How Does an HEI Distributor Work?by Don Bowman
The High Energy Ignition (HEI) used on most General Motors products prior to the use of fuel injection was a hall effect type of distributor. It did away with the conventional points and condenser and used a magnetic pickup. This system consists of a star-shaped wheel on the center shaft, which passes in front of a magnetic pickup. Every time the point of the star passes close to the pickup, it produces a pulse that is used to trigger the coil.
The coil is an integral part of the distributor and is located in the distributor cap. The control unit for the magnetic pickup is a module located in the distributor. The distributor needs only one wire to supply the module with power. The module sends out a voltage to the magnetic pickup. The voltage is interrupted every time the star wheel causes a pulse in the voltage. This pulse is returned to the module, where, in turn, it grounds the coil, causing a spark plug to fire.
The magnetic pickup is not subject to irregularities in firing at high engine speeds, as is the point-type ignition. The HEI also has two methods for advancing the spark. It has a vacuum advance that operates the base plate of the distributor. The distributor is in a semi-retarded position when the engine is starting. Once started, the vacuum produced turns the base plate and increases the advance for the spark.
Once the engine is running and when the rpm increases to over 2,000 rpm, the mechanical advance takes over and further increases the advance to 32 degrees. This is accomplished by the use of two counterweights located under the rotor. These counterweights use small springs to hold them in place at low rpm. When the rpm increases to over 2,000 rpm, the centrifugal force overcomes the spring pressure, and the counterweights swing outward, moving another plate to further advance the spark.
The mechanical advance can be adjusted to increase or decrease the rpm at which the mechanical advance overcomes the centrifugal force and advances the spark. The HEI distributor can be purchased at any auto parts store and comes with three different spring tensions, from light to heavy. The advance curve can be changed with the springs.