The GM HEI (High Energy Ignition) electronic distributors have a reputation for hassle-free performance. Used in GM rigs from its inception in 1974, the famed HEI can be retrofitted to almost any GM engine for a dramatic improvement over a point-style distributor. However, their moving parts do wear over time, and additional power can literally be locked under the cap-even in fresh, factory-spec HEIs.
To explain this in simplistic terms, an engine's piston travels faster than the spark, so, the spark needs a head-start (or advance) to arrive at the proper time for optimal combustion. Inside the head, the rate that the flame moves across the combustion chamber is affected by compression ratio, turbulence of the air/fuel mixture, spark-plug indexing, design of the combustion chamber, and other factors. A killer spark is usually needed for optimum combustion and maximum power.
In engines with an HEI distributor, variable ignition timing is controlled by engine vacuum advance and centrifugal advance. Vacuum is usually consistent in vehicles that have safe-and-sane cam profiles, whereas lumpy cams transfer the bulk of the timing burden to the centrifugal weights. In other words, all of this means that optimizing spark advance is often trial and error.
Professional distributor curving is now almost a lost art. But four-wheelers who like to tinker can tune their HEIs at home-or even in the boonies as part of trail-testing.
Inside an HEI, replacement springs and weights are available to control the centrifugal advance, and adjustable vacuum-advance canisters allow part-throttle spark timing to be tuned. Proform, a company that introduced the HEI Tune-Up Kit (PN 66945) some years ago, includes various weights and springs in the aforementioned kit; these items are also available la carte in an HEI Advance Curve Kit (PN 66948C) for people who don't need a cap, pickup coil, module, rotor, condenser, capacitor, wiring harness, and adjustable vacuum advance (available separately as PN 66952C).
Before pulling out your distributor, first make a mark on the valve cover that points to t
Use a distributor wrench to unbolt the hold-down bracket, and save the bracket unless you'
The new Proform billet distributor is custom-curved at the factory with medium-tension spr
Snug the distributor into place with the hold-down (the Proform kit includes a billet plat
Secure the cap to the distributor with the lock latches, and then transfer the spark-plug
Referring to a shop manual is necessary for the initial timing specs. Check the timing wit