Meet the Subaru Impreza STI CS400. Feel free to forget that rather clunky name if you wish, because you wouldn't use it anyway, not when you can simply call thie car the Impreza Cosworth or Cosworth STI. The subject of rumour and gossip since its existence was confirmed by a brief announcement a couple of months ago in the UK, this car is now a reality.

Cosworth Impreza STi CS400 

The 2.5 litre engine has been tuned to 295kW and 540Nm, a significant 78kW jump over the regular STI, allowing the 0-100khp time to drop by 1.5 seconds to a claimed 3.7 seconds. That is seriously quick, with acceleration to match a Lamborghini Gallardo LP 560-4 and is way ahead of its traditional rivals, the Mitsubishi Evolution and Ford Focus RS, with an electronically limited top speed of 250km/h.

Subaru, anxious to restore some much needed sporting credibility to the Impreza, approached Cosworth about a possible collaboration 15 months ago. This may seem an unusual partnership given Subaru's historic ties with Prodrive in the UK, but Cosworth already had experience with the turbocharged flat-four through its US tuning division. Based in California, it has been modifiying boxer engines for the West Coast tuning scene for several years, often extracting between 450 and 600kW.

Cosworth STiFive Imprezas were delivered to Cosworth's unassuming headquarters in Northampton for work to be carried out. Complete details on the work done to the engine are below, but power increase was mainly due to significant internal improvements and enhanced ECU mapping (we can't tell you the details of the ECU enhancement, but we at MRT know Cosworth used the best ECU software!) . Its also worth bearing in mind at this stage that although it reputation has been built on engine development, Cosworth's talents run deeper. The firm took on responsibility for the entire development process, the result of which is a car with new Eibach spring and Bilstein damper units, a 10mm front suspension drop, a 10mm wider rear track and AP Racing front brakes with six-pot calipers gripping 355mm discs with a unique groove pattern.

The aim isn't to creat a trackday special, but a fast road car, on that retains the driveability of the standard STI but, in the words of Kenyon Neads, Subaru UK's marketing director, "takes it to the next level. When you drive it, you'll find that the dynamics, the composure, stability and power delivery has created a very different and better car."

The understated appearance of the CS400 backs up his assertions. The rear wing is unaltered, the wheels are still "only" 18s, and the colour choices are limited to sliver, red and dark grey. Look close and you might spot the new mesh grille, badges (besides the Cosworth branding, each car will wear an engine signature and a numbered plaque on the door sill) and the darkened glass.

Cosworth STi InteriorInside, the seats have been re-trimmed and feature fatter bolstering, while liberal application of piano-black inserts has helped lift the ambience a fraction, although it doesn't look or feel like a $100K car. Which is what the CS400 is, priced at £49,995 is a near doubling of the £27,590 a standard STI costs in th UK, leaving a yawning price gap between the two.

But not necessarily for much longer. "This is the ultimate manifestation of where we think we can take the STI" Neads says, "but this partnership with Cosworth is ongoing." In other words, the two firms have invested heavily in this car, too heavily for them to call it quits after just 75 units. So expect to see another model, probably with around 260kW.

One final thought about the CS400. The reputations of both firms are on the line: for Subaru this is a chance to prove it hasn't lost its touch, that the hatchback Impreza platform can play host to a driver's car every bit as good as the old saloons. For Cosworth, this is the first time its name has appeared on a road car since the demise of the Escort Cosworth in 1996. Both are determined to get it right.

Personally, we'd love to see how it fares against Mitusbishi's FQ400 Evo X UK model. And for both of them to be available here in Australia from a Subaru or Mitsubishi dealer.

Interview with Bruce Wood, Technical Director of Cosworth, on the engine upgrades to the CS400

As Cosworth's technical director, Bruce Wood is a man who can answer a question that has haunted many Subaru enthusiasts. Is the 2.5 litre flat-four used in European, American and Australian Imprezas less robust than the Japanese market 2.0 litre?

"The basic architecture of the 2.5 is very strong" says Wood. "You can see a lot of evidence of where core work has gone into improving emissions and durability. Its by no means less tuneable, just harder work."

So how did Cosworth go about getting 295kW from the 2.5? "We started by running what we calll a Finite Element Analysis and saw immediately that the piston and rod would not provide the durability we wanted. So we designed our own, forged on site here" says Woods.

Cosworth STiIts the bottom end where most work has been done, including re-grinding the bores, but the head gaskets and oil pump are also new, as are other engine internals, plus the exhaust and turbo. The ECU is also remapped.

Larger capacity manifolds and wider-bore downpipes reduce back pressure, while the turbo is a unique combination of parts built around a Garrett centre section. None of the modifications will be sold as aftermarket parts.

"What's crucial to us is that the CS400 drives, feels and behaves like an Original Subaru STI model" says Woods. "It's fine developing an 600kW car in the US because there's every expectation that it will expire at some point. Buy one from a Subaru UK dealer and there's every expectation that it won't. Durability is crucial."

Sources: CarAdvice and Evo UK Magazine