Subaru's EJ251 and EJ252 were 2.5-litre horizontally-opposed (or 'boxer')
four-cylinder petrol engines.
For Australia, the EJ251 engine was first
introduced in the Subaru BE/BH Liberty
in 1998 and subsequently offered in
and Subaru SG Forester
Subaru BL/BP Liberty
and BP Outback
, the EJ251 was replaced by the EJ252 engine.
For the EJ252 engine, changes included:
- According to Subaru Australia, 80 per cent of engine components were
- A lighter cylinder head and block were achieved by 'eliminating
- Thinner and lighter cylinder liners with improved roundness;
- A re-shaped intake manifold;
- Introduction of a 4-2-1 system with equal length header pipes; and,
- Electronic throttle control ('drive-by-wire').
Beyond these changes, however,
the EJ251 and EJ252 engines are understood to have the same general
Please note that
this article considers the EJ251 and EJ252 engines as they were supplied in
Australian-delivered vehicles; specifications for other markets may vary.
Block and crankcase
The Subaru EJ251 and EJ252 engines had an aluminium alloy block with 99.5 mm
bores – with cast iron dry-type cylinder liners – and a 79.0 mm stroke for a
capacity of 2457 cc. The cylinder block for the EJ251 and EJ252 engines had
an open-deck design whereby the cylinder walls were supported at the three
and nine o’clock positions.
The crankcase for the EJ251 and EJ252
engines had five main bearings and the flywheel housing was cast with the
crankcase for increased rigidity. Like other EJ Phase II engines, the
crankshaft thrust bearing was positioned at the rear of the crankshaft to
reduce the transfer of natural engine frequencies to the transmission and
The EJ251 and EJ252 engines had cast aluminium pistons. Relative to their EJ
Phase I predecessors, the pistons for the EJ251 and EJ252 engines had
reduced piston pin offset and a molybdenum coating to reduce friction. Other
features of the pistons included solid-type piston skirts, flat top
combustion surfaces and reduced top land to cylinder clearance.
The EJ251 and EJ252 engines had an aluminium alloy cylinder head with
cross-flow cooling. The intake ports for the EJ251 and EJ252 engines created
a ‘tumble swirl’ motion for air as it entered the cylinder for better
air/fuel mixing, more uniform flame travel and faster combustion.
EJ251 and EJ252 engines had a hollow-type single overhead camshaft (SOHC)
per cylinder bank. Due to the cylinder head offset, the left camshaft was
longer than the right camshaft to align the cam belt sprockets. Both
camshafts were driven by a single belt which had round profile teeth for
quiet operation and was constructed of wear-resistant double canvas and heat
resistant rubber materials with a wire core.
For the EJ251 and EJ252
engines, the four valves per cylinder were actuated by shim-less type
buckets (i.e. one-piece, solid valve lifters).
Injection and ignition
The EJ251 and EJ252 engines had multi-point sequential fuel injection and
centrally located spark plugs. The EJ251 and EJ252 engines had two ignition
coils (one for each pair of cylinders, i.e. 1-2 and 3-4) which fired the
spark plugs directly twice per cycle. The ignition knock control system had
‘fuzzy logic’ that enabled the maximum ignition advanced angle to be used
without detonation since the programme continually adapted to changes in
environmental conditions and fuel quality.
The EJ251 and EJ252
engines had a compression ratio of 10.1:1; the injection and firing order