Q:Last month we described how to remove a first-generation (’74-’80) High Energy Ignition (HEI) distributor from your Corvette, disassemble it, test its individual components, and rebuild it. This month we pick up our discussion by examining how to upgrade a points distributor to an HEI system.
A:The demands for better emissions and fuel economy in the mid 1970s necessitated leaner fuel mixtures. These leaner mixtures required greater voltage and more spark to ignite. This challenge led GM engineers to design the HEI distributor, which effectively replaced the points-style systems that had been in service for decades.
The GM HEI distributor incorporated a magnetic pick-up assembly that contained a permanent magnet, a pole piece with internal teeth, and a pick-up coil. This device senses pulses emanating from the distributor as it rotates, then provides a low-voltage digital on-off signal to the ignition module, where it is interpreted as an rpm signal. The ignition module uses this information to determine when to fire the ignition coil.
The HEI distributor is an inductive-discharge-style ignition, but with the points replaced by a solid-state switching device called an electronic ignition-control module. The module works like an electrical switch, turning power on and off to the ignition coil. This, in turn, causes the coil to generate spark.
One of the reasons HEI is able to deliver more spark is that, thanks to the electronic ignition-control module, it can safely operate at much higher current loads. On a points-style distributor, the ballast resistor wire limits voltage and reduces current to the distributor. It does this because points can’t handle more than about 2 amps of primary current without having their operating life significantly reduced.
Another function of the control module is the management of the dwell circuit. Dwell is the amount of time the primary circuit is closed to let current flow through the ignition coil between each spark. Long dwell times are needed with inductive ignitions, to allow the ignition coil to become fully charged before firing. This is especially important during high-rpm operation, because there is less time to charge the coil between each spark.
Since an HEI system produces more voltage and amperage output than a points-style distributor, a larger-diameter distributor cap is needed to prevent voltage from cross-firing inside the cap. If you’re thinking about an upgrade, make sure your vehicle has enough clearance between the engine and firewall. Note that these larger distributor caps also typically incorporate enough room for the ignition coil to be housed internally, making the HEI distributor a self-contained unit.
Upgrading from Points to HEI
There are a lot of options when upgrading from a points-style distributor to an HEI one. The first is to obtain a good salvage-yard replacement. Remember, the 1974-1980 HEI distributors used the same advance weights, springs, and stops as most of the points-type units, making this a great, economical upgrade. (Be sure to select a replacement distributor with a four-pin control module. The fifth pin was only used on vehicles equipped with knock sensors.)
Some of the early HEI distributors have a reputation for breaking up at around 5,000 rpm, so you may want to avoid units made in 1974 and 1975. Fortunately GM rectified this problem early on by upgrading the ignition modules and coils to offer greater spark energy at these higher engine-rpm levels.
Note that it’s critical to provide key-on battery voltage to an HEI distributor. Remember, on a points-style distributor, an inline ballast resistor or resistance wire decreases the voltage from the battery to around 9.5 volts when running.
Points-style distributors do use 12 volts when cranking. This is achieved with a bypass wire located between the starter-solenoid (R) terminal and the coil (+) terminal. This terminal is only “hot” when the key is in the crank position.
On most 1970-and-earlier vehicles equipped an external voltage regulator, there will be two wires providing key-on voltage to the coil. One wire is hot when the ignition switch is in the crank position, providing battery voltage to the coil. The other wire is hot when the ignition switch is in the run position, providing approximately 9.5 volts.
Starting in 1971, vehicles using an alternator equipped with an internally regulated voltage regulator usually did not use an inline ballast resistor, but instead used a resistance wire.
When converting from a points-style distributor to an HEI unit on most 1970-and-earlier vehicles, it’s a good idea to run a new 12-gauge wire from the BAT terminal in the HEI distributor cap, then splice into the ignition key on the main feed wire (usually the pink one). Be sure to install an inline fuse in this lead, and solder and shrink-wrap all the connections.
If the wiring harness has an inline ballast resistor, or the ignition switch is located in the dashboard, you may need to use the cranking bypass wire to supply the distributor with battery voltage when the switch is in the crank position.
You can use a test light or voltmeter to see if your newly installed key-on main feed wire has battery voltage when cranking. If necessary, splice the bypass wire with the new 12-gauge ignition-feed wire and eliminate the ballast resistor.
It’s important to match the ignition coil and control module. For example, if you decide to run an aftermarket coil, it’s a good idea to replace the module with one built (or at least recommended) by the same manufacturer. Most aftermarket ignition companies sell matched coil-and-module sets to ensure optimal ignition performance.
In testing aftermarket HEI conversions, I’ve found that a spark-plug gap of approximately 0.040-0.045 inch (up from the factory-recommended 0.035-inch) typically provides the best possible engine performance. Remember to also install new HEI plug wires.
All 1981-and-later HEI distributors employed Electronic Spark Timing (EST), meaning they were controlled electronically by the Electronic Control Module (ECM). The ECM monitored information from various engine sensors, computed the required spark timing, and then signaled the distributor to change the timing accordingly. With this system there was no longer a need for vacuum or mechanical advance. This makes these second-generation distributors impractical for use as a simple HEI replacement.
Installing HEI Conversion Kits
If you’d prefer to keep your engine looking stock, there are a few HEI conversion kits—such as the Pertronix Ignitor kit shown—that will fit inside your existing points distributor, where the points and condenser are mounted. This kit is essentially a Hall Effect trigger, with no external control module. In fact, the only evidence that you’re running an electronic ignition is a small wire running to the positive side of the coil, and even this can be routed unobtrusively.
When using the Ignitor kit, Pertronix also recommends installing one of its Flame-Thrower coils. These coils are black, so if you peel off the label, they look similar to the factory piece.
If you plan on running your engine much past 5,000 rpm, the company recommends one of its full HEI ignition upgrades. These kits are good up to 7,500 rpm and come with everything you need to perform an HEI conversion, including the following:
- Adjustable vacuum advance with a range of 0 to 22 degrees
- A new ignition module with wiring harness and capacitor
- A selection of advance springs and weights
- An ignition coil
- A distributor cap with solid-brass terminals and ignition-rotor button
If you don’t like the conversion idea, and a factory HEI distributor won’t clear the firewall on your application, you could always install one of the custom billet distributors available in the aftermarket. The process of installing a billet distributor is similar to the stock-HEI conversion method.
I’m still an “old-school” guy at heart and believe in leaving everything stock. Even so, I have to admit that there are a lot of advantages to an HEI conversion. As for whether it’s right for your Corvette, only can make that decision.
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