
high energy ignition

high energy ignition, also known as h.e.i., is an electronic ignition system designed by the delco-remy division of general motors. it was used on all gm vehicles, at least in the north american market, 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 called "unitized ignition" was optional on 1972 and 1973 pontiacs. 
most - but not all - hei systems have the ignition coil mounted in the distributor cap. a control module and magnetic pickup are mounted in the distributor, in place of a conventional ignition system's breaker points and condenser.
control modules.
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 in conjunction with fixed-timing distributors, as the computer controls the ignition timing.
upgrading.
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 for surer ignition of a fuel/air mix that may not be optimal. the hei setup has also become a popular swap into non-gm vehicles.