Introduction
Subaru's EJ257 was a turbocharged, 2.5-litre horizontally-opposed (or 'boxer') four-cylinder engine. For Australia, the EJ257 engine was introduced in the Subaru GD Impreza WRX STi in 2005 and subsequently powered the GE/GH Impreza WRX STi and V1 WRX. Effectively replacing the 2.0-litre EJ207 engine, the EJ257 engine was a member of Subaru's Phase II EJ engine family; key features included its:
- Aluminium alloy block and cylinder head;
- Belt-driven double overhead camshafts;
- Turbocharger and air-cooled intercooler;
- Active Valve Control System (AVCS) for variable intake valve timing on the GD Impreza WRX STi, and dual AVCS for variable intake and exhaust valve timing for the GE/GH and V1 WRX STi; and,
- Compression ratio of 8.0:1, subsequently raised to 8.2:1 for the GE/GH Impreza WRX STi.
Please note that this article considers the EJ257 engine as it was supplied in the Australian-delivered GD, GE/GH and V1 Impreza WRX STi; specifications for other markets may differ.
EJ257 block
The EJ257 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 EJ257 engine were 'dry type', meaning that their outer surfaces were in complete contact with the cylinder walls. The EJ257 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.
Crankshaft, connecting rods and pistons
For the EJ257 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 for accurate mating.
The EJ257 engine had cast aluminium pistons with an Alumite coating for the piston head and ring grooves, and a molybdenum coating for the piston skirts. To reduce piston-to-bore clearances (and thereby reduce the amount of unburnt gas that could accumulate between the cylinder wall and piston head), piston pin offset for the EJ257 engine was reduced.
Cylinder head and camshafts
The EJ257 engine had a die-cast aluminium cylinder head that was mounted on a head gasket which consisted of three stainless steel sheet layers. The EJ257 engine had double overhead camshafts (DOHC) per cylinder bank that were driven by a timing belt which had a 125,000 kilometre replacement interval. The single timing belt 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 EJ257 engine was reduced by 1700 grams through the use of hollow shafts and sintered cam lobes.
The EJ257 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
The EJ257 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.
Active Valve Control System (AVCS): GD Impreza WRX STi
For the GD Impreza WRX STi, the EJ257 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.
Dual AVCS: GE/GH and V1 Impreza WRX STi
For the GE/GH Impreza WRX STi and V1 WRX STi, the EJ257 engine had dual AVCS which provided variable intake and exhaust valve timing.
IHI VF48 Turbocharger
For the Impreza WRX STi, the EJ257 engine had a single-scroll, IHI VF48 RF55 turbocharger which provided maximum boost pressure of 103 kPa (14.93 psi). Key specifications of the IHI VF48 turbocharger are given in the table below; for comparative purposes, the Mitsubishi TD04L turbocharger for the EJ255-powered GD/GG Impreza WRX is also included.
Intercooler
The EJ257 engine had an air-cooled intercooler that was mounted on top of the engine. While the intercooler for the GD Impreza WRX STi had a cooling capacity of 14.0 kW, a larger intercooler was introduced for the GE/GH Impreza WRX STi.
Injection and ignition
The EJ257 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 EJ257 engine had an ignition coil for each cylinder that was positioned directly above the platinum-tipped spark plug; the replacement interval for the spark plugs was 112,500 kilometres.
The EJ257 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. For the GD Impreza WRX STi, the EJ257 engine had a compression ratio of 8.0:1. For the GE/GH Impreza WRX STi and V1 WRX STi, however, the compression ratio was increased to 8.2:1.
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Source: http://australiancar.reviews/Subaru_EJ257_Engine.php
