Pumping Up Your Stock HEI Distributor With An MSD Ignition Module

HEI distributors have been around since the very early ‘70s and lasted until the distributorless GM engines of the 90’s.  For the street, the HEI distributors worked fine, but for the high RPM guys, the HEI distributors didn’t hold up their end of the bargain.  The idea behind the HEI system was great, to incorporate an electronic ignition module, magnetic trigger device and ignition coil into a single assembly that would deliver a hot spark.  Stock HEI distributors provided a decent enough spark that made for easier starting engines and a longer spark plug life.  There was some performance gains realized by being able to fire plugs with much wider gaps which created better flame propagation in the combustion chamber.  But gear heads wanting more top end grunt from their motors realized that HEI’s didn’t perform as well in RPMs over 5,500 and they started yanking the HEI distributors out in favor of more traditional ignition systems.


About the same time, a young company was getting started with a new idea for ignition systems and hot rodders migrated to the Multiple Spark Discharge Ignition philosophy.  It was inevitable that MSD Ignition would eventually tackle the HEI rpm issue with a redesigned HEI ignition module capable of high rpm MSD performance in a stock type application.

MSD's new HEAT Digital HEI Module.

Once racers got ahold of the MSD ignition systems, the HEI type distributors were pulled out and left in cardboard boxes or tossed in the garbage.  The advantage of the new CD type of ignition systems was too powerful for performance minded drivers to ignore.  The stock HEI distributors were forgotten to all except those running in strictly stock type classes.  Never willing to dismiss the needs of a racer, MSD developed a module to replace the stock HEI module in a stock HEI Distributor.  Needless to say, the new module has all the advantages of an MSD ignition coupled with the size and look of a stock ignition module.  Silver Gomez of MSD Ignition explained to us that “We wanted to give the dirt track guys that are required to run an OE style distributor some MSD technology that they could use.  This Heat HEI module also works great for the hot rod and street guys that want to keep their engine bays looking stock”.

We got our hands on  the MSD HEAT Digital HEI Module (Part number 83647) to see how easy this replacement part would be to install.  Follow along with us as we upgrade our vintage 70’s distributor with some of MSD’s digital mutliple spark technology.

MSD Digital technology in an HEI module, complete with adjustable rev limiter.

Features of the MSD HEAT Digital HEI Module:

  • Easily installs to non-computer controlled (4-Pin) HEI housings
  • Built-in adjustable rev limiter can easily be set externally without removing the cap
  • Advanced dwell control for improved high rpm output
  • Produces 7.5 amps of juice
  • Uses multiple sparks on startup
  • Fits in stock HEI distributors or in MSD’s Pro-Billet HEI Distributor (PN 8365).
  • A Stock GM Coil can be used or MSD’s HEI Coil (PN 8225).

The quick and easy installation starts by removing the Coil, Distributor Cap and Rotor.

Installing the MSD HEAT Digital HEI Module

The MSD HEAT Digital HEI Module kit comes complete with the HEI module, heat sink compound, green tach wire, a low resistance rotor bushing and mounting screws with washers.  Everything you need to swap the spark dropping stock HEI unit out with the MSD Digital MSD unit.  We are installing our MSD Module on a stock HEI distributor that was removed and replaced with a MSD Billet distributor on one of our street/strip project cars.  There was no plans to use the old distributor because of it’s performance limitations and rather than toss it in the dumpster, we decided to breathe new life into this old warrior by adding some new technology into the beast.  If you are starting out with a distributor that is currently installed into an engine, we highly recommend that you mark the location of the spark plug wires on the cap prior to removal and mark the location of the rotor in relation to the cap and the engine.  Believe us, it will make life easier when you re-install your distributor and turn the engine over.

Cleaning up the old distributor, especially the module mounting face on the distributor.

Keep it Clean

A good cleaning of the distributor prior to removing any component will help protect the internal bushings of the distributor in good condition.  Road grime and crud can easily find it’s way into the distributor housing and eventually into the bushings.  Remove the cap, rotor, and ignition module from the distributor.  It’s ok to toss the stock module away, after all, it’s forty year old technology and deserves to be retired.  If you are replacing the cap and rotor, which we strongly encourage, you can toss these in the trash as well.  Locate the supplied tube of heat sink compound and apply a coat of the compound to the aluminum base of the MSD Ignition Module.

Putting a liberal coat of heat sink compound on the aluminum surface of the MSD module.

Installing the Module

Route the wiring harness out of the distributor housing and attach the short black wire to the ground with one of the supplied mounting screws.  The module can then be mounted to the distributor with the supplied screws and star washers.  When the module is securely mounted, the rotor and distributor cap can be reinstalled and the HEI module harness connected to the distributor cap followed by the 12 volt (battery) wire.  MSD suggests using at least a 14 gauge wire for the 12 volt power wire.  The long black wire from the HEI module harness can be connected to the engine block for a solid ground.

Using the supplied hardware, the module is tightened down, the wiring harness routed and the ground secured to the distributor.

Low Resistance Bushing

MSD supplies a low resistance rotor bushing with the HEAT HEI kit, and while it is not required for proper operation, we highly recommend installing the bushing to improve the spark transfer from the coil to the rotor tip.  The bushing replacement is as easy as removing the coil cover and lifting the coil out of the housing.  Pulling the old rotor bushing out and replacing it with the MSD low resistance bushing and reinstalling the coil and cover completes the replacement.

The low resistance bushing is installed under the coil to help supply higher voltage to the plugs.

Setting the Rev Limiter

The HEAT HEI module harness has a green wire that provides a 12 volt, square wave, 20 degree tachometer signal that triggers most tachometers.  This green wire connects to the tachometer’s trigger input wire.  To set the HEAT HEI module’s programmable rev limiter a tachometer is required.  Setting the rev limit is as simple as running the engine at half of the desired RPM and then momentarily grounding the green tach wire output from the HEAT HEI module harness.  The default rev limit is automatically set to 10,000 RPM.  To confirm the rev limit value, turn the ignition key to the On position (without cranking the engine). The rev limit value will be displayed for two seconds on the tachometer.

The Rotor, Distributor Cap and Coil are reassembled and reinstalled to complete the upgrade.

The Final Word

MSD’s Silver Gomez told us that the HEAT HEI module has “only been available for about 3 months and has proven itself to be a very accurate ignition module for HEI applications, and is very easy on the budget”.  We’d like to add that the kit is easy to install and comes with well written instructions.  For a price tag of around one hundred and thirty bucks, it would be hard to find another digital HEI ignition system with an adjustable rev limiter and the juice to hold voltage up to 10,000 RPMs.

The wiring harness is plugged into the coil. All that's left is to install the distributor into an engine and attach the black grounding wire and plug wires. The green wire is used to set the adjustable rev limiter.

About the author

Bobby Kimbrough

Bobby grew up in the heart of Illinois, becoming an avid dirt track race fan which has developed into a life long passion. Taking a break from the Midwest dirt tracks to fight evil doers in the world, he completed a full 21 year career in the Marine Corps.
Read My Articles

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