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.
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
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.
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
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.
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
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
Subaru BN Liberty
, 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
- Intake resistance was reduced by around 26 per cent due to a larger
- 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.