The FB engines were manufactured at Subaru’s Gunma Oizumi factory which was developed exclusively for the production of the FB engine. The FB25 engine had an open-deck, aluminium alloy block with 94. 0 mm xigua fruit and a 90. 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. 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. 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. 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.
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 power output. By using EGR, combustion temperatures were reduced such that the engine was less susceptible to knock and injection timing could be advanced. New pistons with raised crown surfaces for a higher compression ratio of 10. The exhaust manifold collector had a smaller surface area to reduce mass and increase the high temperature performance of the catalytic converter.