The FB20 engine had an open-deck, aluminium alloy block with 84.0 mm bores
and a 90.0 mm stroke for a capacity of 1995 cc; within the cylinder bores,
the FB20 engine had cast iron liners. Due to its revised connecting rods and
valvetrain components, the FB20 block was the same size as its EJ204
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 FB20 engine had
asymmetrical, diagonally-split connecting rods. Compared to its EJ
predecessor, the FB20 engine achieved an approximate 30 per cent reduction
in frictional losses due to its lighter connecting rods, lighter pistons and
wrist pins, and lower piston-ring tension.
The FB20 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 FB20 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 FB20 engine had double overhead camshafts that were driven by a
maintenance-free chain. For the FB20 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 FB20 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 EJ204 engine
were in the intake manifold,
the fuel injectors for the FB20 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 FB20 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 FB20 engine, cooling around the spark plugs was improved for a
higher knock limit, increased power and improved fuel efficiency.
For the FB20 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 FB20’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.