Audi A6 VS Audi S4

Audi claims the 45 will sprint from 0-100km/h in six seconds, and the 55 in just over five. So, quick, and very quick.

Both are super responsive in the mid-range with peak torque available from just 1600rpm in the 45 and less than 1400 in the 55.

As is increasingly the norm with Vee engines from the ‘Big Three’ German brands the 55 TFSI’s single, twin-scroll turbo is located in the V6’s ‘hot V’ to shorten gas paths from the exhaust to the turbo, and from the turbo into the inlet side.

The aim is to sharpen throttle response and deliver power in a smooth, linear flow. And with maximum torque available from so low down in the rev range, that’s exactly the way it feels.

Select Sport mode, squeeze the right-hand pedal, and the 55’s V6 delivers a firm, consistent shove in the back. The 45 is less urgent in terms of acceleration, but more than adequate for easy highway cruising and confident overtaking.

Both are quietly quick, thanks in part to low-noise acoustic glass and comprehensive use of sound absorption materials around the cabin, remaining composed and relatively subdued as speed rises.

The seven-speed dual-clutch delivers ultra-smooth shifts at around-town speeds and crisp, positive changes in manual mode.

In normal, suburban-style conditions the quattro system decouples the rear axle and sticks with front-wheel drive economy. If all-wheel drive is required, a tricky clutch instantly activates it, in certain situations predicatively.

On top of that, in aggressive cornering torque vectoring by braking (Audi calls it ‘Wheel-Selective Torque Control’) retards the near-side wheels before they slip.

Suspension is a five-link set-up front and rear, with much of the hardware made from aluminium to fine tune response and reduce unsprung weight.

Electronically controlled adaptive dampers are standard on the S line models, with the switch between dynamic and comfort settings swift and pronounced.

Rims are 19-inch on the 45 and 20s on the 45 S line and 55 S line, but all variants are comfortable. Never floaty or unwieldy, just refined and well damped.

The electromechanically assisted steering points accurately but the assistance is overdone and road feel isn’t one of the A6’s strongest suits.

Brakes are 375mm ventilated discs at the front, clamped by six-piston alloy calipers, with 350mm rotors at the rear. They inspire confidence, with progressive feel and more than enough to confidently arrest the 1.8-tonne A6's progress.

The S4 is terrific fun. It's that simple. The new engine is an absolute belter, with all 500Nm of torque available at just 1370rpm. Lag is almost indistinguishable as the turbo spools up and rockets you along to 100kmh in under five seconds, that huge half-ton of torque sweeping you down the road.

With dynamic mode switched on, the car muscles up with firmer suspension and a slightly growlier exhaust. The steering really weights up, too, which takes a little getting used to after the lighter, friendlier setting in Comfort mode - thankfully you can set up an individual mode to dial the assistance back in while everything else is set up for go-fast.

The key to the fun is the rear-biased quattro system - while it's never going to match the purity of BMW's rear-wheel drive 340i, what you lose in steering feel and ultimate adjustability, you gain in off-the-line and mid-corner grip its German rivals could only dream of.

Going fast in an S4 is easy, leaving you to concentrate a little more on your line and gear selection, listening to the distant bark of the engine and the occasional turbo whistle. All of this is purely down to driving taste, of course but the point is, the S4 offers something a little different to the other two. 

For most of the time the S4 defies its 1700kg-plus weight but there is the occasional hesitancy when changing direction through a challenging set of bends, as though the front tyres (245s all around, if you're interested) want to scrub and the quattro system makes a quick adjustment to stop it happening. You don't feel that's what's happening, of course - Audi Sport is better than that - but it's part and parcel of all-wheel drive. It takes a lot to find understeer and for most people, that just won't ever happen.

Further confidence comes from the terrific brakes - 350mm up front and 330mm at the rear, the big forward rotors are gripped by six-pot fixed calipers. Performance is epic and in hard, fast driving they stood up to a fair amount of punishment without fading.

The ride is excellent in all modes, which is quite an achievement given the fat rubber and big wheels, although big bumps at speed make a huge metallic thunk without actually upsetting progress. The eight-speed ZF is brilliant as always and unless you're really motoring, you don't even need its sport mode, which brings impressively fast and positive shifts.

The best thing is, if you don't look in the rear vision mirror, you can't really tell if you're driving sedan or Avant. That might be because it's quite absorbing on a launch drive, but I couldn't split the driving experience between the two.

The only black marks I could easily identify on the S4 is tyre noise on some surfaces and perhaps the steering could be a little more lively like its competitors.