Ford Truck Enthusiasts Forums - Duraspark II ignition with GM HEI module?

the hei conversion is just if you are converting from efi to carb and you don't have the know how or the time and patience to rewire the hole duraspark ignition box. do you actually have your truck at 4000 rpms when you drive it. anyway i am going to do this soon with my tbird. here is the info that i got for the conversion. this is exactly what the ford aftermarket hei distributors are made. A lot of guys when converting from fuel injection to carb use the Duraspark II system. This requires pulling out the TFI distributor and coil and replacing them with the Duraspark distributor, coil, module, starter solenoid, and ballast resistor. Yeah its complicated and can get spendy but what else is there besides an even spendier aftermarket system? Well, if you don't mind a little cross breeding you can use a GM HEI ignition module. All that's required is a Duraspark distributor, HEI module, and coil. Because of the dwell control and current limiting circuits of the HEI you can use just about any coil you want, including the TFI coil, and no ballast resistor is needed.
If you are converting from fuel injection all you will need to buy is a Duraspark distributor, HEI module, and some wire and connectors to hook it all up. The cheapest place to get the Duraspark distributor is at a junk yard or swap meet, usually $20 or less. Duraspark distributors came on Ford cars and trucks from the mid seventies through the mid eighties. Make sure you get one from a 302, the 351w and 351c-460 look similar but are different. You could also get a rebuilt one at the auto parts store, generally $40 or more. Make sure you get one with the right gear for your cam. Flat tappet cams use a cast gear where roller cams use a steel gear. If you look at the shank above the teeth a cast gear will be a rough cast finish where a steel gear will be a smooth machined finish. If you want a distributor with a cast gear tell the parts guy you want one for a 5-speed '84 Mustang GT. For a steel gear tell him you have a 5-speed '85 Mustang GT. If the guy at the parts store has two distributors listed and doesn't know which is which, the one with the cast gear is always considerably cheaper than the one with the steel gear.
For the module you want a GM 4-pin HEI. They came on GM vehicles from the mid seventies through the early eighties. You can buy one new at any auto parts store. Any good parts guy will know exactly what a four pin HEI module is. Although good parts guys are hard to find so you will probably need to ask for an ignition module for a '78 Comaro with a 350. Since they are so cheap you don't need to go to the junk yard. Besides, all the junk yards around here wouldn't even sell me just the module, they wanted me to buy the whole HEI distributor to get it. The module in the picture is a Car Quest #21040 and cost me $17.77. As you can see there are four pins labeled W, G, B, and C. The G pin is a 3/16" (0.187") male quick disconnect and the rest are 1/4" (0.250") male quick disconnects.
The HEI module gets pretty hot, whatever it's mounted to needs to act as a heat sink and carry heat away from it. Mount it securely to a flat metal surface away from the headers or other heat sources. There are two pins on the back of the module that you need to break off so it will sit flat. The module will come with some heat sink compound, smear it evenly over the back of the module before bolting it down. The compound aids heat transfer. You could also mount the module to a big heat sink. I bolted mine to a heat sink from a slot type computer processor. If you're a computer geek like myself then you probably have one laying around. If not then you can buy one at Radio Shack or a computer store. Again you will want to use the heat sink compound between the module and heat sink.
Once you have it mounted you need to wire it up. The B pin goes to the positive coil lead which gets power when the key is on, and C goes to the negative coil lead. The tach. also hooks to the negative coil lead. If you are converting from fuel injection then the coil is already wired for power (red) and tach. (green). Splice into these wires. Don't remove them from the coil just add some wires to them and run them to the module. On an older car you will need to bypass the ballast resistor so you get full power to the coil and module. The Duraspark distributor has a funky three pin connector. You can just plug into it with standard 3/16" female quick disconnects. The purple wire is run to the G pin on the module, and the orange wire is run to the W pin. Run the black wire to one of the mounting screws on the module. The black wire provides a solid ground connection for the module. The module must be grounded or it will not work properly.
Like I said before you can use just about any coil you want. Basically the only coils that you can't use are aftermarket coils which are for CD (capacitive discharge) ignitions only. I recommend the TFI coil, its the square looking coil used on fuel injected Fords. They are dirt cheap and work great because they were designed for the Ford TFI which is basically the same thing as the GM HEI. If the TFI and HEI modules are the same then why can't I use the TFI? The TFI distributor uses a Hall sensor instead of a magnetic pickup so the TFI module will not recognize the "analog" signal from a Duraspark distributor. However, you could hook a TFI module to breaker points since they put out a "digital" signal like the Hall sensor. No matter what coil you use you will not need a ballast resistor. The purpose of a ballast resistor is to add series resistance to limit current through the coil. The HEI actually measure the current and when it reaches a certain point resistance is added using the switching transistor to prevent excessive current draw. So basically it has a built-in self adjusting ballast resistor.
That's all there is to it. Whether you are converting from fuel injection or piecing together a weekend warrior the HEI is hard to beat. Its easy, cheap, and a great performer. You could spend five times more on an ignition and chances are you won't see much, if any, improvement.