/Cosworth Impreza CS400 Technical Details

Cosworth Impreza CS400 Technical Details

cosworth sti 1 at Cosworth Impreza CS400 Technical Details

Yesterday Cosworth and Subaru revealed their limited edition Impreza STi CS400 and since it received such a warm welcome, they’ve released further technical details and specs of the car. This 4WD 400 hp car is a the fastest accelerating hatchback in the world and in this special matte grey color, it’s also one of the best looking. All 75 units of this special edition Impreza will go on sale in the UK.

Hit the jump for full technical details on this magnificent car…

cosworth sti 2 at Cosworth Impreza CS400 Technical Details

Engine delivers a 33 per cent power hike

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.

The overall result is a huge increase in torque higher up the rev range, and significantly improved in-gear acceleration.  This not only makes the Impreza more tractable for day-to-day road use, but also a far sharper tool on the race track.

New turbocharger and ‘breathe easy’ exhaust systems

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.

Manual transmission and All-Wheel Drive system

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.

The Cosworth Impreza’s full-time All-Wheel-Drive transmission with front and rear limited-slip differentials, features a 50/50 front to rear torque split as the default, however, the torque split is varied electronically according to changing conditions.  The multi-mode Driver Control centre differential with viscous coupling senses which axle has the better grip and varies the split ratio in milliseconds to distribute torque accordingly, thus maximising traction and minimising the possibility of wheelspin.

Fine-tuned suspension

Improving the Cosworth Impreza’s handling, steering feel and driver feedback – without compromising ride comfort – was a primary target for the development team.

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 10 mm at the front.

The benefits of this fettling are twofold. Firstly, there is a big improvement in ride quality generally, making the Impreza easier to live with on a day-to-day basis.  Secondly, its on-the-limit behaviour is more focused and composed.  The damping upgrades mean that there is less pitch and roll when turning into corners, meaning that keen drivers can get back on the power earlier and make the most of the car’s Symmetrical All-Wheel-Drive system.

Upgraded ‘racing’ brakes enhance stopping power

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 difference in performance is immediately noticeable.  The harder pads help to minimise fade, meaning that drivers can count on the AP stoppers for effective retardation time and time again.

The standard Impreza WRX STI’s rear brake discs, overall brake balance and original ABS system have been retained.

‘Stand-out’ looks, plus new style wheels

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 new shape front bumper, new fog lights, upper and lower mesh grilles and a new rear 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.

Although the new model’s alloy wheels are a similar overall size to those of the regular Impreza WRX STI model – 18 x 8.5 inch – they are lighter and stronger.  Called ‘Pro Race’ and finished in anthracite the latest wheels from the Team Dynamics range were selected as compatible with the Subaru’s five-stud design.  These wheels have a 43 mm offset (the standard STI wheel’s offset is 55 mm) and extend the track by 12 mm – improving handling and filling the car’s wheel arches more effectively for a more aggressive stance.

The new wheels are fitted with high performance, low profile Michelin SP3 tyres chosen for their dry- and wet-road grip, durability, and low noise levels.  Tyre size is 245/40 R18 and the tread pattern is directional.

Interior modifications and generous premium equipment

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.

Standard features carried-over from the WRX STI model include: LED (light-emitting diode) tail lamps, front and rear electric windows, 10-speaker, six-disc radio/CD player, satellite navigation, Smart-entry and push-button start, plus front, side and curtain airbags.

Full ownership benefits

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.

This is in addition to three years’ membership of Subaru Assistance – a comprehensive home and roadside repair and recovery package throughout the UK and Europe administered by Mondial Assistance.

Body and Chassis

Five-door, five-seat hatchback, with all-steel unitary construction body shell.  Longitudinally-mounted petrol boxer engines driving the front and rear wheels (AWD) via a six-speed manual transmission.

cosworth sti 3 at Cosworth Impreza CS400 Technical Details


STI 2.5-litre / 400 PS
Name Subaru Boxer
Type DOHC, horizontally opposed ‘flat four’
Capacity 2.5-litres, 2457 cc
Bore & Stroke 99.5 x 79.0 mm
Compression ratio 8.2:1
Max power 400 PS (295 kW) @ 5,750 rpm
Max torque 542 Nm (400 lb.ft) @ 3,950 rpm
Valves 16 (4-per-cylinder) with Dual AVCS
Fuel system Multi-point injection
Construction Aluminium cylinder block and heads
Emissions class Euro Stage 5
Manual Six-speed
Gear Ratios
1 3.636:1
2 2.235:1
3 1.590:1
4 1.137:1
5 0.891:1
6 0.707:1
Reverse 3.545:1
Final Drive (rear) 3.900:1


Full-time all-wheel drive with centre differential and viscous coupling, plus Vehicle Dynamics Control, Rear limited-slip differential, Front limited-slip differential and Si-Drive adjustable centre differential

Suspension & Damping
Front Fully independent subframe-mounted MacPherson Struts, with Eibach coil springs and gas-filled Bilstein shock absorbers. Anti-roll stabiliser bar
Rear Fully independent subframe-mounted Multi-Link, Eibach coil springs and Bilstein gas-filled shock absorbers (adjustable) anti-roll bar
Type Electric power-assisted rack & pinion
Gearing 2.5 turns lock-to-lock
Turning circle (m) 11.0
Front 355 x 32 mm ventilated discs with 6-pot callipers
Rear 316 x 20 mm vented discs with single-pot callipers
ABS Four-Channel anti-lock system with EBD
Wheels & Tyres
Alloy 18 x 8.5 inch, with 245 / 40 R18 directional tyres
Dimensions (mm)
Overall length 4,415
Overall width 1,795*
Overall height 1,475
Wheelbase 2,625
Front track 1,530
Rear track 1,540
Ground clearance 140
*excluding door mirrors
Interior (all models)
Front Rear
Headroom 1,024 951
Legroom 1,091 960
Shoulder room 1,430 1,426
Hip room 1,384 1,370
Capacities (all models)
Fuel tank 60 litres
Luggage (VDA) 301 litres rear seats up
1,216 litres rear seats down
Weights (kg)
Curb weight (min.) 1,505
Gross weight 1,980
Top speed / mph 155 (electronically limited
0-to-62 mph / sec 3.7
Fuel Economy (mpg)
Combined cycle [tbc]
CO2 g/km [tbc]

(Founder / Chief Editor / Journalist) – Arman is the original founder of Motorward.com, which he kept until August 2009. Currently Arman is our chief editor and is held responsible for a large part of the news we publish.