First look: Subaru’s super-STI by Cosworth

SUBARU and Cosworth have joined forces to produce the wildest production Impreza STI ever seen, powered by a 295kW turbocharged boxer engine that propels the exclusive new WRX to 100km/h in a claimed 3.7 seconds.

That makes the Cosworth CS400 a match some of the world’s finest supercars, as well as a full second and a half quicker than the standard Impreza WRX STI hatch.

The bad news is production will be limited to just 75 vehicles, exclusively for the UK market from June. The right-hand-drive Cosworth Impreza STI CS400 will be priced at £49,995 ($A87,130) on the road in Britain – well up on the standard STI, which is priced from $61,990 in Australia.

For the money, the first genuine Cosworth-developed road car since 1996 – which was formally announced at the Geneva motor show in March and has now been revealed for the first time – comes with an array of engine and chassis upgrades that combine to deliver claimed quarter-mile (400m) acceleration in 12.75 seconds and a top speed of 250km/h.

Claimed to be “one of the world’s most accelerative hatchbacks”, the Cosworth Impreza employs a 33 per cent more powerful version of the STI’s 2.5-litre turbocharged four-cylinder flat four to drive all four wheels via a six-speed manual transmission.

The horizontally opposed all-alloy turbo four, which continues to feature twin overhead camshafts per cylinder bank and Subaru’s Active Valve Control System (AVCS), is removed from the vehicle by Cosworth, which replaces a range of internal and ancillary components with Cosworth-specified parts.

Subaru says a pre-requisite for the engine rebuild was to keep the enhancements as simple, efficient and as close as possible to the original equipment design to guarantee reliability and simplify maintenance at regular Subaru dealerships.

That includes high-performance pistons, manufactured using the same methods Cosworth uses to produce its Formula One pistons, reducing weight, friction and carbon build-up, forged connecting rods made from high-strength 4340 (EN24) steel, special Cosworth engine bearings, heavy-duty cylinder-head studs, high-performance multi-layer head gaskets and a higher-pressure oil pump.

Cosworth says the result is a substantial increase in torque at high revs and significantly improved in-gear acceleration, making the CS400 more driveable on the road and quicker on the racetrack.

Fitted to the 400hp turbo four is an all-new Cosworth forced induction system comprising a new compressor and wastegate actuator to increase boost pressure, complimented by a unique exhaust system with larger capacity tubular manifolds and larger-diameter downpipes.

Combined with a remapped ECU, the changes are claimed to liberate no less than 295kW at 5750rpm (versus 221kW at 6000rpm for the standard model), along with a cracking 542Nm at 3950rpm – up from 407Nm.

Subaru says that remains “within the torque capacity design limits of the original Impreza drivetrain”, despite the fact Cosworth also applies carbon synchromesh to the top three ratios in the six-speed manual gearbox and an upgraded single-plate clutch with revised pressure plate bearing and disc, plus a short-throw gearshift linkage for quicker shifting.

The gearbox, final drive ratios and full-time all-wheel-drive transmission with front and rear limited-slip differentials, viscous centre coupling and default 50/50 front/rear torque split is otherwise unchanged.

Chassis changes are aimed at improving handling, steering feel and driver feedback, via the fitment of special Eibach coil springs and new Bilstein dampers built to Cosworth specifications, including a 10mm lower front ride height.

Cosworth says the result is improved ride quality and more focussed and composed on-the-limit handling, along with less pitch and bodyroll.

Also fitted are larger 355mm ventilated front brake discs gripped by motorsport-style six-piston callipers with bespoke pistons and larger brake pads, all supplied by AP Racing. The standard STI’s rear brakes and ABS set-up remain unchanged.

Completing the upgraded mechanical package are stronger, lighter 18 x 8.5-inch ‘Pro Race’ alloy wheels similar in size to the regular STI’s but with 43mm offset (instead of 55mm) and 12mm wider wheel tracks. They are shod with high-performance directional Michelin Pilot Sport 3 tyres (245/40 R18), said to have been chosen for their dry and wet-road grip, durability and low noise levels.

External cosmetic extras for the CS400 include standard Xenon headlights, a new front bumper with revised foglights and upper and lower mesh grilles, a new rear spoiler, discreet Cosworth badging on the lower grille, front seats, brake callipers and floor mats, and individually numbered plaques on the engine and door sills.

Inside, the Cosworth Impreza, which comes with a numbered certificate of authenticity and a full three-year/36,000-mile new-car warranty, features new Recaro front seats, a piano black centre stack, privacy glass and Cosworth-branded black leather for the rear seats and steering wheel.

As with the garden-variety UK-spec STI, the CS400 features LED tail-lights, power windows, a 10-speaker/six-CD sound system, keyless entry and starting, satellite-navigation and six airbags. Kerb weight is 1505kg.

In the UK, Subaru also offers a ‘330R’ pack for the STI, which for no extra cost until June 30 includes 18-inch five-spoke ‘Lustric’ alloy wheels and a ‘Prodrive Performance Pack’, which boosts peak power to about 245kW at 5400rpm and maximum torque to 470Nm at a lower 3400rpm – delivering claimed 0-100km/h acceleration in 4.8 seconds.

The Prodrive pack is also available at no extra cost for the standard WRX in the UK until June 30, increasing power from the British-spec model’s standard 170kW to 195kW (the same as Australia’s WRX) and torque from 320Nm to 390Nm at just 3000rpm – well up on our Rex’s 343Nm.