Both the Duraspark system and the GM HEI have been around since the '70s. The stock Duraspark is by most accounts a rugged and reliable system, but unfortunately the originals are aging and let's face it, the parts store replacements are less than reliable. Just ask me how I know.
One popular and cheap ignition option for Ford enthusiasts is using a GM 4-pin HEI ignition module to trigger the coil. Though it is considered high blasphemy by some in the Ford community, the idea is almost as old as the technology itself. Just do an internet search. It is all done while retaining the Duraspark distributor and using any number of aftermarket coils. Many use the later Ford TFI coils. The HEI runs off of 12 volts, and when using an aftermarket or TFI coil there is no ballast-resistor needed. Users of this setup tell tales of a stronger spark, better starting, and smoother idle. The big plus is that the HEI module costs 15-20 bucks and a spare can sit in your glove box.
When my parts store Duraspark box died, I decided to try out an HEI module instead of replacing it (again). I saw on a few different websites where guys have gutted an old Duraspark case and used that as a mount and heat-sink for the HEI module. It keeps a clean, stock-looking appearance while improving your ignition somewhat. In my searching, I found this to be the subject of a few message boards out there and decided to compile the info in one place while I did this conversion on my own car, a 1985 GT Mustang.
First, you'll need an old Duraspark box. Please don't gut a working stock one, as that would be true blasphemy...even to me. The end result you're looking for is an aluminum box that bolts in place of the Duraspark and serves as a protective cover and heat sink for the HEI module. A crappy parts store replacement that has already failed will be ideal.
Here is a parts store, presumably made in China, Duraspark box
In order to “gut” the internals of the box, you need to bake the box. Yes, just cook it in the oven. The circuit board is entirely surrounded by a potting compound that is poured into the box where it dries around the board. This forms both a moisture-proof and impact resistant mold around the electronics...pretty cool really. This is what you see on the back of the box-- that rubbery-feeling stuff. In its dry form it is nothing short of a bitch to get out of there, but once you bake it (I did 350F for about 20-25 minutes) it comes loose from the aluminum case and you can pry it right out.
This is what comes out when you “bake” the Duraspark box. I used aluminum foil both under and on top just to be safe in case anything got too gooey. It didn't even stink up the kitchen. This pic might as well have been snapped mid-toss to the trash bin.
This is the empty case. The finned aluminum creates a problem for a heat sink, as it really needs to be a flat surface. That is an easy problem to solve, though. Keep reading.
Once that is done, all you need to do is mark placement of the HEI module in the box and figure out where to drill mounting holes. I took the easy way out and drilled all the way through, using 6-32 machine screws and nuts. I also cut up a flat piece of aluminum I had in the shop to use as the actual heat sink to go between the module and the finned surface of the case. You must have some sort of heat sink to mount the HEI module to as it gets very hot. You also have to use a quality heat sink compound between the surfaces to aid in the heat transfer.
Here's how I aligned the HEI module in the case. As stated, the finned surface won't do.
The case with machine screws in it and aluminum plate mounted.
This is the heat sink compound I used from Radio Shack
HEI module mounted up. I used the Radio Shack compound between both the plate and case, and module and plate.
Wiring the HEI to the distributor and coil is pretty straight forward. The HEI module has 4 pins, B&C on one end, W&G on the other. Pins B, C, and W are .25” blade style while G is .187” blade...so shop accordingly for terminals. There are only 5 wires involved. 2 to the coil (one is ignition source), and 3 to the distributor. You'll need a 12 volt ignition start/run source that actually puts out 12 volts, ie. doesn't go through the ballast-resistor. Another wire you may want to hook up from the car is the tach wire (if you have one).
Here's a simple diagram of how to wire it.
The HEI ignition wired up inside the Duraspark case. I have not wrapped the wires yet. I used Weather Pack terminals to allow quick disconnect/removal from the rest of my “harness” to the distributor and coil.
I manged to use a small scraper to remove (and not destroy) the rubbery backing on top of the old circuit board. It made a nice protective backing for the new setup.
With all the wiring done and the “box” mounted up, I turned the key and it fired right up. I have an AutoMeter tach and it worked fine off of the coil (-) as usual. I'm not going to address the whole “well, it feels like it pulls harder” or “yeah, it turned over with just a bump of the starter” claims of doing this conversion. I will say that it is at least as good as the Duraspark for the kind of driving I do. Remember, you will retain the same limitation of no rev-limiter as with the Ford system. I'm quite happy with the simplicity of it all, and the ease of carrying a spare just in case. Now that I have done such a blasphemous act, I will take my leave...happy in the fact that I have my car running again.
The Final Product