Subaru EJ207 Engine

Subaru's EJ207 engine was a 2.0-litre horizontally-opposed and turbocharged four-cylinder engine. For Australia, the EJ207 was first offered in the Subaru GC Impreza WRX STi in 1999 and subsequently powered the Subaru GD Impreza WRX STi. A member of Subaru’s Phase II EJ engine family, key features of the EJ207 engine included its:

• Aluminium alloy block and cylinder head;
• Belt-driven double overhead camshafts;
• Turbocharger and air-cooled intercooler; and,
• Compression ratio of 8.0:1.

Please note that this article considers the EJ207 engine as it was supplied in the Australian-delivered GC Impreza WRX STi and GD Impreza WRX STi; specifications for other markets may differ.

The EJ207 engine had a die-cast aluminium block with 92.0 mm bores and a 75.0 mm stroke for a capacity of 1994 cc. The cast iron cylinder liners for the EJ207 engine were ‘dry type’, meaning that their outer surfaces were in complete contact with the cylinder walls.

For the GC Impreza WRX STi, the EJ207 had an open-deck design whereby the cylinder walls were attached to the block at three and nine o’clock positions.

For the GD Impreza WRX STi, however, a semi-closed deck design was used which had additional attachment points at the top and bottom of each cylinder liner (i.e. the twelve and six o’clock positions) so that the bores were less susceptible to distortion. It is understood that the EJ207 block for the GD Impreza WRX STi also had additional crankcase reinforcement ribs. For the EJ207 engine, the crankshaft was supported by five main bearings and, like other EJ Phase II engines, the crankshaft thrust bearing was positioned at the rear of the crankshaft. The connecting rods were made from forged high carbon steel, while big end cap dowel pins and set screws were used to improve mating accuracy.

Both the GC Impreza WRX STi is understood to have forged aluminium alloy pistons, while the GD Impreza WRX STi certainly did. For both engines, the pistons had solid slipper-type skirts with a molybdenum coating to reduce friction. Each piston had two compression rings and one oil control ring; of these, the top piston ring had an inner bevel, while the second piston ring had a cut on the bottom outside to reduce oil consumption. To prevent interference with the valves, the piston crown was recessed.

The EJ207 crankcase had oil jets that squirted oil on the underside of the pistons to aid cylinder wall lubrication and piston cooling. The EJ207 engine had a die-cast aluminium cylinder head that was mounted on a head gasket which consisted of three stainless steel sheet layers. The EJ207 engine had double overhead camshafts (DOHC) per cylinder bank that were driven by a timing belt which had a 100,000 kilometre replacement interval. Each camshaft was supported at its three journals, held in position by three camshaft caps and had a flange which fitted the corresponding groove in the cylinder head to receive thrust forces. To increase wear resistance and anti-scuffing properties, the noses of the cam lobes were subjected to a ‘chill’ treatment. For the GD Impreza WRX STi, each intake camshaft had teeth at its rear end for the variable valve timing position sensor. The EJ207 engine had four valves per cylinder – two intake and two exhaust, in a cross-flow valve configuration - that were actuated by shim-less valve lifters. The intake valves had hollow stems to reduce mass and inertia, while the exhaust valve stems were filled with sodium. At high temperatures, the sodium would liquefy and its motion within the stem would effectively transfer heat from the valve head to the valve stem, contributing to faster cooling of the valve head. For the GC Impreza WRX STi, intake duration was 242 degrees, exhaust duration was 248 degrees and valve overlap was 19 degrees (see table below).

For the GD Impreza WRX STi, the EJ207 engine had Subaru’s ‘Active Valve Control System’ (AVCS) which adjusted the opening and closing timing of the intake valves by changing the phase angle of the camshaft sprocket relative to the camshaft within a maximum range of 35 crankshaft degrees. Under the control of the ECM, an oil flow control valve would move its spool to switch the hydraulic passage to/from the advance and retard chambers in the camshaft sprocket to vary the phase angle between the camshaft sprocket and camshaft.

Based on input signals from the air flow sensor, engine coolant temperature sensor, throttle position sensor and camshaft position sensors, the engine control unit could use three computer maps to achieve the following -

• Optimum valve timing for stable idling: minimal intake and exhaust valve overlap);
• Improved fuel consumption at medium engine speeds and low loads: intake valve timing was advanced to reduce intake air blow back and improve fuel consumption. Furthermore, increasing intake and exhaust valve overlap enhanced exhaust gas recirculation (EGR) for a reduction in NOx emissions. When engine load increased, advancing the intake closing time utilised the inertia of the intake air to create a supercharging effect; and,
• Maximum power at high engine speed and load: intake valve timing was further advanced to maximise overlap and utilise the scavenging effect produced by exhaust gas pulsations to draw intake air into the cylinder. Since the intake valve was closed at the end of the intake stroke, air intake efficiency was improved and power increased.

For the GD Impreza WRX STi, intake duration was 242 degrees and exhaust duration was 248 degrees, while valve overlap range from 4 to 39 degrees (see table below).

Like other EJ Phase II engines, the intake ports for the EJ207 engine created a ‘tumble swirl’ motion for the intake air as it entered the cylinder for better air/fuel mixing to achieve more uniform flame travel and faster combustion.

The EJ207 engine did not have a Tumble Generator Valve (TGV), but the TGV partition was still present in the intake manifold. For the GC Impreza WRX STi, the EJ207 engine had a single scroll, roller bearing turbocharger which provided maximum boost pressure of 825 mm Hg (15.95 psi, 1.1 bar) from 3000 rpm. It is understood that the GC Impreza WRX STi had an IHI VF28 turbocharger.

The EJ207 engine had a wastegate to limit boost pressure. A simple flap valve that was opened by a diaphgram, the wastegate enabled exhaust gas to bypass the turbine. For the GC Impreza WRX STi, the EJ207 engine had a single scroll turbocharger which provided naximum boost pressure of 848 mm Hg (1.13 bar, 16.4 psi) at 4000 rpm. It is understood that the GD Impreza WRX STi has an IHI VF35 RHF55 turbocharger.

Compared to the GC Impreza WRX STi, the compressor wheel to housing seal for the GG Impreza WRX STi reduced clearance by 66 per cent. As a result, pumping losses were reduced for faster boost accumulation.

The EJ207 engine had an air-cooled intercooler that was mounted on top of the engine. For the GD Impreza WRX STi, key metrics for the intercooler are given in the table below; for the WRX STi, its intercooler was 39 per cent larger than that in the EJ205-powered Subaru GD/GG Impreza WRX. For the GD Impreza WRX STi, the driver could press a button which sprayed water - for two (2) seconds - from a diffusion-type nozzle in the intercooler duct for additional cooling under high engine loads or high ambient temperatures. The water tank was positioned in the boot.

For the GD.II Impreza WRX STi, intercooler efficiency was improved by altering the shape of the air baffle plate inside the scoop and by increasing the spray pattern from the water nozzles The EJ207 engine had multi-point fuel injection with an injection and firing order of 1-3-2-4. The pentroof combustion chambers had centrally positioned spark plugs and a wide ‘squish area’ for increased combustion efficiency. Furthermore, the EJ207 engine had a compression ratio of 8.0:1.