I converted my original '69 distributor to use an HEI module back around 1984. Great plug life, and smooth operation at high loads and RPMs. The module is mounted to a small heat sink under the shielding.
I think we would all like to see some kind of pictures/write up of that if you have the time.
Well, I've struck out trying to find some pictures on the web of the parts I'm using in my distributor, and I'm just too lazy to pull the shielding and cap/rotor off the distributor, so I'll try to describe what I have, and then take some pictures down the road when I put the new engine in.
Some history. In 1974 I bought an electronic ignition conversion kit (Stinger Ignition, IIRC), made by Hayes. The kit included an extruded, 8 pin reluctor wheel that pressed over the distributor points cam, a pickup coil (resembling some of the old Chrysler type pickup coils), and a potted up PCB control box.
The system worked fine for a half dozen years (and most of that was vehicle storage time while I was in college), and then I started to notice drivability/misfire issues under load and higher (> 3500) RPM. I pulled the whole system out of the car, and brought it into work one day to measure what was going on. The waveform out of the distributor looked steady and predictable over RPM. The output of the box, however, was erratic at times. A component on the PCB (printed circuit board) was apparently breaking down. As the control box was all potted up, there was no way of probing inside to see where the faulty component was. That was the start of my distrust of underhood ignition systems that used PCBs.
So, what to do? I wasn't thrilled about shelling out the money to buy another control box, but I didn't want the car sitting in the garage unable to run, either. I did notice while testing the distributor that the pickup coil signal looked essentially identical to what an HEI distributor pickup coil puts out. So, what about using an HEI module instead? They're cheap, available, and stone reliable. I hooked up a four terminal HEI module to the distributor and coil on the lab bench, and shazam, it ran like a champ!
I went home and reinstalled the distributor, and with a small custom wiring harness, I installed the module and mini heatsink inside the chrome ignition shielding. As the HEI module automatically controls the primary current level and dwell time, I disconnected the stock ballast resistance wire and ran a new wire to the fuse box. (In hindsight, I could have left the ballast wire still hooked up, as it wouldn't affect the operation of the ignition.)
The car fired right up. I drove it around, and no more misfire at higher loads and RPMs.
Now, the dry engineering stuff: The stock coil specs were roughly 1.9 ohms, and perhaps 8 mH inductance on the primary windings. A stock HEI coil has about half an ohm (.5) resistance, and about 6-8 mH of inductance. This means that the HEI coil will charge very fast (not requiring much dwell time/angle), while the stock coil, due to its higher resistance and inductance, would require longer dwell times. Looking at the dwell signal with a 'scope showed that the laws of physics were still in effect on my engine. The oilcan coil required more dwell time/angle to charge up to what the HEI module was trying to attain. The HEI module is calibrated to modify the dwell under all conditions to try to draw 5.6-5.8 amps in the primary windings. This is easy to do with a low resistance, lower inductance coil, but more difficult with a higher resistance and inductance coil. The module "compensates" in this case by increasing the dwell time/angle to allow more time for the primary current to charge the coil. So, the net result is, I have additional primary current (and energy, versus the original points setup) over the complete RPM range due to the variable dwell circuit increasing the dwell at higher RPMs (40 degrees at higher speeds).
"Optimizing" the circuit would entail changing the coil to a lower resistance, lower inductance unit to allow the current to ramp up faster. I've got a variety of "pre-driven" coils gathering dust on the shelf waiting for a opportunity when I can devote time to doing some actual electrical measurements on the engine, and determine which coil I want to swap in to replace the original coil.
Given that both engines I've had in this car have run smoothly with this setup all these years, I haven't had any feelings of urgency on this, and I've just spent my free time trying to update the chassis, instead.
Well, I'm getting a blister on my typing finger, so I'll stop for now, Please feel free to ask any followup questions.
