Subaru EJ255 Engine

Introduction

Subaru’s EJ255 engine was a turbocharged 2.5-litre horizontally-opposed (or ‘boxer’) four-cylinder engine. For Australia, the EJ255 engine was introduced in the Subaru SG Forester XT in 2003, but subsequently offered in the GD/GG Impreza WRX and BL Liberty GT (see table below). Developed in conjunction with the more powerful EJ257 engine, key features for the EJ255 engine included its:

• Aluminium alloy block and cylinder head;
• Belt-driven double overhead camshafts;
• Turbocharger and air-cooled intercooler; and,
• Active Valve Control System (AVCS) and, for the BM Liberty GT, Dual AVCS.

Please note that this article considers the EJ255 engine for Australian-delivered vehicles; specifications for other markets differ.

EJ255 block

The EJ255 engine had a die-cast aluminium block with 99.5 mm bores and a 79.0 mm stroke for a capacity of 2457 cc. The cast iron cylinder liners for the EJ255 engine were ‘dry type’, meaning that their outer surfaces were in complete contact with the cylinder walls. The EJ255 engine had a semi-closed deck design whereby the cylinder walls were attached to the black at the twelve, three, six and nine o’clock positions.

Compared to its EJ207 predecessor, the EJ255 cylinder block had a new high strength, cast steel alloy rear main journal to reduce bearing oil clearance during cold operation and, as a result, reduce vibration and bearing rumble noise during the warm-up phase. Furthermore, this reduced bearing oil clearance was maintained when the engine was at operating temperature.

Crankshaft, connecting rods and pistons

The crankshaft for the EJ255 engine was supported by five aluminium alloy bearings and the crankshaft thrust bearing – positioned at the rear of the crankshaft – had a metal flange to support thrust forces. Each corner formed by a journal or pin and a web underwent a fillet-rolling process to increase its strength.

The connecting rods for the EJ255 engine were made from forged high carbon steel, while big end cap dowel pins and set screws were used for accurate mating.

The slipper skirt pistons were made from high tensile strength aluminium alloy (UAC12H) and had ‘dough nut’ shaped crowns to prevent interference with the intake and exhaust valves; it also had engraved marks to identify piston size and direction of installation. The piston head and ring grooves were coated with Alumite, while the piston skirts had a molybdenum coating to reduce friction. Compared to the EJ207 engine piston pin offset for the EJ255 engine was reduced to accommodate tighter piston to bore clearances and reduce unburnt gas build-up between the cylinder wall and piston head.

Each piston in the EJ255 engine had three rings: two compression rings and one oil control ring. Of these, the top piston ring has inner bevels and the second piston ring has a cut on the bottom outside to reduce oil consumption.

Cylinder head and camshafts

The EJ255 engine had a low pressure cast aluminium cylinder head that was mounted on a head gasket which consisted of three stainless steel sheet layers. The double overhead camshafts (DOHC) per cylinder bank were driven by a single timing belt which had round profile teeth for quiet operation and was made from a strong flexible core wire, wear resistant canvas and heat resistant rubber.

Each camshaft was supported at 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. Relative to the EJ207 engine, it is understood that camshaft mass for the EJ255 engine was reduced by 1700 grams through the use of hollow shafts and sintered cam lobes.

The EJ255 engine had parallel flow cooling system whereby coolant flowed into the block under pressure, crossed the gasket to the cylinder head and then passed through holes adjacent to each cylinder.

Valves

Like the EJ257 engine, the EJ255 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.

Active Valve Control System (AVCS)

With the exception of the BM/BR Liberty GT, the EJ255 was equipped with Subaru’s ‘Active Valve Control System’ (AVCS) which provided variable intake valve timing by changing the phase angle of the camshaft sprocket relative to the camshaft. When introduced in theSG Forester XT, the AVCS for the EJ255 engine had a 20 degree range of adjustment. However, the range of adjustment was subsequently increased to 35 degrees for the GD/GG Impreza WRX and 50 degrees for the GE/GH Impreza WRX; the ranges for the other models in the table below

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 (ECU) would determine optimum valve timing and send an electrical signal to an oil control valve that was positioned at the end of each intake camshaft sprocket to control oil pressure to the advance and retard chambers within the AVCS actuator. The ECU 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.

Dual Active Valve Control System: BM/BR Liberty GT

For the BM/BR Liberty GT, the EJ255 engine was equipped with Subaru’s Dual Active Valve Control System (AVCS) which provided variable intake and exhaust timing; it is understood that both the intake and exhaust camshafts had a 40 degree range of adjustment.

Intake

Like other EJ Phase II engines, the intake ports for the EJ255 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.

Unlike the EJ207 engine, the EJ255 had a Tumble Generator Valve (TGV) in the intake manifold to improve exhaust gas emissions at low engine speeds. The TGV caused intake airflow to be redirected by closing a butterfly valve in the intake manifold, creating a tumble air motion at low intake air speeds to enhance air/fuel mixing.

Turbochargers

Subaru SG Forester XT (2003-05): Mitsubishi TD04L turbocharger

The Subaru SG Forester XT had a Mitsubishi TD04L turbocharger which provided peak boost pressure of 600 mmHg (11.6 psi); key specifications are given in the table below. The intercooler for the SG Forester XT had a core capacity of 3.2 litres and cooling capacity of 11.9 kW.

Subaru SH Forester XT (2008-12): Mitsubishi TD04L turbocharger

It is understood that the Subaru SH Forester XT continued to use Mitsubishi TD04L turbocharger, though the shape of the turbine wheel and impeller were changed to improve torque at low-to-medium engine speeds. The Subaru SH Forester XT also introduced a larger intercooler with a core capacity of 3.8 litres and cooling capacity of 12.1 kW.
 

GD/GG Impreza WRX (2005-07): Mitsubishi TD04L Turbocharger

The GD/GG Impreza WRX had a Mitsubishi TD04L turbocharger which provided peak boost pressure of 0.93 bar (13.5 psi); key specifications are given in the table below. Furthermore, the intercooler for the GD/GG Impreza WRX had a cooling capacity of 11.9 kW.

GE/GH Impreza WRX (2008-14): TD04 and VF52 turbochargers

Upon its release, the GE/GH Impreza WRX had a Mitsubishi TD04 turbocharger which provided peak boost pressure of 78 kPa or 11.3 psi; the intercooler had a cooling capacity of 12.1 kW.

From December 2008, however, the GE/GH Impreza WRX was fitted with an IHI VF52 turbocharger which provided greater boost pressure; in separate press releases, Subaru Australia listed boost pressure as 92 kPa and 103 kPa, though the latter is considered to be incorrect since 103 kPa was the boost pressure for the EJ257 engine in the Impreza WRX STi. The post-December 2008 GE/GH Impreza WRX also had a bigger intercooler (cooling capacity of 14.0 kW) and a revised exhaust system which achieved a 30 per cent reduction in gas flow restriction.

BL/BP Liberty GT (2006-09): IHI VF46

The BL/BP Liberty GT had an IHI VF46 turbocharger which provided peak boost pressure of 95 kPa (13.78 psi); key specifications are given in the table below.

BM/BR Liberty GT (2009-14): IHI VF45

The BM/BR Liberty GT had an IHI VF45 turbocharger which provided peak boost pressure of 87 kPa (12.62 psi); key specifications are given in the table below. For the BM/BR Liberty GT, the turbocharger was positioned directly underneath and at the front of the crankshaft so that it was closer to the cylinder head exhaust ports – this reduced the length of the path that the exhaust gas had to flow to reach the turbine, reducing energy losses and increasing response time. According to Subaru, maximum boost pressure was achieved 30 per cent faster than in the BL/BP Liberty GT. Other changes relative to the BL/BP Liberty GT included:

• A larger compressor wheel and air pump to force a greater volume of air into the cylinders;
• A 25 per larger intercooler; and,
• The catalytic converter was positioned immediately after the turbocharger so that it reached operating temperature sooner, thereby reducing cold-start emissions.

Injection and ignition

The EJ255 engine had multi-point fuel injection with an injection and firing order of 1-3-2-4. The pentroof combustion chambers had a wide ‘squish area’ for increased combustion efficiency and centrally positioned spark plugs. The EJ255 engine had an ignition coil for each cylinder that was positioned directly above the spark plug. While the SG Forester XT had Iridium spark plugs, it is understood that all other models in the table above had platinum-tipped spark plugs.

The EJ255 engine had an ignition knock control facility with fuzzy logic that enabled the maximum ignition advance to be used without detonation by constantly adapting to changes in environmental conditions and fuel quality.