The Mopar electronic ignition system introduced in 1972 was cutting edge in its day. While everyone else was using the ancient points-type ignitions, Chrysler scooped 'em all with the electronic unit.
A magnetic pickup coil in the distributor is triggered by one of eight tips of the rotating reluctor, sending the signal to the ignition control box to fire the coil via internal transistorized electronics. It was a giant leap forward over points, which were nothing but mechanical switches handling the primary voltage of the ignition system. That was a long time ago, and the Mopar electronic ignition concept has changed little since then. As a basic ignition-triggering system, however, it just doesn't get any better. Reliability, accuracy, and high spark energy were all hallmarks of the original system, and over the years, the system has been refined with better and more modern electronics.
Do you get the feeling that we kind of like Chrysler's electronic ignition?
Our points-equipped '68 'Cuda...
Our points-equipped '68 'Cuda was ripe for a low-buck ignition upgrade, so we reached into our trough of cores and harvested a factory small-block electronic distributor for a high-performance build.
Chrysler's factory electronic...
Chrysler's factory electronic distributor was state-of-the-art in its day, and as good as it gets for high-performance street applications. The plan was to strip and rebuild this tired distributor and modify it with performance upgrades in mind. Step one is removing the reluctor, which just pries up with a couple of screwdrivers. Take care not to lose the roll pin, which indexes it to the shaft.
The magnetic pickup assembly...
The magnetic pickup assembly and vacuum advance unit can come off next. They come undone by removing the four screws on the outside of the housing and unhooking the vacuum canister's advance arm from the plate.
The Mopar electronic system is comprised of three key components, each of which contributes to the performance of the system. First, of course, is the distributor. The distributor's function is simply to act as a switch, sending a signal to the control unit to fire the coil, and routing the spark energy to the plug wires.
Now that we've already gone through the three major players-the distributor, control unit, and coil-let's have a closer look at the distributor.
As a switch, the distributor's first function is to tell the control unit when it's time to fire. This is timing, and the event occurs when one of the eight tips of the rotating reluctor comes in line with the magnetic pickup unit. Complicating things is the fact that timing has to vary under various running conditions, so the distributor has provisions for altering the timing. This function is handled by the advance mechanisms enclosed in the distributor.
Two systems are employed to alter the timing: the mechanical advance and the vacuum advance. Since the signal to fire is given whenever the reluctor and pickup line up, varying the timing has to be accomplished by changing the relative positions of these two components. The mechanical advance mechanism uses centrifugal weights controlled by springs to rotate the reluctor forward relative to the pickup. The vacuum-advance mechanism uses a vacuum diaphragm to pull the pickup unit, mounted to a pivoting plate, back relative to the reluctor. In a high-performance application, the mechanical advance is the most relevant, since at wide-open throttle, there's no vacuum, so you won't see any vacuum advance. Performance-tuning a distributor mainly involves getting the mechanical advance optimized. There are two characteristics to a conventional mechanical advance system, which boils down to "how much?" and "when?"
To remove the shaft assembly,...
To remove the shaft assembly, the roll pin at the retaining collar is punched out with a pin punch. Don't lose the thrust washer shims under the collar.
The shaft assembly with the...
The shaft assembly with the centrifugal-advance mechanism will now pull out of the housing.
The length of the slots on...
The length of the slots on the mechanical advance plate (pointer) determines how many degrees of mechanical advance the distributor will deliver. In a high-performance application, it's typically desirable to shorten the amount of mechanical advance so more initial advance can be dialed in while keeping the total (max) mechanical advance constant.
The shaft is comprised of...
The shaft is comprised of two pieces, which must be separated to disassemble the mechanical advance mechanism. A small hard-to-get-to wire retainer inside the upper shaft is spread open while the advance plate is levered upward.
Most factory advance plates...
Most factory advance plates are stamped on the underside with the number of degrees of advance they deliver in distributor degrees. The plate in our distributor was stamped 15, meaning it provides 15 distributor degrees at full advance, which equates to 30 degrees at the crank. In our case, if we set up our small-block for full mechanical advance in the neighborhood of 35 degrees total-typically about ideal for max power-we would have only 5 degrees BTDC at idle, much less than ideal for a big-cammed small-block.
We wanted to run about 15...
We wanted to run about 15 degrees of initial advance (at idle), and have the engine timed for 35 degrees total (at full mechanical advance). That means the distributor needs to provide 20 degrees of mechanical advance instead of the 30 in the factory plate. To shorten the mechanical advance, the slots need to be shortened. The slots in our plate measured .480 inch long. We zapped the ends of the slots with a MIG welder to get some of the advance out of our plate.