FB25 Subaru Engine
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Subaru FB25 Engine

Introduction

Subaru's FB25 was a 2.5-litre horizontally-opposed (or 'boxer') four-cylinder petrol engine. Effectively replacing the EJ253, the FB25 engine was a member of Subaru’s third generation 'FB' boxer engine family which also included the FB20, FA20D, FA20E and FA20F engines. The FB25 engine first offered in Australia in the 2011 Subaru SH.II Forester.

Key features of the FB25 engine included its:
  • Aluminium alloy cylinder block with open-deck design;
  • Aluminium alloy cylinder head with double overhead camshafts;
  • Four valves per cylinder actuated by roller rocker arms;
  • Subaru’s dual ‘Active Valve Control System’ for variable intake and exhaust valve timing; and,
  • Cooled exhaust gas recirculation.

The FB engines were manufactured at Subaru’s Gunma Oizumi factory which was developed exclusively for the production of the FB engine. Please note that this article considers the FB25 engine as it was supplied in Australian-delivered vehicles; specifications for other markets may vary.
Subaru FB25 engine
Model Engine Trans. Power Torque Years C.R.
Subaru SH.II Forester 2.5-litre petrol F4 5sp man.,
4sp auto
126kW at 5800rpm 235Nm at 4100rpm 2011-12 10.0:1
Subaru SJ Forester 2.5-litre petrol F4 CVT 126kW at 5800rpm 235Nm at 4100rpm 2013-on 10.0:1
Subaru BM/BR Liberty 2.5-litre petrol F4 6sp CVT 127kW at 5600rpm 235Nm at 4100rpm 2012-14 10.0:1
Subaru BR Outback 2.5-litre petrol F4 6sp CVT 127kW at 5600rpm 235Nm at 4100rpm 2013-14 10.0:1
Subaru BN Liberty 2.5-litre petrol F4 6sp CVT 129kW at 5800rpm 235Nm at 4000rpm 2014-on 10.3:1
Subaru BS Outback 2.5-litre petrol F4 6sp CVT 129kW at 5800rpm 235Nm at 4000rpm 2014-on 10.3:1

FB25 block

The FB25 engine had an open-deck, aluminium alloy block with 94.0 mm bores and a 90.0 mm stroke for a capacity of 2494 cc; within the cylinder bores, the FB25 engine had cast iron liners. Due to its revised connecting rods and valvetrain components, the FB25 block was the same size as its EJ253 predecessor, despite its smaller bore and longer stroke. According to Subaru, the longer stroke improved fuel efficiency by enabling faster air induction and reduced unburnt fuel during cold starts.

The FB25 engine had separate cooling circuits for the cylinder block and head to improve coolant distribution. Around the block, the flow rate was limited to maintain a high temperature for the cylinder liner oil, thereby reducing friction from the motion of the pistons.

Connecting rods and pistons

To reduce engine width and enable its longer stroke, the FB25 engine had asymmetrical, diagonally-split connecting rods. Compared to the EJ253, the FB25 engine achieved a 28 per cent reduction in frictional losses due to its lighter connecting rods, lighter pistons and wrist pins, and lower piston-ring tension.

Intake

The FB25 engine had a plastic intake manifold with reshaped branches to reduce pressure losses and thereby increase power; the valves were also revised to reduce drops in pressure when open and to increase tumbling when closed. Upstream of each intake port, a metal partition acted as a tumble generator valve (TGV) to increase air tumble and create vortices within the combustion chamber. Furthermore, the size and shape of the resonators in the intake manifold were streamlined.

Cylinder head

The FB25 engine had an aluminium alloy cylinder head with separately cast camshaft carriers so that cores in the cylinder head could be omitted for a reduction in metal thickness. As noted above, cylinder head cooling was enhanced by using separate circuits for the cylinder block and head.

Camshafts and dual AVCS

The FB25 engine had double overhead camshafts that were driven by a maintenance-free chain. For the FB25 engine, a chain drive was adopted because it enabled a narrower included valve angle and a reduction in the sprocket diameters of the crank and camshaft for reduced width. The four valves per cylinder were actuated by roller rocker arms (previously valve lifters for the EJ253).

The FB25 engine had Subaru’s dual Active Valve Control System (AVCS) which provided variable intake and exhaust valve timing.

Injection and ignition

Whereas the fuel injectors for the EJ253 engine were in the intake manifold, the fuel injectors for the FB25 engine were moved to the cylinder head. According to Subaru, positioning the injectors in the cylinder head enhanced the flow of atomised fuel, thereby improving fuel efficiency and reducing exhaust gas emissions.

The FB25 engine had coil-on-plug ignition with an integrated igniter for each cylinder. The spark plug caps, which provided contact to the spark plugs, were integrated with the ignition coil assembly. For the FB25 engine, cooling around the spark plugs was improved for a higher knock limit, increased power and improved fuel efficiency.

Exhaust

For the FB25 engine, the diameters and lengths of the exhaust manifold were modified to improve catalytic converter warm-up and increase power output. Furthermore, the more free-flowing exhaust system contributed to higher power output.

The FB25’s exhaust gas recirculation (EGR) system included a cooling circuit that enabled greater exhaust gas volumes to be recirculated than occurred in the EJ engines. By using EGR, combustion temperatures were reduced such that the engine was less susceptible to knock and injection timing could be advanced.

BN Liberty and BS Outback: FB25 changes

For the Subaru BN Liberty and BS Outback, a range of changes for the FB25 engine were introduced, including an ‘Automatic Stop Start’ function which enabled the engine to shut down when the vehicle was stationary in traffic to conserve fuel. Other changes included:
  • A thinner cylinder block (base thickness was reduced from 3.5 mm to 3.2 mm);
  • Intake resistance was reduced by around 26 per cent due to a larger intake path;
  • A new high-tumble intake port shape;
  • Larger intake valves (36 mm diameter, previously 34 mm);
  • Increased valve pitch (41 mm, previously 39 mm);
  • Revised position and ports for the Tumble Generator Valves (TGVs) to increase their tumble effect. The new TGVs also had a unified resin design rather than being produced from separate aluminium parts;
  • New pistons with raised crown surfaces for a higher compression ratio of 10.3:1 (previously 10.0:1). Furthermore, the centre weight location and skirt shape were optimised to reduce vibration noise;
  • A new piston skirt coating to lower friction;
  • A mass was added to the left-hand exhaust cam sprocket to reduce noise from timing chain engagement;
  • A dedicated circuit was provided for EGR cooling and a separate high-efficiency cooler was adopted;
  • The exhaust manifold collector had a smaller surface area to reduce mass and increase the high temperature performance of the catalytic converter. Furthermore, pressure loss was reduced while power output was improved with increased diameter manifold pipes;
  • The exhaust pipe assembly included a lighter rear chamber and pipe diameter was changed to reduce booming noise; For rust resistance, stainless steel was used in the rear flange and bracket; and,
  • A ‘new generation’ ECU.

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