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.
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.
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
GD Impreza WRX
, 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
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
- 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
GE/GH Impreza WRX STi
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
-powered GD/GG Impreza WRX is also included.