Earlier this year Daniel Stern wrote an article that looked at General Motor’s HEI ignition system. The GM HEI ignition is well known among car enthusiasts, but it was far from the first attempt at electronic ignition by an American auto maker. Throughout the 1960’s GM, Ford and Chrysler all experimented with different versions of electronic ignitions, or as they were commonly known then, transistorized (or just transistor) ignition. Just as the transistor revolutionized radios, it did the same with automobile ignitions. Nevertheless, the road to a modern electronic ignition was a storied one. This three part series will examine the evolution of electronic ignitions used by American manufacturers from the 1960s until they became the industry standard in the 1970s.
Before we go into detail on the electronic ignition, it’s important to go over the basics of how a breaker-point operated inductive ignition system works. What exactly is that mouthful? This is just a big elaborate name for the breaker-point ignition system that most cars used up until the 1970s. A breaker-point ignition system is comprised of several key components. These include the distributor, the ignition coil, a resistor (either a ballast or resistor wire), the spark plug wires and of course the spark plugs. The ignition has a primary and secondary circuit. These two circuits meet inside the ignition coil. A coil, which is pulse transformer, consists of primary and secondary windings. The primary windings have about 200 to 300 turns of larger wire, while the secondary windings have about 20,000 to 30,000 turns of very fine wire. In the center of the coil is an iron core.
So how does it all work? When the breaker points in the distributor are closed, the current from the battery flows through a resistor which reduces the voltage from 12 volts to approximately 8 volts. These 8 volts go through the primary windings on the ignition coil then through the closed breaker points and finally to ground, completing the circuit. When the points are closed there is no spark at the spark plug.
It is important to remember that when electrical current flows through the primary windings in the coil, it creates a magnetic field. So when the breaker points open, there is no longer a direct connection for the current to flow to ground. This causes the magnetic field in the coil to collapse very rapidly. The magnetic flux lines cross from the primary to the secondary windings, and this induces a much higher voltage (about 25,000 volts) into the secondary windings. The high voltage current induced in the secondary windings (about 25,000 volts) flows from the coil to the distributor. The distributor then routes the current to the correct spark plug wire and produces the ignition spark at the spark plug.
It should be noted that the time to charge a coil to full saturation (dwell time) is fixed. However, as engine RPM increases, the time between the points opening and closing is reduced. This means at higher RPM, the coil does not have enough time to become fully saturated. This results in a reduced ignition output, as the lower primary voltage, creates a lower secondary voltage. Some manufacturers got around this by using dual points, which increased the dwell time at higher RPM.
The basic breaker points ignition system works reasonable well, but it does have a number of serious limitations. Firstly, the current must all pass through the breaker points. To obtain reasonable life out of a set of points, the amperage must be limited, typically to somewhere around 3.5-4.0 amps. Increasing the current beyond these levels causes a rapid degradation of the breaker point life. This is the reason why the voltage is reduced to about 8 volts by a ballast resistor or resistor wire (the current is reduced along with the voltage). This reduced power allows for a reasonable compromise for breaker point life and ignition energy. The only exception is when the engine is starting the resistor is bypassed while the starter is engaged. This allows a full 12 volts to be delivered for the coil for maximum ignition energy. This brief period doesn’t have a major effect on points wear. Secondly, each time the breaker points open and close, the arcing that occurs causes point wear. In addition, the rubbing block on the distributor also wears. This wear slowly reduces the accuracy of the points over time, hence the need for frequent tune-ups. Finally, high RPM use causes “point bounce” which can cause misfires and poor ignition performance.
Due to the limitations and the high maintenance of points ignitions, in the early 1960’s American auto manufacturers started to look at ignition solutions that used transistors in place of the points. The use of transistors for the switching mechanism had the potential to be a much faster and more efficient the rather basic mechanical breaker points switch. General Motors and Ford were the first of the big three to experiment with a transistorized ignition with both having systems available for the 1963 model year.
Ford offered a transistorized ignition system on the Ford Thunderbird and 427 powered vehicles for the 1963 model year. This system was the most basic form of electronic ignition and it actually still used a set of breaker points. So, exactly how does an electronic system with a set of points actually offer any improvements? Well, the points remained in the ignition as the trigger for the coil, however, they were now essentially used like a relay.
A relay allows a low current circuit to trigger a higher current circuit. You can use a switch on a low current circuit and connect it to a high current circuit through a relay. Once that switch closes, the relay will “trigger” the high current circuit. Consequently, a relay allows a switch with low current to control a high current circuit without having the high current flowing through it. For the Ford transistor ignition, the breakers points were like that low current switch.
The Ford transistor ignition added a new device called the amplifier. An amplifier is just an old name for an ignition module and it contained the super-fast switching transistors inside. The amplifier is connected between the ignition switch (power supply) and the ignition coil, and that supplies the power to the coil. It is the “master switch” and controls when the coil is in saturation or collapse. Basically it does the job of the points in a conventional ignition.
The way it works is the points are used to trigger the amplifier. The amplifier does not know when to switch the coil from saturation to collapse. This job remains that of the breaker points and the rubbing block. The breaker points still open and close based on the distributor rotor’s location. When the points open, this triggers the amplifier which will use transistors to open the primary circuit between the coil and ground. This causes the secondary circuit be induced, and the spark plug will fire. Since the breaker points are no longer supplying power to the coil, Ford reduced the voltage to 3 volts (which also reduces the current) through the use of a resistor. The low voltage significantly reduced the points wear resulting in a substantially longer point life. This means the points will be more accurate for a longer time and require less adjustment. The amplifier allowed for an increase in primary circuit voltage and faster switching of the coil, which increases the ignition’s energy and high RPM performance.
Despite the lower maintenance requirements and greater overall ignition energy, the Ford transistor ignition was not a popular option. It remained standard on the 427 until 1967, but didn’t even last that long on the Thunderbird option list leaving after 1966. I explained the basics of the Ford Transistor ignition above, but it was actually a fair bit more complex, as Ford also incorporated a cold start mode. While it may have been somewhat beneficial to a person racing a 427 Ford, to the average owner the lower maintenance was probably the only big advantage. However, this rather costly option was far more complex than a conventional ignition and when it did break, it was far more costly and difficult to repair.
The Delco-Remy division began to design a new ignition system around 1962 with the goals to improve reliability, increase component life, and require less maintenance. The result was the Delcotronic Transistor Controlled Magnetic Pulse Ignition. This Delco transistor ignition was very advanced for its day and unlike the Ford system, it completely eliminated the breaker points from the distributor. Instead of a rubbing block which open and closed a set of mechanical points, this new ignition system used a magnetic pulse generator (magnetic pickup) inside the distributor to trigger the transistors in the amplifier. An iron timer core replaces the rubbing block and it has the same number of equally spaced projections (or vanes) as the engine has cylinders. The pickup assembly has a ceramic permanent magnet, a stationary pole piece and the pickup coil. The stationary pole piece also has equally spaced teeth, one for each cylinder.
As the distributor shaft rotates and the teeth of the stationary pole piece and the timer core approach alignment, the output voltage increases as positive polarity. As the teeth pass through alignment, the output voltage abruptly reverses and passes through zero to produce a negative polarity. As the voltage crosses the zero point, it signals the amplifier to turn off the coil’s primary current, which induces the secondary circuit and fires the spark plug. This voltage signal that is being created is really just a small AC current. The amplifier’s solid state technology determines when to turn the primary circuit back on. And as the teeth on timer core and pole piece move further away, a positive voltage is created again, stating the cycle over. The pickup coil’s signal provides a highly accurate switching point for accurate spark timing that is not affected by temperature or vibration.
Unlike the Ford system, this ignition had no mechanical parts to wear out. The use of a magnetic pulse generator to trigger the amplifier was a big step forward towards modern electronic ignition that most electronic distributor based systems would eventually adopt. Delco-Remy offered this ignition as an aftermarket conversion kit as well, but the kit used breaker points to trigger the amplifier box, and it operated just like the Ford system. This made the system less costly and much simpler to install as the old distributor could be reused.
The Delco transistor ignition was first introduced as an option for Pontiac in 1963 on the 389 and 421 engines. It was added to the option list for Corvette in 1964 under the option code K66. It was available on other high-performance Chevrolets during the 1960s but it was most commonly found on Corvettes. It was the most common early electronic ignition. Although a complex and expensive ignition, the Delco system was an excellent ignition for high-RPM engines and was mandatory on several high performance engines. Racers and high performance enthusiast frequently used or retrofitted this ignition because it performed so well. 1971 was the last year the Delco transistor ignition was available, last being used on the Chevrolet LT1 and LS6 high performance engines. However, it too was plagued with the similar low cost-benefit of the Ford ignition, offer few big advantages for the average driver, while being more costly, far more complex in design and much more difficult to repair.
Although Chrysler would be the first of the big three to adopt electronic ignition across its vehicle line in 1973, it didn’t have much to offer prior to that. In 1966, Dodge offered a Motorola Transistor ignition system for its fleet models only. The big appeal of transistor ignition for fleet operators in was the reduced maintenance costs. Motorola’s ignition was an aftermarket system, it was designed to be easily retrofitted to other cars. So, like the Ford system and the add-on Delco system, the Motorola system also used points to trigger an amplifier box and functioned in the same manner.
These earliest versions of electronic ignition had some of all of the following objectives: Higher secondary (spark plug) voltage, reduced maintenance and better high speed operation. Ford and General Motors used these ignitions on high performance vehicles, in particular the high RPM engines. This is where the transistor ignition had a clear advantage over the typical breaker-points setup, with the fast switch transistors allowing for more coil saturation time. Dodge, on the other hand, offered the transistor ignition for fleet use only which was obviously an attempt at reducing maintenance costs rather than increase high RPM performance.
Although these electronic ignitions offered improvement as over breaker points ignitions, there were a number of significant drawbacks. The cost of the ignition system was far higher, there were more components, they required specialized parts such as special ignition coils, and they were far more complex. The complexity meant that fixing these systems was required more skill and knowledge. And if the system did fail, it usually did so suddenly, and parts were often not readily available.
In the next installment I will continue to cover some additional early electronic ignition systems and the first mainstream electronic ignitions used by the Big Three.
A special thanks to Daniel Stern for supplying some of the research material on vintage ignition systems.