High energy ignition

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High energy ignition, also known as H.E.I., is an electronic ignition system designed by the Delco-Remy Division of General Motors, and introduced on some GM vehicles in 1974, including the Camaro Z28 Special High Performance (with 3123 units factory installed)[1] and 1974 Buick Century Gran Sport Stage 1 (distributor code 1112521). It was used on all engines from 1975 through the mid-1980s. There were many design variations over the years, and provisions for computer controls were added for some applications starting in the late 1970s. A predecessor system was optional on Pontiacs as "code 704 UPC K65 unitized ignition system" for the 1972 and 1973 model years.

HEI is characterized by the incorporation of the ignition coil into the distributor cap for most applications. The system consists of a control module and a magnetic pickup mounted in the distributor. This eliminates ignition points and the coil wire. The control module performs the same function as the breaker points in a points and condenser system.

Control modules[edit]

There are four basic types of HEI control modules: four-, five-, seven-, and eight-pin. The four-pin module was used on carbureted engines and uses conventional mechanical timing controls (vacuum and centrifugal advance mechanisms). The five-pin module was introduced in 1978 and was an early attempt at electronic timing control; it contains a provision for connecting a knock sensor. The seven- and eight-pin modules are used on early computer-controlled engines. The seven- and eight-pin modules contain no mechanical timing control mechanisms as the computer controls ignition timing. Late 1980s GM cars and trucks used a slim distributor cap HEI with a separate ignition coil on throttle body (TBI), PFI, and SFI injection equipped engines. These distributors were not stand-alone units because they did not contain mechanical timing controls.


HEI distributors are a popular swap on older GM cars originally equipped with points and condenser type ignition systems. The HEI system produces a more powerful spark, which allows for a wider spark plug gap. The wider gap ensures ignition of air/fuel mixtures that are excessively lean, and/or have a high percentage of exhaust gas, causing misfires and high exhaust emissions. The wider gap also leads to a more reliable and quicker startup, especially in cold weather, when the engine is harder to turn over and pulls battery voltage down.

The HEI setup has also become a popular swap into other models of vehicles that did not originally come with an HEI ignition. Cases where the cranking speed of older GM engines is not sufficient to cause the inductive trigger coil of the HEI to cause the module to energize the high voltage spark coil are known. If in doubt, an ordinary electronic tachometer can be connected to the HEI distributor in question. By turning the distributor with a variable speed drill the minimum crankshaft speed at which spark will occur can be ascertained. Temporary use of a point-type distributor in the engine to be converted will allow its cranking speed to be measured and the problem isolated. Changing to a faster-cranking starter may be desirable. Some enthusiasts have attached small magnets to the harmonic balancer that cause a 3-wire-type hall-effect chip mounted in a wooden cube to switch the module at a fixed timing during cranking. Releasing the starter also releases a small ("cube") relay that reverts the HEI to normal inductive triggering.

Spark amplifiers[edit]

Add-on ignition spark amplifiers allow for the conversion of a HEI distributor system to a VHEI (Very High Energy Ignition) system, further increasing the net output energy of a HEI spark. A well-tuned HEI can produce up to 40,000 volts, compared to about 18,000 volts for the points system. Another reason for this is the self-contained plug-and-play nature of these systems. All the system needs is an ignition switch controlled 12 volt source that is live in both the "crank" and "run" positions.

Ballast resistor[edit]

To wire the system, the ballast resistor unit (early vehicles) or ballast resistor wire feeding the points-type coil needs to be bypassed with regular copper wire. This is because the points system used this resistor to reduce the voltage to the coil to around 9 volts while the ignition switch is in the "run" position to prevent overheating the points. The points system has a "bypass" wire from the ignition switch or the starter solenoid to deliver full battery voltage to the coil during cranking. The HEI system needs the full battery voltage at all times to work its best. One common hookup method is to replace the ballast resistor or wire with a normal copper wire and attach both wires to the HEI's power input terminal. This ensures the HEI receives full power while running.


The choice in HEI system type is vitally important based on the engine it will be used with. Each GM division had its own engine design. Thus, a Chevrolet ignition may not be identical to an Oldsmobile, Pontiac, Buick, or Cadillac engine. Typically, the ignition system compatibility followed the engine size. Most GM engines (of the same size) between 1974 and 1988 used a similar distributor ignition system. The module type is important as well. For non-computer controlled carburetor engines, the 4-pin unit is used since it uses mechanical timing controls (vacuum and centrifugal advance). The 5-pin module is like the 4-pin, with an added knock/detonation sensor input to the module so the ignition could be deliberately retarded and the power output dropped to the spark plug to aid disruption of the knock condition. The 7-pin module is designed for computer timing controls. It incorporates 2 magnetic signals from the pickup sensor, and 3 signals shared with the ECM engine control module), while the remaining 2 pins provide signal and power to the coil. Normally the 7-pin is not used on non-computerized engines since it does not contain mechanical timing controls, but some early designs used a seven-pin distributor module in conjunction with mechanical and vacuum advance in the distributor body, so the seven-pin module would be backward compatible.


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