Road Test: Audi A8 S8 Quattro 4dr Auto

This is a story about the engine rather than the car. Sorry if you were expecting to get chapter and verse on the new Audi S8, but take it from us the engine is more important (and more interesting) than the car here. Audi will sell a handful of S8s in Europe, a few more in the States, and probably a clutch in China. For a firm that sells 1.5 million motors a year, the S8 is chicken feed.

But the engine, the new 4.0-litre twin turbo, that's the thing. This is the engine that will soon power the Bentley Conti GT and will probably be tweaked a bit for the next Audi RS6. It will underpin anything large and fast from the VW Group, period (with the exception of those die-hards at Porsche). So to that end it has cylinder cut-out, it has eco modes, it can do 27.7mpg and still send a 5.2-metre long limo to 62mph in 4.2 seconds. That's ridiculous, isn't it?

Sure the all-new V8 twin turbo has 513bhp, but torque's the thing here - 479lb ft anywhere and everywhere, from 1,700-5,500rpm. That's amazing. You put your foot down, there's a slight delay, a bit of softness to the engine response that can probably be dialled out for the more sporting cars this 4.0-litre will sit in, and off it goes. There's a bit of rumble and burble (to be honest we'd like a bit more noise, but we appreciate the S8 isn't that sort of car) and soon you're doing a deeply illegal speed. That's what happens when effortless speed meets effortless refinement. You just don't know how fast you're going.

OK, so we lied - we will come clean about the S8. Promise not to be disappointed? Good. Because the S8's not very interesting. Engine note aside I don't know why you'd have it over a diesel A8. Both major on torque and refinement. Audi claims the S8 is sporty and sure, it's faster in a straight line and I'm sure they have figures that show it's superior in every measurable test, but it doesn't feel exciting to drive.

There are flaws, too. The car always seems to be a beat behind your inputs. Hit the brakes and the car pauses a fraction before responding, same when you try to change gear manually in the eight speed auto or turn the steering. In fact the steering is the most curious of all. It has something called Dynamic Steering, which, when you approach the limits of grip can adjust the steering angle without you turning the wheel.

You only notice it working in extremis, but hey, Audi took us to a racetrack and it was horrible. You turned into a corner and tried to hold a steady line, only for the car to start moving around under you. I'm sure it's got some safety benefits, but it doesn't boost confidence. In fact the combination of hot weather, a two ton car and tight corners ensured the S8 battled understeer all the way round, despite the standard fit Sports Differential - which incidentally does do a pretty good job of feeding more power to the rear wheels.

That aside, circuit work did the S8 no favours. This is a big, heavy car that's designed not for track days, but for fast road work. There it makes more sense. It's stable, quiet, you don't feel the steering working weirdly (although it's still slow to react), but instead marvel at the fact you can really get 30mpg from a 500bhp limousine - or 15mpg if you want to enjoy the muted roar and the way the S8 scampers through the rev band. And scoots through corners. It is a very composed and accomplished thing. But not an involving one.

Despite the modest visual overhaul (tweaked bumpers, new badges, extra brushed aluminium, more toys), the clever engine tech, the new seats and everything else that's designed to make the limo feel a fraction - but not too much - more sporty, the S8 simply doesn't feel different enough to a standard A8. It's in a weird position, a niche within a niche, but for our money it needs more of a USP in order to stand out from the crowd. By itself, the new engine is not enough.