Subaru EJ253 Engine

Introduction

Subaru’s EJ253 engine was a 2.5-litre horizontally-opposed (or ‘boxer’) four-cylinder petrol engine. For Australia, the EJ253 engine was first introduced in the Subaru SG Forester in 2005. Compared to the EJ252 engine which it replaced, changes for the EJ253 included:

• Reinforced casting around the engine’s main bearing journal;
• Alterations to cylinder liner shape; and,
• Introduction of Tumble Generator Valves (TGVs) to reduce start-up emissions.

For Subaru models introduced after August 2007, the EJ253 engine was equipped with Subaru’s ‘Intelligent Active Valve Lift’ (i-AVLS) which provided increased valve lift for one intake valve per cylinder.

Please note that this article considers the EJ253 engine as it was supplied in Australian-delivered vehicles; specifications for other markets may differ.

EJ253 block and crankcase

The Subaru EJ253 engine 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 EJ253 engine had an open-deck design whereby the cylinder walls were supported at the three and nine o’clock positions.

The crankcase for the EJ253 engine 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 EJ253 engine had cast aluminium pistons. Relative to their EJ Phase I predecessors, the pistons for the EJ253 engine 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 EJ253 engine had an aluminium alloy cylinder head with cross-flow cooling. The EJ253 engine had a hollow-type single overhead camshaft (SOHC) for each 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 EJ253 engine, the four valves per cylinder were actuated by shim-less type buckets (i.e. one-piece, solid valve lifters).

Intelligent Active Valve Lift System (i-ALVS)

Subaru’s Intelligent Active Valve Lift System (i-AVLS) was first introduced on the post-August BL/BP Liberty and BP Outback. With i-AVLS, one intake valve for each cylinder which could utilise a low lift camshaft lobe profile or a high lift camshaft lobe profile. At low engine speeds, the operation of the low/mid lift camshaft profile on one intake valve increased the speed of the air entering the combustion chambers from that port and created an imbalance in pressure as air entered the cylinder. This pressure imbalance created a swirling pattern and better air/fuel mixture formation, thereby increasing torque output.

At high engine speeds, the rocker arms of each cylinder’s two intake valves were locked together such that high-profile camshaft lobe acted on both of them. With the higher lift, intake resistance to air as reduced to enhance top-end power. Based on engine load, driving requirements and atmospheric conditions, the ECM would determine which camshaft lift profile to engage.

Intake

The intake ports for the EJ253 engine 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 EJ253 engine was also fitted with a Tumble Generator Valve (TGV) upstream of each intake port. At low intake air speeds a butterfly valve would close to be redirect the intake air such that it created a tumbling motion to enhance air/fuel mixing and reduce exhaust emissions. At higher engine speeds, the valve would open to reduce intake resistance.

Injection and ignition

The EJ253 engine had multi-point sequential fuel injection and centrally located spark plugs. Initially, the EJ253 engine 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.

It is understood that the EJ253 engine had a MAF (mass airflow) sensor rather than a MAP (manifold absolute pressure) sensor for more accurate measurement of intake air volume. Furthermore, the EJ253 engine had a compression ratio of 10.1:1; the injection and firing order was 1-3-2-4.

2009 Changes: BM/BR Liberty, BR Outback & Liberty Exiga

With the release of the Subaru BM/BR Liberty, BR Outback and YA Liberty Exiga models, a range of changes were introduced for the EJ253 engine –

• Five per cent lighter pistons;
• The shape of the intake duct, ports and manifold were revised to improve airflow characteristics for low-speed torque;
• The aluminium intake manifold was replaced with a resin-based plastic manifold to reduce mass;
• The i-AVLS was revised for greater low-speed torque and fuel efficiency;
• New ‘high ignition’ spark plugs were introduced and the sequential ignition coils were replaced with direct coil-on-plug ignition;
• Cooling was enhanced by increasing coolant flow around the spark plugs;
• A lighter catalytic converter; and,
• A single 16-litre muffler was introduced to reduce the mass of the exhaust system by 6 kg.