Doing it the English way: Cosworth Impreza STI CS400

Written by Cheryl Tay on . Posted in Cheryl Tay Blog | Blog Post of Cheryl Tay

9tro issue #5
By Cheryl Tay


After coming out all the way from Central London to Northampton to visit the Cosworth factory, I had to make the visit as worth its while as possible.


So when I was at Cosworth’s manufacturing facility, I also got to experience the 2010 Cosworth Subaru Impreza WRX STI CS400.


As the name obviously suggests, this is a collaboration between Subaru and Cosworth – Subaru’s car with Cosworth technology. And the 400 in the name comes from the increase in power: originally 300bhp to 400bhp after the Cosworth upgrade.





The Cosworth Impreza STI CS400 looks similar to the usual Impreza STI with its standard Xenon headlamps, but there are slight aesthetic differences: Lowered ride height, wider track, new shape front bumper, new fog lights, upper and lower mesh grilles and a new rear spoiler.

rear_spoiler_MDLIt’s hard not to realise that this is a Cosworth-branded car with the Cosworth badging on the lower grille, front seats, brake calipers and floor mats.


Individually-numbered plaques are found on the engine and door sills, and each car comes with a numbered certificate of authenticity.


Fitted with Pro Race’s lighter and stronger 18 x 8.5 inch alloy wheels, the wheels have a 43 mm offset compared to the standard STI wheel’s offset of 55mm). Track is also extended by 12 mm to improve handling.


Tyres have been changed to high-performance low-profiled 245/40 R18 Michelin PilotSport3 with directional tread pattern, which give good dry and wet grip, resilience and low noise levels.




Sitting beneath the bonnet of the CS400 is an upgraded version of Subaru’s horizontally-opposed, all-aluminium Boxer engine.


Taking half a day to remove the stock ‘flat-four’ power unit, the engine is then rebuilt and hand finished over the next 30 hours with several internal and ancillary parts replaced with Cosworth-specified components.


High-performance pistons by Cosworth, forged connecting rods made from high-strength 4340 (EN24) steel and special Cosworth engine bearings are fitted in.


All these help to reduce carbon build-up, minimise friction and reduce weight – the minimum curb weight of the car is now 1,505kg.


Heavy duty cylinder head studs, high-performance multi layered head gaskets and a higher-pressure oil pump are also fitted.


Careful not to overdo it, the engine upgrade is to stay as close as possible to the original equipment design to ensure trouble-free maintenance at regular Subaru dealerships.


The Cosworth turbocharging system in the CS400 features a new compressor design and wastegate actuator for up-rated boost pressure and increased power.


A unique exhaust system was also developed to “allow the engine to breathe more easily” and larger capacity tubular manifolds along with increased diameter downpipes help speed up the evacuation of hot gases. In addition, the first catalyst is a non-restrictive design and the larger tail pipes give the exhaust note a distinctive tone.


With this Cosworth performance upgrade and ECU re-map, the CS400 generates 395bhp of power at 5,750rpm – that’s 33 per cent more power than the standard Impreza STI at 296bhp at 6,000rpm.


Torque has also been increased to 542Nm higher up the rev range at 3,950rpm. Acceleration to 100km/h from standstill now takes just 3.7 seconds, compared to 5.2 seconds before the upgrade.


To manage the increased power of the Cosworth-tuned engine, the six-speed manual transmission has carbon synchromesh applied to the top three ratios and a short throw linkage for quicker shifting.


An upgraded single-plate clutch with a revised pressure plate, bearing and disc has also been added, but the gearbox and final drive ratios remain unchanged.


Subaru’s original suspension layout and major components are kept the same, but a special ride-handling-steering package was customised by Cosworth, Bilstein and Eibach, to match the increased performance.


Eibach coil springs and new Bilstein inserts with unique damping characteristics were manufactured to Cosworth’s specifications. The ride height of the car was lowered by 10 mm at the front too.


Ride quality is generally improved, keeping the car suitable for daily use. On-the-limit behaviour is more focused and composed as the damping upgrade cause less pitch and roll when turning into corners, allowing drivers to get back on the power earlier and fully utilising the car’s Symmetrical All-Wheel-Drive system.


Upping the stopping power is a specially-commissioned bespoke AP Racing brake package with six-pot calipers and larger diameter (355mm) ventilated front discs operating on bigger brake pads.



However, the stock 316mm vented rear brake discs with single-pot calipers, overall brake balance and original ABS system remain.





The CS400’s cabin sports Recaro front seats, Piano Black centre stack and trim accents, and Cosworth-branded black leather trimmings for the rear seats and steering wheel.


Standard features brought over from the stock WRX STI model include LED tail lamps, front and rear electric windows, 10-speaker, six-disc radio/CD player and front, side and curtain airbags.



Only available for the UK market, there are only 75 units of the CS400. Once bookings were open, all units were sold out quickly at the price of GBP 49,995 (over and above the stock one at GBP 27,000).


The first few owners of this limited-edition Cosworth-unique STI collected their keys in early September.


I was asked if I think this car would do well in the Asia market. That was quite a tough question, I replied; there is something called JDM after all.


 Cosworth Impreza STI CS400
Engine: 2,457cc DOHC 16-valve 4-cylinder
Transmission: Six-speed manual
Max Power: 395bhp at 5,750rpm
Max Torque: 542Nm at 3,950rpm
0-100km/h: 3.7 secs
Top Speed: 155mph (electronically limited)

Cheryl Tay

Twitter Facebook Google Plus Linked In
Singapore’s only female full-time motoring and motorsports photojournalist. Independent automotive consultant and prominent local motorsports personality Cheryl Tay is uniquely passionate about all things cars and motorsports. She also has a strong passion to share her interest and knowledge, hence having a dream to become the ‘Oprah Winfrey of cars and motorsports’ and create a multimedia platform for her sharing. - A female in a male-dominated world, Cheryl Tay is Singapore’s only female full-time motoring journalist and motorsports blogger and she regularly writes for prominent titles in Singapore, Asia and internationally. (See full list of titles that Cheryl Tay writes for here.)

Contact us

For invites, request of services, pitches, story ideas, feedback or any other enquiries, kindly email

For submission of press releases and latest news, kindly email

Tips for sending news releases:
- Include text of the release as part of the email message
- Attach press release in MS Word/PDF format
- Include low-res pictures in the email
- Hi-res pictures downloadable via a link or available upon request