The combination of Cosworth and Subaru plus the model name Impreza STI CS400 leaves little doubt that the net result will be high performance and advanced technology.
A Subaru U.K.-approved limited edition (75 units) of the Impreza available only in the U.K., it has a Cosworth-tuned all-aluminum 2.5-L turbocharged flat-four engine that produces 295 kW (396 hp) — some 33% more power than that of the standard car. It drives all wheels through a six-speed gearbox.
Claimed performance figures include 0-100 km/h (0-62 mph) in 3.7 s, and a standing-start quarter mile covered in 12.75 s at 172 km/h (107 mph).
Cosworth has engineered the Subaru's turbocharger, exhaust, suspension, brakes, and wheels. It has also added some styling changes.
It is the engine, which features DOHC plus Subaru's AVCS (Active Valve Control System) that gets most attention in this collaboration. Standard cars are delivered to Cosworth’s facility north of London and the engine removed. The engine enhancements have been developed to be as close as possible to the original equipment to ensure required reliability and for servicing at Subaru dealerships.
High-performance pistons are designed to reduce reciprocating mass, minimize friction, and reduce carbon buildup. The connecting rods are forged in high-strength (EN24) steel, and the bearings feature a proprietary Cosworth design, metallurgy, and engineering. Heavy-duty cylinder head studs are fitted plus high-performance multilayered head gaskets and a higher pressure oil pump.
These changes help achieve a claimed maximum torque figure of 542 N·m (400 lb·ft) at 3950 rpm.
The turbocharger system includes a new compressor design and wastegate actuator to support up-rated boost pressure. Larger capacity tubular manifolds and increased diameter downpipes plus larger diameter tail pipes are fitted, and the first catalyst is a nonrestrictive design. The ECU is remapped to accommodate the intake and exhaust changes.
Carbon synchro rings are used for the top three ratios of the car’s six-speed manual gearbox. The carbon surface enables faster gear change at high rpm and also acts as a braking material to stop the gear more quickly during shifts compared with standard synchro rings. Gear selection is via a short-throw linkage. The single-plate clutch is upgraded, with revised pressure plate, bearing, and disc.
The car’s all-wheel-drive system has a 50:50 default setting for front-to-rear torque split, varying according to changing conditions. The car includes a multimode center differential with viscous coupling.
Although Cosworth met requirements for enhanced handling, steering feel, and feedback, it also needed to ensure required levels of comfort. Cosworth worked with chassis teams from Bilstein and Eibach which supplied suspension inserts and coil springs, respectively. The car's ride height is lowered by 10 mm (0.39 in) at the front.
Front suspension also includes subframe-mounted MacPherson struts. At the rear is a subframe mounted multilink system. The changes are stated by Subaru U.K. to improve ride comfort and give composed on-the-limit behavior. Steering is electric-assisted rack and pinion.
AP Racing-developed front brakes are used, with 355-mm-diameter (14-in) rotors and six-pot calipers featuring special pistons and larger brake pads. Rear brakes are unmodified from those of the standard car.
Wheels have a 43-mm (1.7-in) offset, and the track is extended 12 mm (0.47 in). Tires are 245/40 R18 Michelin SP3 with directional tread pattern.
Styling changes to the STI CS400 include a newly shaped front bumper, new fog lights, upper and lower mesh grilles, and a different rear spoiler.