Tech

Ask Away! with Jeff Smith: Replacing an Antiquated Distributor on a Small Block

Here is a simple schematic drawn up by our buddy Eric Rosendahl showing how we wired the HEI module. It’s very simple and yet completely effective. (Image/Jeff Smith)

I have a 302 Ford in my street rod and have been using the Mallory Unilite distributor for years. Lately it’s been acting up and I can’t buy parts for this system anymore. What would be a good replacement for my small block?

K.S.

Jeff Smith: The answer is a little bit like the question of life—the easiest route will be the most expensive, and the least-expensive way will require the most effort.

So the answer is—there’s no cheap and easy way to go.

We’ll assume you don’t want to regress to a points-type ignition. That’s like living in a cave and hunting wild animals for survival. The next simplest route would be to invest in either a Davis Unified (DUI) distributor, or a Ford HEI-style distributor that uses one of DUI’s HEI-style modules. This is easy since all you do is hook up a switched 12-volt wire to the distributor and the ignition is ready to fire. This distributor obviously also includes an integrated coil, so aside from a new set of spark plug wires you might need, this is all you’ll have to buy.

Another option is an MSD ready-to-run distributor that doesn’t require a separate capacitive discharge ignition box. This distributor is built around a billet aluminum body which is one reason for the high cost. This will require a separate MSD coil so consider that and add roughly another $100 to the investment.

Pertronix offers a similar distributor called the Flamethrower using the Ignitor II ignition conversion. This distributor also is a ready-to-run distributor and like the others will require a separate coil.

All of these distributors (except the Davis HEI) will use a brand-specific ignition module to control the spark. While all are probably durable, the issue is that if one were to fail, there’s no easy way to fix the problem on the road. You could buy a spare module and keep it in the glove box, of course. That would work and probably guarantee that you’d never need it!

Our next idea is to combine a stock Ford Duraspark distributor with an equally easy-to-find and inexpensive stock GM HEI four-pin module. This plan does require some minor fabrication and wiring skills, but nothing difficult and certainly easily accomplished in a few hours.

Here’s how it works:

The Ford Duraspark distributor uses a very simple magnetic pickup. This just happens to be very similar in design to the GM HEI distributor’s pickup. The pickup’s job is to signal to the module when to trigger the spark. But instead of using a replacement Ford Duraspark module—which exhibit questionable durability characteristics—we will instead substitute the durable and reliable GM HEI module.

The wiring is simple. The two wires coming from the pickup are a signal wire and a ground. On most of the Ford pickups, the Ford violet or purple wire is the ground while the orange wire is the signal. The orange signal wire connects to the “W” terminal on the HEI module while the violet or purple wire from the distributor connects to the “G” terminal on the HEI module. On the other side of the four-pin module, the “C” terminal is wired to the negative (-) side of the coil while the “B” terminal connects to the positive (+) side of the coil. Connect switched power from the battery to the positive sides of the coil and you have an ignition system.

HEI module platform with aluminum plate

We built a simple HEI module platform with aluminum plate and wired it to work with this Ford Duraspark distributor. By spinning the distributor with a coil wired to a battery, we have created an ignition system. (Image/Jeff Smith)

It’s best to mount the HEI module to a piece of aluminum plate that acts as a heat sink to keep the module cool. Use some heat sink material like Radio Shack PN 2760418. This is a white paste that enhances heat transfer to keep the module cool. Any HEI module will work and there are plenty to choose from.

Do not use MSD’s Spark Guard for a heat sink material. It is the exact opposite of a thermal transfer material—acting instead like an insulator. That’s what you don’t want for this application.

You can even wire in a relay to control the switched power that will not pull power for the module through the ignition switch and perhaps add a little voltage. We made one up with a small wiring gang connector, but that part isn’t really necessary. You can mount this anywhere under the hood and if the module fails, you could literally plug in a new one in a matter of minutes.

This trick will work with any magnetic pickup distributor so a Mopar or an AMC will work the same way. All you have to do is determine which wire is ground and which is the signal. Using a a volt-ohm meter, determine which one is ground.

For those with a CD ignition system, you could make one of the HEI boards and wire it to connect to the MSD distributor which is also a two-wire magnetic pickup. This way, if the MSD CD box failed, you could use an adapter from the distributor two-pin connector to plug directly into this HEI plate and with a couple of wiring connectors, you’d be back on the road in minutes.

Part of being a good hot rodder is understanding how systems work.

Once you have that knowledge, then you can begin to see how you can modify systems to make them work in ways their original designers never intended. The original GM designers of the HEI module probably never thought a guy could wire it up to work with a Ford Duraspark distributor.

That’s what makes hot rodding and playing with cars so much fun.

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8 Comments

  1. Ty Gross. aka:: MOTORHEAD TY says:

    I Agree,
    I just modified a 1987 B250 Dodge van with a318,Wieand Action Plus intake,Feel brick 600cfm performance Carburetor, Headman Headers. This model Dodge had the”infamous Black Box “ECM/Spark Control, which I eliminated. I went with the ” “Speedmaster”HEI Distributor (I got from JEGS for $60) The best $60 I ever spent,they also carry a wide variety of application’s.
    This unit use’s stock GM replacement parts, real nice set up.
    {This Dodge is ready to RUN!!!} Drop in, 1wire hook up.

  2. I can only see the left half of the wiring diagram. It sounds like a very interesting idea and I would like to try it.

  3. Gianluca says:

    I have a question … i replaced original delco distributor with an MSD 8528 and the original coil with a MSD Blaster 2. If I will install an MSD 6AL I may get any improvement, more reliability or what? The car is a 1971 Firebird 350.

    • Jeff Smith says:

      Gianluca – the ready-to-run distributors use a similar type of amplifier for spark control. Essentially, all of these like an HEI are what is called inductive ignition systems. The short version of a very long answer is that inductive ignitions offer a much longer spark duration compared to capacitive discharge ignitions. That’s why MSD can offer multiple spark discharge (MSD) of three strikes of the spark at idle – it can recharge the coil that fast. While this is advantageous for very high engine speeds, I don’t see a real advantage to CD at street engine speeds below 6,000 rpm. So the answer is you are better off, in my opinion, to stick with what you have. Where you could gain some drivability improvements is by tuning the amount of vacuum advance or the mechanical advance curve. These will be minor slight improvements but still advantageous.

  4. Michael Schreiner says:

    Extremely useful information. Enjoyed reading and learning. Keep up the great work. All info was true and correct‼ Thanks

  5. Kevin Hull says:

    Anyway to modify the Mallory Unilite distributors to work with an HEI module?
    I have 2 of them and would rather not buy another distributor.

  6. I had the same Mallory Unilight set up in my 302 street rod. I switch over to a HEI small block Ford unit. It was very affordable, and it cleaned up the engine bay with the elimination of the coil and resistor block. Great ignition system for cruising and easy to install. One hot wire and a tach wire and that’s it.

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