 |  | | | Virtually all GM V-8 engines from the mid-'70s up to the distributor-less era of the '90s were equipped with HEI distributors. The HEI is an elegant design incorporating an electronic ignition module, magnetic trigger device and ignition coil into a single assembly. It delivered a hot spark for easy starting and long spark plug life. In fact, it could fire plugs with up to .060-inch gaps at low rpm. However, performance enthusiasts replaced HEIs with traditional ignition systems because the stock HEI didn't perform well above 5,000 rpm.
Extended Range | Back To Top
Now HEI ignition systems are being used for everything from hot street machines to drag and circle track cars. The performance aftermarket developed a special ignition module and a high performance coil for the HEI that would extend its potential rpm range clear to 9,000 rpm. This gave the HEI a new lease on life and an entire industry has sprung up around HEI ignition systems. Today you can purchase a totally new high-performance HEI from a number of aftermarket manufacturers, and there are many companies offering various stages of rebuilds on O.E. cores. In fact, one company has adapted HEI components to a Ford distributor housing, so even Ford fanatics can use this great ignition system.
Modified Core | Back To Top
If you currently have an HEI distributor core in good shape, you can modify it yourself for about $100 in a couple of hours. Or if you don't have a core you can usually find one at the local auto-recycling yard or through the many automotive online auctions. Cores usually go for $30-$50 depending upon condition. The one shown in this rebuild is for a Buick 455 and cost $40 from an online auction. CAUTION! Make sure you have a pre-computerized HEI. The easiest way to tell is to look for the vacuum advance canister. Computerized HEI's don't have vacuum advance canisters. Also, if you're buying a used core, look inside of the cap to make sure everything is there. The condition under the cap is somewhat unimportant because you are going to replace everything except the distributor housing, shaft, magnetic pickup and the distributor gear. The ignition module, advance weights and springs, rotor, vacuum canister, cap and coil will all be replaced.
Heat Sink | Back To Top
Our Buick HEI looked pretty bad with the cap removed. A little rust and years of scum had built up, so we disassembled the entire unit, bead-blasted the housing and the top part of the distributor shaft. You most likely won't have to do this, but if you do, just drive the drift pin out that connects the gear to the shaft and the gear comes off allowing you to pull the shaft up and out of the distributor housing. There's usually a thin metal shim or two between the gear and housing, so make sure you keep them and put them back when you reassemble the unit. The first step is to put the heat-sink silicone on the area where the module sits to transfer heat from the module to the housing. This is a very important step to ensure long life for the HEI module.
Vacuum Advance | Back To Top
Next, the new centrifugal weights and springs are installed. Most modification kits come with various springs to control the advance curve. We selected the softest springs that allowed full advance at the lowest possible rpm. A chart is typically supplied to show the advance curves for the various colored springs. Be sure to add the new weight bushings and a little grease to the bottom of the weights. Our HEI performance upgrade kit included a new adjustable vacuum advance. It can be adjusted with an Allen wrench for the amount of total vacuum advance, a great feature with today's lower octane gasoline. Most ignition pinging or spark knock occurs when you transition from light-throttle cruise to acceleration. The vacuum advance pulls in a lot of spark advance at cruise; and, when you open the throttle to accelerate, there's just too much advance. By limiting the amount of vacuum advance, you can eliminate this problem.
Install the new cap and high-output coil and your ready to rev it up. Your tuned-up HEI should be capable of sending 50,000 volts to your spark plugs. And with the quicker advance curve, throttle response should be a lot stronger than it was. With this much ignition power on tap, we opened up our spark plug gaps to .045-inch. Wider plug gaps have shown increases in horsepower on the dyno. Don't forget to install a quality set of spark plug wires. You don't want those 50,000 volts arcing all around your engine compartment!
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