First introduced in 1993, all Subaru Impreza car engines are derived from the EJ series engine. This engine was originally from the Subaru Legacy. In choosing the horizontally-opposed engine configuration, Subaru reasoned that a boxer engine, as such engine layouts are also called, provide several advantages over more conventional engine configurations. For one, a flat engine that is inline with the transmission minimizes body roll owing to a lower center of gravity. The boxer design itself minimizes engine vibrations because the opposing pistons counteract the induced vibration of its opposite. In its use in a front-drive vehicle, torque steer is also mitigated because equal-length driveshafts can be used. For all wheel drive applications, the manufacturer is allowed a low-profile, balanced and symmetrical drivetrain package. The overall result is a better-balanced car with exceptional agility.
The EJ series was designed by engineers from Subaru’s parent company, Fuji Heavy Industries. The team was made up of engineers Masayuki Kodama, Takemasa Yamada and Shuji Sawafuji.
All Subaru Impreza engines are water-cooled, 16-valve, horizontally-opposed 4s and are made in single- or twin-cam configurations. Other refinements include turbocharging and, in later years, Subaru’s Active Valve Control System (AVCS). It is Subaru’s version of variable valve timing technology pioneered by Honda for mass production. This system rotates the camshaft through hydraulic pressure to optimize the cam timing at any given moment. It uses crank, throttle position, airflow, oxygen and air-fuel ratio sensors to feed data to the ECU, which operates the system’s control mechanisms.
Although the most commonly used, and most popular, variants are the EJ20 (which powers the famous turbocharged 280-320 horsepower WRX versions) and the EJ25 (a 2457 cc variant which is used for naturally-aspirated applications), there is a bewildering number of variations, depending on the market (Asia, Japan, USA or Europe) and level of trim (base Impreza, Impreza WRX, Impreza WRX STI, Japan-only RA, etc.). These engines themselves are also used in light aircraft, kit cars and engine swaps into other cars. It is a popular swap into the Volkswagen Type 2 van, which is originally powered by an air-cooled horizontally-opposed four cylinder.
The lowliest EJ engine is the EJ15, which is a 90 horsepower single-cam 1.5 liter. Then there is the EJ16, followed by the EJ18 and then the EJ20, EJ22 and finally the EJ24. The under 2-liter variants were mostly single overhead cam variants, but also included the EJ22. The EJ20 found in the WRX cars were turbocharged, dual overhead cams designs that proved themselves in several years of campaigning in the World Rally Championship. It was in the WRC where Subaru gained the respect and credibility accorded its cars today as affordable and reliable road rockets. Even today you can find first-generation Impreza track cars whose engines have been modified and boosted to produce outputs in excess of 800 reliable horsepower.
Some 2005 and later Subaru Impreza engines, specially the turbo engines, use the CAN (Controller Area Network) bus, normally running at 500k baud, as the singlular input channel for speed readings.
When the speed signals are absent, the ECU will force the engine into limp home mode. This has proven to be troublesome to some people who want to use the Subaru Impreza engines in aircraft applications, engine swaps or as engine replacements for older Subaru vehicles.