Whereas the EJ20G engine had a Mitsubishi TD05 turbocharger, the EJ20K engine was fitted with a smaller capacity Mitsubishi TD04 turbocharger. The TD04 turbocharger was introduced due to a change in WRC rules that manufacturers no longer had to produce at least 5000 vehicles of a production model for competition eligibility. The lower-inertia TD04 turbocharger is understood to provide slightly greater boost pressure than the TD05 which is understood to provide 11-12 psi. As a result of the TD04 turbocharger, peak power of 155 kW occurred 400 pm earlier (i.e. at 5600 rpm), while peak torque increased by 20 Nm to 290 Nm (still at 4000 rpm).
To prevent excessive boost pressure, which could cause knocking and heavier thermal loads on the pistons, the EJ20K engine had a wastegate valve. Once boost pressure reached its maximum, the wastegate valve would open so that part of the exhaust gas would bypass the turbine and flow into the exhaust pipe.
The turbocharger was lubricated by the engine oil and used full-floating type bearings to form lubrication films. Furthermore, engine coolant from the coolant drain hose (under the engine cylinder head) flowed to a coolant passage in the turbocharger bearing housing. After cooling the bearing housing, the coolant flowed into the coolant filler tank via a pipe.
The EJ20K engine had an air bypass valve to prevent the suction noise that can otherwise occur when the throttle valve is suddenly closed and causes a sudden rise in air pressure between the turbocharger and the throttle body. The air bypass valve was actuated by the vacuum created by the closure of the throttle valve and allowed the suction air to bypass the turbocharger and flow upstream, thereby lowering the pressure in the air passage.
Since the turbocharging process increased the temperature of the intake air, it was then passed through an air-cooled intercooler that received cooling air via the bonnet duct. The intercooler was mounted on top of the engine and cooled the intake air to increase its density.
Injection and ignition
The EJ20K engine had pentroof combustion chambers which featured a wide ‘squish’ area. The EJ20K engine had multi-point fuel injection via gallery-type (or side-feed type) fuel injectors. For the GC/GM Impreza WRX, the EJ20K engine used a hot-film type mass air flow sensor to calculate intake air volume. The injection and firing order for the EJ20K engine was 1-3-2-4.
The EJ20K engine had centrally mounted spark plugs and a compression ratio of 8.0:1. Furthermore, a piezo-electric type knock sensor installed on the cylinder block which converted knocking vibrations into electric signals.