The HEI Distributor: Clearing The Confusion And Debunking Myths

When discussing reliable, easy-to-use ignition systems, the HEI distributor is one that securely tops the list. A single 12-volt wire is all that is required to function, and it can be easily stabbed into any distributor-fired Chevrolet engine. It’s a staple among hot rodders. GM’s HEI distributor was envisioned and ultimately built by the Delco-Remy Division of General Motors and found its way into production vehicles starting in 1974.

These spark ignitors worked so well that they were used on all General Motors engines. You can find them in everything from inception until being phased out in the mid-1980s. In fact, the HEI is such a well-thought-out design that the folks at Performance Distributors have taken the concept and made it better — for more than just General Motors engines.

When discussing a factory-stock HEI, the module is probably the most misunderstood piece of the puzzle. (From right to left) The four-pin module was used on carbureted engines with a traditional vacuum and centrifugal advance. The five-pin module was introduced in 1978. It was an early attempt at electronically controlling ignition timing. The extra pin is connected to a knock sensor. The seven- and eight-pin modules are used on early computer-controlled engines that use fixed-timing distributors.

The HEI distributor consists of a control module and a magnetic pickup inside the housing that replaces the previously used contact points and condenser. The control module’s function is to provide the same ignition pulse as the breaker points previously found in a points and condenser system.

However, early HEI distributors proved to be less than reliable in high-performance applications and got a bad reputation. Unfortunately, the units were infamous for not delivering a solid spark at engine speeds above 5,000 rpm. It didn’t take GM long to rectify the situation by modifying the modules and coils to offer increased spark energy at higher engine speeds. When used in OE applications, these changes proved to be the perfect solution. Unfortunately, enthusiasts are never satisfied with OE applications.

HEI distributor

Although GM did designate a particular position for the Number One plug wire, as long as the firing order is correct, the distributor does not care which “lug” is used as Number One.

HEI Goes High-Performance

Today, there’s a dizzying number of aftermarket high-performance modules, coils, and complete ready-to-run HEI distributors available to performance junkies. These modern takes on a vintage ignition have fueled the market and fired the combustion of an untold number of engines.

Like many parts and products offered to consumers, there always seems to be untruths and misinformation that get spread like wildfire. Compounding that is the fact the internet affords anyone with an opinion a worldwide venue to spout what they think is true. Because of that, I thought maybe it would be a good time to clear the fog so to speak. In fact, if you would like to read a little more about troubleshooting an HEI distributor, click here.

To get some solid information you can store in your noggin, we reached out to Steve Davis of Performance Distributors. We wanted his input to debunk some of the more popular rumors and innuendo. With Steve’s help, we have compiled the following most-common misconceptions about the ignition and the truths that need to be told.

Untruth: A long-running rumor that is often told relays that an inductive ignition — like an HEI — is not as good for use in high-performance engines as a capacitive discharge (CD) ignition.

Truth: “This myth is only true when the comparison is to a stock HEI distributor,” says Davis. “Today, we make high-output modules and coils that saturate fast enough to fire consistently at high RPM. This firing ability also allows the use of wider spark plug gaps. In fact, we suggest .050- to .055-inch with our D.U.I. distributor. The benefit of a wider plug gap is a more complete burn of the fuel mixture.”

HEI distributor

One common misconception says the HEI’s coil can get hot, shortening its lifespan since it’s covered. This is not true as an HEI coil generates less heat than a traditional oil-filled “can” coil.

Untruth:  Since an HEI distributor coil is mounted in the cap and is covered, it can overheat?

Truth: “This is probably the oldest HEI distributor myth of all,” Steve quips. “First, HEI coils only draw two to three amps. Compare that to an external, oil-filled coil that draws roughly six amps, and there is no way it can get as hot when working properly. The lower amperage draw actually results in a much cooler running HEI coil. An HEI coil also runs cooler because it is encapsulated in thermal epoxy. This epoxy dissipates heat more efficiently than oil-filled coils. Also, an oil-filled coil can leak. The solid epoxy eliminates the possibility of leaks. You can also lose a small amount of spark intensity when you run a coil wire to an external oil-filled coil. The spark has to travel a longer path of resistance.”

Untruth: A billet distributor housing is far superior to a cast piece.

Truth: “There is no benefit to a billet distributor housing in terms of increasing performance — castings are stable and straight,” according to Steve. “Internally, the bronze bushings utilized in Performance Distributor’s D.U.I. distributors are oil-impregnated and extremely durable.

Untruth: A stock HEI will do a good job only when used in low- to mid-RPM-range performance applications.

Truth: In reality, the advance curve typically found in a stock HEI distributor is not fully advanced until 4,000 to 4,500 rpm, which is very slow. If the camshaft in your engine has a powerband that begins at 2,500 rpm (or below), then your advance curve will not be matched to your camshaft’s power band. This will result in a significant horsepower/torque loss.

In addition, low- to midrange-performance engines (and even stock engines) benefit from a more intense spark from idle, all the way through the entire RPM range as fuel is burned more completely.

HEI distributor

Although some feel that using a solid core wire will help deliver a better spark, with an HEI unit, that is not a good idea. A spiral-core wire will help prevent electronic interference.

Untruth: Connecting, or not connecting, a vacuum advance can add or hurt peak horsepower.

Truth: “A distributor’s vacuum advance does not lead to gaining or losing horsepower,’ Steve states. “The advancing of engine timing, via the vacuum advance, immediately begins to decrease as soon as you accelerate. In fact, at wide-open throttle, there is no vacuum advance. However, we do recommend connecting your vacuum advance hose to a direct manifold vacuum. This is because it will provide you with more vacuum advance at idle. This can help to keep your plugs cleaner. Some engines will idle too fast with the line connected to a manifold vacuum. If this happens, you will need to connect your vacuum advance to ported vacuum.”

Untruth: The Number One plug wire must be located at a specific terminal on the distributor cap for the engine to run correctly.

Truth: As long as your Number One cylinder is at top-dead-center on the compression stroke, it does not matter which terminal you use for your Number One plug wire. What that means is, whatever position you stab the distributor into the engine, as long as you can move the distributor to advance or retard timing as needed, any terminal can be the Number One connection.

Untruth: If you have a high-compression engine — or one making a lot of horsepower, you must use solid-core plug wires.

Truth: HEI systems will run better by using spiral-core spark plug wires. The spiral core prevents internal wire vibration and curbs electronic interference.

The dielectric grease placed under a module is not there to protect the module from heat. It is utilized to help the module transfer the heat it develops to the distributor housing.

Untruth: The silicone grease you place under an HEI module is designed to insulate the module from heat.

Truth: It is not. Actually, the silicone transfers the heat produced by the module to the distributor housing. In effect, the HEI housing actually becomes a heat sink for the module.

Untruth:  If you decide to eliminate the vacuum advance module you have to phase the rotor in the distributor.

Truth: According to Steve, “if you install your vacuum advance eliminator using the same holes in the housing that were originally used for the vacuum advance mounting, there is no cause for rotor phasing. It will already be phased correctly due to the original design.”

These myths and untruths have been perpetuated for decades, and it’s hard to believe that many still believe them. Thankfully, we were able to get the truth and clear the air, thanks to Performance Distributors.

Article Sources

About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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