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.
Crankshaft, connecting rods and pistons
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.
Cylinder head and camshafts
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
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.
Valve timing: GC Impreza WRX STi
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).