Subaru EJ251 & EJ252 Engines

Introduction

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 the BH Outback, GD/GG Impreza RS and Subaru SG Forester.

For the 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 redesigned;
• A lighter cylinder head and block were achieved by ‘eliminating excessive reinforcement’;
• 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 attributes.

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 driveline.

Pistons

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.

Cylinder head

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.

The 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 was 1-3-2-4.