Two types of ignition systems are used on your Subaru. A conventional system using breaker points and condenser was used from 1970 through 1976. In 1977 a breakerless ignition was introduced on the California models, the others continued to use the conventional ignition. All models from 1980 use the breakerless ignition.
Both systems employ a distributor which is driven by the crankshaft, a high voltage rotor, a distributor cap, spark plug wiring, and an oil-filled conventional type coil.
The two systems differ in the manner in which they convert electrical primary voltage (12 volt) from the battery into secondary voltage (20,000 volts or greater) to fire the spark plugs.
In the conventional system, the breaker points open and close as the movable breaker arm rides the rotating cam eccentric, thereby opening and closing the current to the ignition coil. When the points open, they interrupt the flow of primary current to the coil, causing a collapse of the magnetic field in the coil and creating a high tension spark which is used to fire the spark plugs. In the breakerless system, a distributor shaft mounted reluctor rotates past a magnetic pick-up coil assembly causing fluctuations in the magnetic field generated by the pick-up coil. These fluctuations in turn, cause the ignition control unit to turn the ignition coil current on and off, creating the high tension spark to fire the spark plugs. The ignition control unit electronically controls the dwell, which is controlled mechanically in a conventional system by the duration which the points remained closed.
Both the conventional and breakerless ignition systems are equipped with dual advance distributors. The vacuum advance unit governs ignition timing according to engine load, while the centrifugal advance unit governs ignition timing according to engine rpm. Centrifugal advance is controlled by spring mounted weights contained in the distributor, located under the breaker point mounting plate on conventional systems and under the fixed base plate on breakerless systems. As the engine speed increases, centrifugal force moves the weights outward from the distributor shaft, advancing the position of the distributor cam (conventional) or reluctor shaft (breakerless), thereby advancing the ignition timing. Vacuum advance is controlled by a vacuum diaphragm which is mounted on the side of the distributor and attached to the breaker point mounting plate (conventional) or the magnetic pick-up coil assembly (breakerless) via the vacuum advance link. Under light acceleration, the engine is operating under a low load condition, causing the carburetor vacuum to act on the distributor vacuum diaphragm, moving the breaker point mounting plate (conventional) or the pick-up coil assembly (breakerless) opposite the direction of distributor shaft rotation, thereby advancing the ignition timing.
The turbocharged engine employs a vacuum advance/pressure retard type of diaphragm. It functions in the normal way until the driver applies enough throttle so that the turbocharger supplies air/fuel at more than atmospheric pressure. At this point, ignition timing is retarded past the normal timing progressively as manifold pressure rises. In addition to the knock prevention function of the pressure retard, an electronic pickup senses engine knock type vibration in one cylinder head and retards the timing electronically still further, should knock occur.
Some earlier models use vacuum advance at normal running throttle openings, but combine it with manifold vacuum retard at idle. The retard mechanism is contained in the rear part of the vacuum diaphragm chamber. When the engine is operating under high vacuum conditions (deceleration or idle), intake manifold vacuum is applied to the retard mechanism. The retard mechanism moves the breaker point mounting plate (conventional) or pickup coil assembly (breakerless) in the direction of distributor rotation, thereby retarding the ignition timing. Ignition retard, under these conditions, reduces exhaust emissions of hydrocarbons, although it does reduce engine efficiency somewhat.