SUBARU's fastest and most powerful Impreza yet - the WRX STi WR1 - is about to enter dealer showrooms, with more than half of the 500 cars in the limited run already spoken for.

The special edition Impreza WRX STi celebrates Petter Solberg winning the driver's title in last season's World Rally Championship. It features a power upgrade to 320 PS plus a host of technical and visual upgrades, and costs £29,995 on the road.

First unveiled in January at the Autosport show, the Prodrive-developed WR1 has just received official Subaru (UK) engineering approval after recording the best performance figures for a road-going UK-spec Impreza.

For example, 60mph is reached in an astonishing 4.25 seconds with the 0-100mph time an equally impressive 10.67 seconds. Prodrive technicians also logged a quarter mile time of 12.8 seconds, while the top speed is limited to 155mph.

By comparison, the standard 265PS Impreza STi is quoted with a 0-60mph time of 5.1 seconds and a top speed of 151.5mph.

Peak power for the WR1 is 320PS at a relatively low 5,800rpm, with maximum torque or pulling power of 309.8 lb/ft at 4,000rpm.

The standard STi produces 265PS at 6,000rpm and 253 lb/ft torque at 4,000rpm while the optional Prodrive-developed Performance Pack liberates 305PS and 299 lb/ft torque at the same revs and reaches 60mph in 4.6 seconds and 100mph in 12.2 seconds.

Other test track statistics see the WR1 shaving 0.7 seconds off the STi's 30-50mph time in 4th gear - down to 4.0 seconds - and 0.8 seconds off the 50-70mph time in 5th - now 4.6 seconds.

Finally, illustrating its supercar-like performance, the WR1 boasts an 80-100mph time of a staggering 3.6 seconds in fourth gear and 6.5 seconds in 6th (4.6 and 7.8 seconds for the 265PS STi).

The extra power and torque are achieved thanks to a revised engine control unit (ECU), high-flow catalyst and sports silencer.

Also guaranteed to excite enthusiasts is a driver-controlled centre-differential via a switch mounted next to the handbrake.

It means the torque distribution between front and rear wheels can be manually selected. This allows the choice, for example, between a sharper cornering turn-in or more stable straight-line running. The system defaults to "automatic" mode whenever the ignition is restarted.

Also new are special, Prodrive-developed uprated springs and bump-stops with a 25mm lower ride-height. Other handling features include 18in diameter P-FF7 (Prodrive Flow Forming seven-spoke) alloy wheels with 225/45 Pirelli PZero tyres.

Externally, the WR1 is instantly recognisable thanks to its new Ice Blue Metallic paint plus driving lamps and a stainless steel mesh grille.

Interior changes include anthracite instead of blue suede-effect seats, a special gear knob, imaged carpet mats and a tax disc holder engraved with the special edition number.

Features shared with all Impreza WRX STi's range from a six-speed gearbox to Brembo brakes and front and rear Suretrac differentials.

Updated: 10:01 Friday, March 19, 2004