It's hard to believe that it's over a dozen years since the anonymous looking Subaru Impreza Turbo shook the automotive world. Since then it has become synonymous with back road burning credentials and has evolved to reflect new technologies and market trends. Typically changes have included power upgrades and cosmetic modifications that have gradually honed the car's capability and built on the strengths whilst addressing weaknesses.

For the 2006 model year the Subaru Impreza has received a large number of upgrades. The spec sheet shows new engine, gearbox, transmission, differentials, suspension and tyres as well as a new appearance and improved interior. Subaru is making a big effort with this latest car.

Externally the facelifted model is a real improvement over the outgoing Impreza. The new nose treatment eradicates the memories of the rather startled looking Mk2 Impreza with a new grille treatment intended to resemble spreading wings, reflecting parent company Fuji Heavy Industry's background in aviation. The large bonnet scoop remains (albeit slightly less gaping), as does the heavily sculpted front bumper. Things are a little different at the rear where the huge spoiler has been joined by a diffuser below the rear bumper and a new smaller (and a good deal more subtle than the tea tray item on the boot lid) wing above the rear window.

Beneath the skin the changes are altogether more serious. The complete driveline has been changed from front to rear with some interesting and decisive hardware changes. Under the bonnet the change to the 2.5-litre quad cam engine was not in the interests of ultimate power - the headline figure has raised by only 11bhp to 276bhp. Critically the peak power figure is now produced lower down the rev range at 6000rpm and this is an indicator of the new engine's persona. The real proof of the pudding lays in the torque figure, up by 36lb.ft (to 289lb.ft) now at 4000rpm. Helped by variable valve timing, this engine offers a broader, flatter torque curve than the 2-litre unit it replaces.

Where the old Impreza WRX STi was a whoosh of boosted frenzy, the new car boasts a more generous spread of accessible power with useful thrust now on tap from around 2500rpm and ever present up to the 7000rpm red line. Smoothness remains a strength although the throbbing exhaust note has been diluted somewhat. It's a shame, but a price worth paying for the benefits the new engine brings.

The enhanced torque curve has enabled the engineers to optimise the gear ratios, with sixth now effectively a leggy overdrive intended to improve refinement and cruising, not to mention increase fuel economy. In gear acceleration in 6th is tardy as a consequence, at least up to around 60mph, but again it's a sensible and worthwhile trade.

Acceleration in the other five gears is on the serious side of quick. Zero to sixty mph in five seconds doesn't seem a huge improvement over the old car, nor does the 0-100mph of 12 seconds, nor indeed the 158mph maximum. What these headline figures don't reflect is the real world difference in pace. The new STi is a much more accomplished overtaker and has significantly more grunt pulling out of corners. A choice of gears is now available where previously only one would have been the optimum solution.

Shift quality has been worked on as well with the upper gears receiving a special coating to improve the speed and feel of the shift; it's another small but noticeable improvement. There is a little reluctance from cold, particularly in engaging second and third gears. What is also slightly odd is the rolling resistance I noticed from cold and some low speed protestations from the new driveline.

The symmetrical four-wheel drive system has received some major changes. These are designed to complement the new suspension setup introduced on the previous generation. For the first time a yaw sensor features for optimising power distribution working in conjunction with steering and throttle sensors and the front, rear and centre differentials. It is the centre differential that is so-to-speak, pivotal.

Drivers can alter the characteristic of this viscous diff by a small dial next to the handbrake enabling him to pick the handling traits dependant on the grip available. More drive can be transferred to the rear to improve turn-in at the expense of straight-line stability. This one button has transformed the Impreza's handling. No longer does one wait for the nose to bite or get frustrated by the early onset of understeer. Now the nose turns in sharply and the sticky Bridgestones chew into the surface, in turn enabling the application of the extra torque on hand. In very greasy conditions understeer remains the order of the day and the driving style needs to be adjusted accordingly but in the dry, damp or wet conditions we drove the car in it was streets ahead of the old Impreza.

Ride quality has deteriorated slightly, partially due to those stiff side-walled tyres no doubt but also down to the stiffer settings adopted in the interests of agility. The steering is also better, particularly when drive is transferred away from the front wheels, thanks in part to an anti-kickback damping valve in the hydraulic circuit. Speaking of hydraulics, the Brembo brake set-up is infallible, but could do with a touch more feedback and feel through the middle pedal.

Inside the new Subaru Impreza is spacious and comfortable, the STi logoed seats fitted to our test car are great, as is the nice to hold Momo wheel. The pseudo suede trim on the seats and doors looks and feels great and is a welcome brightening focal point in what could be a bland interior. The revised dials are clear and are assisted by the move of the dimming control to the indicator stalk allowing for quick and easy adjustment of brightness when switching from dark to light to compensate for switching on the lights.

Overall, the new Impreza WRX STi is a revelation. For some it is still overly brash in terms of appearance, but there is at least a WRX STi spec.D (for discreet, not diesel!) version which I'd personally prefer as it deletes the large rear wing. On the road the difference in real world pace is palpable. In a straight fight the old Impreza wouldn't see which way the new car has gone. It has been improved in every area that counts from a driver's point of view. Handling balance, agility, flexibility, steering, the list is exhaustive and impressive and makes this the best Impreza officially imported onto these shores by a country mile.

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Subaru Impreza UK range overview

Four-door saloon:
- Subaru Impreza 2.0R Sport £16,200
- Subaru Impreza 2.0R Sport Automatic £17,250
- Subaru Impreza 2.5 WRX £20,900
- Subaru Impreza 2.5 WRX SL £22,400
- Subaru Impreza 2.5 WRX-STi Type UK £26,995

Five-door wagon:
- Subaru Impreza 2.0R £16,200
- Subaru Impreza 2.0R Sport £16,700
- Subaru Impreza 2.0R Sport Automatic £17,750
- Subaru Impreza 2.5 WRX £21,400
- Subaru Impreza 2.5 WRX SL £22,900