Subaru EJ20K Engine

Introduction

Subaru’s EJ20K was a turbocharged, 2.0-litre horizontally-opposed (or ‘boxer’) four-cylinder petrol engine. In Australia, the EJ20K engine powered theSubaru GC/GM Impreza WRX from November 1996 (for the 1997 ‘model year’ or MY97) to 1998. For the Subaru GC/GM Impreza WRX, key features of the EJ20K engine included:

• Die-cast aluminium block and cylinder head;
• Double overhead camshafts (belt-driven);
• Four valves per cylinder;
• A Mitsubishi TD04 turbocharger;
• Air-cooled intercooler; and,
• 8.0:1 compression ratio.

Compared to the EJ20G engine which it replaced, changes for the EJ20K engine included:

• The Mitsubishi TD04 turbocharger (replacing the larger TD05 turbo);
• A new intake manifold (with the compressor inlet tunneled below);
• Larger intercooler;
• New pistons;
• New head gaskets;
• For valve actuation, shim-adjustable solid tappets replaced hydraulic lifters;
• A new ECU and boost pressure control system; and,
• New ignition coils.

Please note that this article considers the EJ20K as it was supplied in the Australian-deliveredGC/GM Impreza WRX; specifications for other markets may vary.

EJ20K cylinder block

The EJ20K engine had a die-cast aluminium alloy cylinder block with 92.0 mm bores and a 75.0 mm stroke for a capacity of 1994 cc. The cylinder block had an open-deck design to enhance cooling efficiency and dry-type, cast iron cylinder liners.

Crankshaft and pistons

For the EJ20K engine, the crankshaft was supported by five bearings that were made from aluminium alloy, while the no. 3 thrust bearing had a metal flange to receive thrust force. Furthermore, the corners of the crankshaft journals and webs, and the crank pins and webs, had a fillet-roll finish to increase stiffness.

For the GC/GM Impreza WRX, the EJ20K engine had cast pistons and connecting rods. The piston head was recessed for both the intake and exhaust valves, while the pistons had three rings: two pressure rings and an oil control ring. Of these,

• The top piston ring had an inner bevel design;
• The second piston ring had an interrupt design to reduce oil consumption; and,
• The oil control ring had a slit design.

To reduce mass and sliding, the piston skirt had a ‘slipper’ design. Furthermore, the piston pin was located in an offset position.

Cylinder head and camshafts

The cross-flow cylinder head for the EJ20K was made from die-cast aluminium and had double overhead camshafts (DOHC) per cylinder bank. A single timing belt was used to drive the four camshafts, while the back of the belt also drove the water pump. The timing belt consisted of a strong and inflexible core wire, wear-resistant canvas and heat-resistant rubber material. For quiet operation, the teeth on the timing belt had a round profile. For theGC/GM Impreza WRX, a hydraulic belt-tensioner maintained timing belt tension. The timing belt cover was a made from a synthetic resin moulding and used rubber at the mating surface of the cylinder block to absorb noise and vibrations.

Each camshaft was supported by three journals with three camshaft caps, while each camshaft flange was supported by a groove in the cylinder head to receive thrust force. During their manufacture, the camshaft ‘nose’ was subjected to a chill treatment to increase wear resistance and anti-scuffing properties.

Valve timing

The EJ20K engine had four valves per cylinder that were actuated by solid tappet, hydraulic lifters. Based on the figures below, the EJ20K engine for the GC/GM Impreza WRX had an intake duration of 242 degrees, exhaust duration of 246 degrees and valve overlap of 17 degrees.

TD04 Turbocharger

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.

Intercooler

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.