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?
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?"
"How much?" is the maximum amount of mechanical advance the mechanism delivers. Performance engines like a certain amount of advance through most of the upper operating range, usually referred to by tuners as the amount of "total" advance. Typically, Mopar engines respond best with between 35 and 40 degrees of total advance, depending on the combination. The "total" advance is the sum of where the timing starts, the initial timing setting at idle, plus the amount the mechanical advance mechanism adds in. Stock engines don't need a lot of initial advance to provide smooth idle and good off-idle acceleration without detonation, and typically run initial timing near TDC (top dead center). By contrast, a hot-cammed street sweeper often needs 15 degrees or more just to idle cleanly. If both engines run best with 35 degrees "total," the stocker, starting at 5 degrees initial, will need way more mechanical advance in the distributor mechanism to get to 35 degrees "total" than would a hot mill idling with 15 degrees initial (10 degrees more, to be exact). Setting up the amount of mechanical advance comes down to figuring out what the desired (or required) initial timing setting needs to be, then limiting the mechanical advance to the value required to achieve the optimal total setting.
The second part of the mechanical advance mechanism to consider is the rate at which the advance comes in. This is the function of the advance springs. Stiffer springs bring the advance in more slowly, while lighter ones let the advance come in sooner. The springs answer the question of "when?"
There are many theories on what is best, but often the mistake is to go too quick with the advance rate. Some springs are so light that the mechanical advance will start to swing in at idle rpm, which can cause real driveability problems, erratic idle, or the engine nosing over and stalling when the rpm drops and the engine suddenly loses a fistful of timing at idle. Detonation can also result from too much timing too fast. Considering that in a performance application the engine flashes against the converter over 2,500 rpm, or much higher in a racier car, a super-quick advance curve gains little, if anything. On the other hand, the factory setup feeds in additional timing rather slowly in the high-rpm range-well beyond the point where the "total" should be. What is best for your car depends upon the combination, although a rule of thumb is to have the advance start a few hundred rpm over idle speed and reach full advance by 2,500 rpm or so in a typical street car-higher if it detonates. The curve is easy to map with a timing light and tach.
While the distributor handles the switching and routing chores, it has little to do with the spark energy. This is the primary function of the control unit and ignition coil combination. The control unit affects the amount of primary energy and saturation time to the coil, which influences its output. There are a variety of Mopar Performance ignition-control units available for specific levels of performance and rpm. Ultimately, it is the coil's job to generate the voltage that goes to the spark plugs. A high-performance ignition coil is worth considering in a performance ignition, particularly as demands of rpm and compression go up. Be aware that some race coils are not suitable for prolonged low-rpm street use.
Our recently acquired '68 Barracuda's 318 was sparked by a stock points ignition. Suffice to say that it was one of the first items on the hit list in upgrading this machine. We had stacks of the stock production distributors on hand, and decided to dissect one for a full performance rebuild. Eventually, we plan on beefing up this 318 with a set of ported iron heads, and a big cam and induction combo, so the advance was modified accordingly. Even in a stock-for-stock exchange, electronic ignition offers the kind of reliable, long-term performance that a points ignition can never match.
We had fun and saved some dough tricking out our salvaged sparker, but if you'd rather just bolt one on ready to go, Mopar Performance offers these gems brand new as conversion kits, including the wiring harness, ballast, and control box, or as individual components at a reasonable cost. Either way, when it comes to Mopar ignition systems, the factory stuff packs a lot of performance.
