Overview: Subaru Impreza Cosworth STI CS400

By the time you read this, Subaru will have sold every last Cosworth Impreza. We can be fairly certain of this, because only 75 are being built — or, rather, rebuilt: the Cosworths start out as standard cars before being heavily reworked by Cosworth. The model is being offered only in Britain, and only in right-hand drive.

With Cosworth’s help, the turbocharged Subaru 2.5-litre boxer engine gets a remarkable 33 per cent. boost in both power and torque. The increase in power comes entirely through boosting torque, with the maxima of both curves occurring at lower crankshaft speeds than is the case with the standard car. Headline outputs of 400PS and 542Nm represent volumetric specific outputs of 162PS/l and 220Nm/l, with power- and torque-to-mass ratios of 265PS/t and 360Nm/t. Subaru claims a 0-100km/h sprint time of 3.7s, and we can believe it.

Subaru Impreza Cosworth STI CS400.

Fortunately, the engine drives all four wheels. A conventional six-speed manual transmission is used. The unit has carbon synchromesh applied to the top three ratios, and the single dry-plate clutch has a revised pressure plate, bearing and disc. Gearbox and final drive ratios are unchanged. No additional uprating is necessary, as the gearbox is still operating within its torque capacity.

Both front and rear differentials have limited slip. The viscous coupling centre differential normally splits torque equally between front and rear axles, though the torque split can be varied electronically according to the available traction.

The Cosworth Impreza deploys a considerable number of Cosworth-engineered components. Subaru’s horizontally-opposed, all-aluminium engine is heavily revised, and its control unit is remapped. The power-plant is removed from the car, hand finished, and several internal and ancillary components are replaced with up-rated items. The turbo­charger system is new and of Cosworth origin.

Internal upgrades include high-performance pistons, which are manufactured using the same methods as those employed to create Cosworth’s Formula 1 pistons. They are designed to reduce carbon build-up, minimise friction and reduce weight.

New forged connecting rods are fitted, made from high-strength 4340 (EN24) steel. Cosworth’s own engine bearings are used, as are heavy-duty cylinder head studs, high-performance multi-layered head gaskets and a higher-pressure oil pump.

4340 (EN24) steel
Ni: 1.30-1.70
Cr: 1.00-1.40
Mn: 0.45-0.70
C: 0.36-0.44
Mo: 0.20-0.35
Si: 0.10-0.35
S: 0.040 max
P: 0.035 max

The engine’s layout remains unchanged, with two overhead camshafts per bank and variable valve timing. One of Cosworth’s priorities in uprating the engine was to keep the enhancements simple, efficient, and as close as possible to the original equipment design to avoid any potential reliability problems. The facility for the engine to be maintained at regular Subaru dealerships was also a necessity.

The result of all of the changes is a substantial fattening of the torque curve higher up the rev range — as well as the lower peak — and significantly improved in-gear acceleration.

A unique exhaust system was developed to allow better breathing at high crankshaft speeds. Larger capacity tubular manifolds and increased diameter downpipes speed up the evacuation of exhaust gases. The first catalyst is a non-restrictive design.

Adjustments to the Subaru’s running-gear involved technicians from Bilstein and Eibach. Eibach coil springs and new Bilstein inserts, featuring unique damping characteristics, were manufactured to specifications set out by Cosworth. The ride height was dropped by 10mm at the front. Subaru claims that ride quality has improved significantly over the standard model, while on-the-limit behaviour is described by Subaru as more focused and composed. The damping upgrades reduce pitch and roll when turning into corners.

To increase the Impreza’s braking performance, Cosworth commissioned A.P. Racing to develop more powerful front brakes. Larger diameter (355mm) ventilated front discs were adopted, with six-pot callipers featuring bespoke pistons operating on larger and harder brake pads. The standard car’s rear brake discs, overall brake balance and original ABS system have been retained.

The CS400’s alloy wheels are larger than those of the regular model — 18"x8.5" instead of 17"x8" — and also lighter and stronger. The Cosworth’s wheels have a 43mm offset against the standard car’s 55mm, widening the track by 12mm. Michelin SP3 tyres with directional tread are fitted.

Individually numbered plaques are fitted on the engine and door sills. Each Impreza Cosworth STI CS400 is supplied with a numbered certificate of authenticity.

The CS400 is Cosworth’s first road car since the last Escort Cosworth was built in 1996, when the Company was owned by Ford.

Subaru Impreza
Cosworth
STI CS400
Impreza
2.5 WRX
STI
Cylinders 4HO 4HO
Valves 4 4
Bore/stroke 99.5/79.0 99.5/79.0
Swept volume 2457cc 2457cc
Compression
ratio
8.2:1 8.2:1
PS/rpm 400/5750 300/6000
Nm/rpm 542/3950 407/4000
Maximum speed 155 155
0-100km/h 3.7 4.8
Urban MPG
(l/100km)
TBA 20.5
(13.8)
Combined MPG
(l/100km)
TBA 27.4
(10.3)
CO2 g/km TBA 243
Emissions TBA TBA
Transmission M6 M6
Driven wheels All All
Fuel tank 60l 60l
Kerb mass * 1505 1505
PS/t 265 199
Nm/t 360 270
Length 4415 4415
Width 1795 1795
Height 1475 1475
Wheelbase 2625 2625
Track: front
Track: rear
1542
1552
1530
1540
* DIN. EU kerb mass = DIN + 75kg.
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