The 2.5-litre turbocharged 'Boxer' engine is transformed to produce 33 per cent more power, drawing heavily on Cosworth's extensive Formula 1 experience. The engine, which drives all four wheels via a six-speed manual gearbox, boasts 400 PS and the new model reaches 62 mph in 3.7 seconds, going on to cover a quarter mile in 12.75 seconds at a terminal speed of 107 mph. Top speed is limited to 155 mph.
The Cosworth Impreza STI CS400 features an extensive range of Cosworth-engineered components in the engine, turbo system, exhaust, suspension, brakes and wheels, plus subtle changes to the exterior and interior styling to create a truly distinctive high performance car with everyday hatchback practicality.
At the heart of the new Cosworth Impreza is a heavily revised version of the marque's famous horizontally-opposed, all-aluminium Boxer engine. The 'flat-four' power unit is removed from the car, hand finished, and several internal and ancillary components are replaced with up-rated items, specified by Cosworth. As with all Impreza 'Boxer' engines, the STI Cosworth unit features twin overhead camshafts per cylinder bank (DOHC) plus an Active Valve Control System (AVCS) to deliver variable valve timing and enhance low-speed pulling power.
The focus of the Cosworth engine upgrade was to keep the enhancements simple, efficient and as close as possible to the original equipment design to guarantee reliability and easy maintenance at regular Subaru dealerships. Internal upgrades include high-performance pistons, which are manufactured using the same methods as those employed to create Cosworth's Formula 1 pistons. Special features reduce carbon build-up, minimise friction and reduce weight.
Forged connecting rods are made from high-strength 4340 (EN24) steel and the special Cosworth engine bearings boast a unique combination of design, metallurgy and engineering. Heavy duty cylinder head studs, high-performance multi layered head gaskets and a higher-pressure oil pump are also fitted. An all-new Cosworth turbocharging system features a new compressor design and wastegate actuator to supply up-rated boost pressure and deliver increased power.
A unique exhaust system was developed to allow the engine to 'breathe' more easily and unleash its full potential. Larger capacity tubular manifolds and increased diameter downpipes speed up the evacuation of hot gases. The first catalyst is a non-restrictive design and larger tail pipes give the exhaust note a unique tone.
After the in-depth Cosworth performance upgrade which includes an ECU re-map, the boxer engine in the CS400 generates 33 per cent more power (400 ps at 5,750 rpm, compared with 300 ps at 6,000 rpm for the standard Impreza WRX STI), and 542 Nm of torque, while remaining within the torque capacity design limits of the original Impreza drivetrain.
To handle the increased power of the Cosworth-tuned engine, the Impreza STI's six-speed manual transmission has carbon synchromesh applied to the top three ratios and a short throw linkage for quicker shifting. There is also an upgraded single-plate clutch with a revised pressure plate, bearing and disc. The gearbox and final drive ratios are unchanged.
Retaining Subaru's original suspension layout and major components, Cosworth and a team of chassis engineers collaborated with Bilstein and Eibach technicians to tailor a ride/handling/steering package that was ideally matched to the new model's increased performance. Special Eibach coil springs and new Bilstein inserts, featuring unique damping characteristics, were manufactured to Cosworth's specifications. The ride height was lowered by 15 mm at the front.
To match the new Impreza Cosworth's supercar pace, Cosworth commissioned AP Racing to develop more powerful front brakes. Larger diameter (355 mm) ventilated front discs were adopted, with motorsport-style six-pot callipers featuring bespoke pistons operating on larger brake pads. The standard Impreza WRX STI's rear brake discs, overall brake balance and original ABS system have been retained.
The outstanding performance of the Cosworth Impreza is reflected in its 'stand-out' appearance and standard Xenon headlamps. The lowered ride height and wider track give the car a whole new stance, which is reinforced by a front bumper that now incorporates fog lights, a new front lip spoiler, and upper and lower mesh grilles. Also added is a new rear boot-lid spoiler.
Discreet Cosworth badging is carried on the lower grille, front seats, brake callipers and floor mats. Individually numbered plaques are fitted on the engine and door sills. Each Cosworth Impreza STI CS400 is supplied with a numbered certificate of authenticity.
Inside, the sporty new Recaro front seats, the rear seats and steering wheel are all re-trimmed in Cosworth-branded black leather. Also distinguishing the CS400 are Piano Black centre stack and trim accents, and the fitment of privacy glass.
The new Cosworth Impreza STI CS400 comes with the reassurance of a three-year or 36,000-mile warranty, plus 12-year anti-corrosion cover and three-year paintwork warranty.