Audi S8

Thanks to its thunderous 5.2-litre V10, Audi's S8 is a memorable experience. Under hard acceleration, the Lamborghini engine provides fierce performance, but also allows relaxed cruising. Through corners it's not as much fun as a BMW M5, but it's more sporty than7-Series and S-Class rivals. With one of the best interiors of any luxury car, this is a stylish and sophisticated express.

Big, fast luxury motors are quickly becoming an Audi speciality. In the past few years we've seen the rapid S4, RS4 and S6 prove that the German manufacturer is the master of the executive express. And the even-faster twin-turbocharged RS6 - a car capable of nearly 200mph - finally showed that BMW's M5 and the Mercedes-Benz E-Class AMG faced some real competition.

So it always seemed strange that the model at the top of the German marque's range, the spacious S8, was ordinary by comparison. Never seen as anything more than swift luxury transport, it was a car that couldn't call itself a super saloon by today's standards. But that's changing, thanks to a V10 engine borrowed from the Lamborghini Gallardo. Fitted to the latest A8, it puts Audi firmly on the map as the performance saloon king.

To make sure that the S8 has the best possible chance of success, Audi engineers haven't transplanted the Italian supercar's engine untouched - they've enlarged it from 5.0 litres to 5.2 and added FSI direct-injection. Refinement of the powerplant has been improved with the addition ofa balancer shaft, which damps out vibrations under acceleration.

The result is 450bhp and 540Nm of torque - 90 per cent of which is available from 2,300rpm. Hooked up to a close ratio six-speed automatic gearbox, this car is extremely fast -0-60mph takes five seconds, while top speed is limited to 155mph. On the move, throttle response is instantaneous and searing. But even better is the way the V10 unit sounds, with a hard-edged deep metallic howl. To cope with the extra performance, the S8's air-suspension uses the Sport mode of other A8 models as its base, adding stiffer dampers, a 20mm lower ride height and a quicker steering rack. When combined with the quattro four-wheel-drive system, which biases torque 40:60 in favour of the rear wheels, the S8 proves surprisingly agile for a car that weighs 1,940kg, and it's easy to cover ground very quickly and feel secure at the same time. But although the Audi changes direction more smartly than rival models and body control is better than on less sporty A8s, it doesn't feel as performance-oriented as a BMW M5.

The steering weights up artificially during hard cornering and has little feel, so it's best to let the car do the work, sit back and enjoy the ride, which despite the massive 20-inch alloy wheels, proves almost as comfortable as in a regular A8.

Only over small bumps is the added stiffness jarring for those inside the cabin - although if you switch the adaptive damping to Sport mode, the comfort deteriorates rapidly.

The performance of the brakes is outstanding, especially if they're the optional all-round ceramic discs, which add an extra £5,000 to the £70,000 price tag. They don't fade at all, last 186,000 miles and stop the hefty S8 with ease. However, while the com-pany's engineers have made a number of changes under the surface, stylists have resisted the temptation to overdress the outside of the vehicle.

A large grille, numerous frontal air intakes and wing-mounted V10 badges give a heavy hint that this is no ordinary A8, but with a subtle bodykit and minimal boot spoiler, the S8 looks every inch the suave performance express.

Inside, the leather-lined cabin has two-tone seats, carbon fibre trim and brushed aluminium for the gearshift paddles, which are fitted behind the three-spoke steering wheel. And, as befits a top-of-the-range luxury car, you'll find a DVD satellite navigation system, a BOSE six-CD stereo, electric seats and adaptive xenon headlights, are all included as standard. It all adds up to a very impressive package.