Audi S8

The S8 does what all fast Audis should: it delivers crushing straight-line performance with minimum fuss. Unsurprisingly, the star of the show is the new 4.0-litre V8, which combines storming pace with a stirring soundtrack. And thanks to an innovative cylinder shut-off system, stop-start and direct injection, it promises the best economy in its class. We’ll wait until we drive a production car on a longer route before passing final judgement on the ride and handling, but this prototype was impressively comfortable – which bodes well for the everyday usability of Audi’s latest executive express.

Nobody does understated high-performance models quite like Audi. And the S8 is the best of the breed, combining the looks of a discreet luxury saloon with the heart of a supercar.

The latest version is set to up the ante even further with an all-new 526bhp 4.0-litre twin-turbo V8, and we had the chance to sample this engine in a prototype car.

The unit will be used in the forthcoming S6 and S7, albeit in a detuned state, as well as in the Bentley Continental GT, and it has a clever trick up its sleeve. When it’s producing between 120Nm and 250Nm of torque, the gearbox is in third or higher and the revs are between 1,000rpm and 3,500rpm, four of the eight cylinders are shut down to cut fuel consumption considerably. Audi claims the car will use 12 per cent less unleaded at a 50mph cruise.

A dash indicator switches in colour to green when the S8 is running as a four-cylinder – but that’s not the only change.

Because a V4 lacks the inherent refinement of a V8, Audi has fitted active engine mounts and active noise-cancelling technology to remove any unwanted sounds and vibrations. When the driver requires more power, the sleeping cylinders fire back up within milliseconds.

For a prototype, this S8 drives impressively like a production model. Where the previous car had you shaking in your seat when driving over poor surfaces, the newcomer is truly cosseting. It grips hard through corners, too.

However, the engine is clearly the star of the show. From idle, it feels taut and smooth at the same time. It boasts a deep, rumbling menace, and this deepens further with every single prod of the throttle.

The Audi is fast, too, shoving you in the back as the smooth eight-speed automatic transmission sends power to all four wheels. It just never seems to run out of punch, spinning freely and vigorously to the red line in every gear.

When the cylinders shut down, it all happens so calmly that you don’t even notice. If anything, the engine gets quieter when operating as a V4. Yet when you ask it to wake up again, it does so instantly.

The auto’s kickdown neatly disguises the short time – three tenths of a second – it takes the cylinders to fire again, making the whole process seamless.

We look forward to driving the full production version, but judging by this prototype, the S8 is a supersaloon with a conscience.