It wasn't all that long ago that GM engineers designed the original HEI system to replace the decades old points-type ignitions. In the mid-'70s, leaner mixtures demanded greater voltage and more spark energy to kick off combustion. The HEI distributor is still an inductive-discharge ignition, but exchanges points for an electronic device called a module. Since this system produces more voltage and amperage output than a point-type ignition, it requires a larger-diameter cap to prevent voltage crossfire inside the distributor cap. The large cap also offers extra space to host the coil, making the HEI distributor self-contained. All you have to do is to apply a 12-volt hot lead to the distributor and you're ready for the road.
Early HEIs were infamous for giving up at speeds above 5,000 rpm-so many avoided them. This was true of the early-model distributors, but it didn't take GM long to modify the modules and coils to offer greatly increased spark energy at higher engine speeds. These days there's a plethora of aftermarket high-performance modules, coils, and complete ready-to-run HEI distributors that can be added to a high-performance engine that'll provide excellent spark energy and rpm potential well in excess of 7,000 rpm.
During the early years of HEI use there morphed a bunch of, well, we'll call 'em myths in regards to these systems-some possibly true, some blatantly false, and some open to interpretation. The following are the 10 most common, along with some views, which are believed to dispel them. Take a look and see what you think.
Myth 1: Inductive ignitions, like HEI - High Energy Ignitions, aren't as good for high performance engines and racing motors as CD-capacitive discharge ignitions.
This myth is only true when compared to stock HEI systems. Today, we are making high-output modules and coils that saturate fast enough to fire constant at high rpm. This firing ability also allows wider spark plug gaps-in the vicinity of 0.050-0.055-inch. The benefit of wider plug gaps is a more complete burning of the fuel mixture.
Myth 2: Internal coils (in-the-cap) are prone to overheat.
This is probably the oldest HEI myth of all. Actually, an HEI draws only 2- to 3-amps, compared to the approximate 6-amps an external oil-filled coil draws. The lower amperage draw results in a much cooler running HEI coil. HEI Coils also run cooler because they are encapsulated in thermal epoxy, which dissipates heat more efficiently than oil-filled coils. The solid epoxy, in comparison to oil-filled coils also eliminates the possibility of leaks. You can also lose some spark intensity when you run a coil wire to an external oil-filled coil as the voltage has to travel a longer path, creating more resistance.
Myth 3: Billet distributor housings are vastly superior.
For the most part there is only minor benefit to a billet distributor housing over a cast housing, at least in terms of better performance. Most castings are stable and straight and modern machining techniques aid in narrowing the benefit gap in this respect. But, in the looks department, you've got to admit that billet looks better.
Myth 4: A stock H.E.I. will do a good job for low to mid range rpm performance motors.
The advance curve on a stock HEI is for the most part not fully advanced until 4,000-4,500 rpm, which is very slow. If your low-end cam power band begins at 2,500 rpm (or below), then your advance curve will not be matched to your camshafts power-band, which will result in a significant horsepower/torque loss. In addition, low to midrange performance motors (as well as stock motors) benefit from a more intense spark from idle, all the way up the entire rpm range as the fuel is burned more completely.
Myth 5: Vacuum advance adds power.
Vacuum advance doesn't lead to more horsepower. Vacuum advance immediately begins to decrease as soon as you accelerate, and at wide-open throttle vacuum disappears. However, it is recommended that you connect your vacuum advance hose to direct manifold vacuum because it will provide you with more vacuum advance at idle, which will help to keep your plugs cleaner. Some motors will idle too fast/rough at manifold vacuum, in which case you will need to connect your vacuum advance to ported vacuum.
Myth 6: There is a specific #1 terminal on the distributor cap.
As long as you are at TDC on the compression stroke, it does not matter which terminal you use as your number one.
Myth 7: Because this is a high-compression high-horsepower motor, use solid core plug wires.
Actually most HEI systems run well with spiral-core wires. The spiral core prevents internal wire vibration and prevents electronic interference.
Myth 8: The silicone grease you place under an HEI module insulates from heat.
Actually no it doesn't, in fact, the silicone transfers the heat produced by the module to the distributor housing. In effect, causing the HEI housing to act as a heat sinc.
Myth 9: An HEI rotor must be phased.
On an HEI, if you use the holes in the housing that were originally there for the vacuum advance, to install your vacuum advance eliminator, there is no cause for rotor phasing. It will already be phased correctly due to the original design.
Myth 10: Not necessarily a myth but a fact. A defective tachometer, can cause an HEI to malfunction.