its Delco-Remy Division, introduced the first real update in ignition systems since Kettering's 1906 invention-the High Energy Ignition system or HEI for short. The HEI system has become the mainstay in Street Rod ignition systems.
Without getting overly technical, we will attempt to describe the HEI system. In the HEI system, the points are replaced with a magnetic reluctor wheel. This star-shaped wheel rotates within a circular magnetic pole with matching points directed toward the center of the wheel. The change in magnetic flux in the reluctor and sensor assembly is detected by a winding at the base of the sensor, providing an input to a transistorized switching circuit in the ignition module. This output is then sent to a coil, similar to the points ignition system. GM elected to install the coil in the top of the distributor cap, making the HEI one piece-one big piece, often too large to fit in a street rod.
Eliminating the points, and the nagging maintenance that went with them, was a great step forward; however, the major improvement in the HEI was in the output voltage. The points ignition system put out about 20-25 thousand volts, while the stock GM HEI puts out about 35-40 thousand volts. If you have ever been bit by an HEI, you know that 35-40 volts can really sting. The increased voltage allows the spark plugs to work more efficiently as the gap increases from wear and to be less prone to fouling from oil and other contaminants in the combustion system. In addition, the higher voltage provides increased spark duration. The longer spark duration of the HEI system is instrumental in firing lean with exhaust gas recalculation (EGR) diluted fuel/air mixtures.