Ford 1970's ignition modules 101:
Breakerless ignition showed up in North American Ford vehicles in 1974. The first ignitions were not called DuraSpark; they were referred to as breakerless ignitions. The modules had a green wire strain relief with two harnesses, one connector with 3 pins, and one with 4 pins. I mention these modules just to make you aware of them; the Duraspark modules are more desirable for ignition swaps.
In 1977 Ford began referring to the breakerless ignitions as DuraSpark ignitions, and released two modules simultaneously; the DuraSpark I for California, and the DuraSpark II for the rest of North America. The DuraSpark I module was more sophisticated in its design, more expensive to produce, and capable of producing a stronger and more consistent spark under varying engine load conditions. Ford found this necessary to meet California emissions standards in 1977. Both DuraSpark modules featured spark retard when starting the motor. I am uncertain if that feature was incorporated into the earlier module. Both DuraSpark modules had two harnesses, one with a two pin connector, and one with a 4 pin connector, 6 wires in total. The earlier module with the green strain relief had 7 wires; it had a blue wire that was not carried over into the DuraSpark modules.
The DuraSpark I module had a red wiring strain relief. It was a very sophisticated design, sensing & controlling the current in the coil primary as a means of controlling the current and spark quality produced in the coil secondary. It was only installed in California cars; therefore many hot rodders were not and are still not aware of its existence, or the difference between it and the DuraSpark II module. In 1978 Ford found it could meet California emissions regulations with the less expensive DuraSpark II module in all of its motors except the 302 V8. So California cars with the 302 V8 received the DuraSpark I ignition, and California cars equipped with all other motors got the DuraSpark II ignition like the rest of North America. The same applies to 1979 California cars. 1979 I believe was the last year DuraSpark I was installed in any Ford car. Worthy of mention is the safety feature built into the DuraSpark I module; if the distributor rotor is not turning, the module shuts off current to the coil primary, the ignition cannot fire, so it is safe to work on the ignition without fear of being shocked. NOT SO with any other Ford module. Disconnecting the wrong wire on any other Ford module while you are working under the hood can result in triggering the ignition and shocking you with enough current to stop your heart.
The DuraSpark II module introduced in 1977 has a blue wiring strain relief. It is not much different from the early module with green wiring strain relief. Its reliable and provides a nice spark, but it is less sophisticated than the DuraSpark I. Current in the coil primary is controlled just like it was with a breaker point ignition, with a ballast resistance in line with power to the coil. To provide a hotter spark while starting the motor, there is a second wire supplying the coil that bypasses the ballast resistor to provide full battery voltage when the ignition key is in the start position. This module is the most common module for retro-fitting breakerless ignition to early cars, simply because many people are not aware of the existence of the DuraSpark I module, or they are not aware of the differences. Its also worth mentioning this module can kill you while you are working on the motor unless precautions are taken to avoid an accidental discharge of the ignition!
After 1977 Ford developed other ignition modules with white, brown or yellow wiring strain reliefs that have other features that are unneeded or even undesirable for retro-fitting breakerless ignition. They are designed to work with input from sensors, such as knock sensors, altitude sensors, MAP sensors, etc, and they are designed to control spark timing based on those inputs, to compensate for knock, altitude, engine load, etc. Or they are designed to work in conjunction with Ford's early "EEC" engine management systems. Some of these modules were called DuraSpark II, others DuraSpark III, and beginning in 1981 there was also the "universal" module.
Suggested Shopping List
(1) Rebuilt & recurved Duraspark distributor (sourced from a 1977 – 1982 351M, 400 or 460)
(2) Duraspark I module (reference a 1977 – 1979 California vehicle with a 302 V8 at the parts counter).
(3) Duraspark compatible coil: Accel #140207, MSD #8205 (MSD #8200 or #8202 are OK as well)
(4) Painless Wiring #30812 Duraspark wiring harness
(5) Ford Racing Perf Parts 9mm plug wires #FMS-M-12259-M301
(6) Autolite Racing #AR24 plugs
Installation Notes:
1. When purchasing a module for your car, buy it from Ford! The Ford manufactured modules are very reliable. Reliability of the aftermarket modules is hit or miss.
2. DuraSpark modules use a high voltage coil made specifically for DuraSpark ignition. Use the Ford DuraSpark coil or DuraSpark specific replacement such as Accel part number 140207 or MSD part number 8205. Do not use a breaker point ignition coil.
3. DuraSpark coils are oil filled and must be mounted oriented vertically, with the top higher than the bottom. They must not be mounted lying horizontally.
4. Painless Wiring sells a wiring harness for DuraSpark II conversions. It should be applicable for DuraSpark I conversions too, as there is very little difference in the wiring between the two ignitions. It is available at Summit Racing under part number PRF-30812, or JEGS under part number 764-30812.
5. DuraSpark distributors for the 351C will be sourced from a 1977 through 1982 Ford vehicle equipped with a 351M, 400 or 460 cubic inch V8.
6. A ballast resistor suitable for DuraSpark II installations is available from NAPA Auto Parts, NAPA part number ICR23 (1.2 ohms).
7. The Duraspark distributor is not the best choice for sustained high rpm use even in perfect working condition, its propensity for spark scatter is infamous. For operation above 6000 RPM the distributor should be converted to centrifugal advance only, the vacuum advance plate should be locked out, the weight pins strengthened and the pick-up indexed with the rotor. Ford SVO once offered a distributor like this, part number M-12127-A301. Any Ford distributor should be expected to wear out within 50,000 miles. For the record the Moroso #72202 is the best distributor money can buy. It is a 351 Windsor distributor so the distributor drive gear will require replacement to make it fit the 351C.
8. The Duraspark modules are very reliable, good for over 100,000 miles and at least 10 years of operation. The Duraspark I module is one of the most sophisticated modules ever offered in a production automobile but it was not designed for operation over 6000 RPM. I’ve never read any reliable information about the high RPM performance of the Duraspark I module, I cannot comment on what happens to its performance at high engine speeds. However, to keep things in perspective, the Duraspark I module is most likely to perform better at high RPM than a dual point distributor. The Duraspark modules have one feature that is very desirable for a high performance motor; they retard the spark advance during starting. They also lack one feature that is very important for performance driving, an engine speed limiter. Ford SVO once offered a module with a programmable engine speed limiter, part number M-12199-A301 (or M-12199-B301 which included electronic tach drive capability), for use with ignition coil #M-12029-A302 and wiring harness #M-12071-A301.
9. If a better ignition is desired, then my recommendation is to look beyond an ignition employing a distributor and consider a distributorless ignition such as the Ford EDIS-8 or Electromotive Engine Controls XDI system. Distributor ignition systems are as obsolete as the slide rule and typewriter.