It takes a keen eye to spot the subtle visual differences that separate the new Audi S4 Avant from its less powerful and considerably cheaper sibling, the A4 Avant. Four exhaust pipes, skimpy side skirts and brushed aluminium door mirror covers are the only indications that this is in fact a Q-car, a veritable wolf in sheep's clothing. A medium-size estate car, the S4 Avant is handsome, but in a rather modest, no-frills kind of way. With its huge goatee of a front grille and LED eyeliner on its headlights, it is unmistakably an Audi, but it is bereft of any aggressive styling cues that would hint at its BMW 335i-bashing performance. Not only does it outperform its arch rival, the 335i, it will give BMW M3 owners a fright and run with minor league Porsche 911s. The key to this is Audi's new supercharged 3 litre V6 that replaces the previous model's 4.2 litre V8 in the interests of less weight, better fuel consumption and lower emissions. Audi has not used a supercharged engine since the fabulous Auto Union racing cars of the 1930s. The new V6 produces 333 bhp and a towering 440Nm of torque, 90 per cent of which is available over a wide rev range for outstanding overtaking. Audi's direct fuel injection system, TFSI (the T confusingly standing for turbocharged), not only provides better control of the fuel/air mix for increased efficiency, it also cools the cylinders allowing a higher compression ratio to be used for more power. The power is transferred to all four wheels via Audi's seven-speed S-tronic dual clutch gearbox. Just as with other dual-clutch systems, such as Volkswagen's DSG or Porsche's PDK, the gearbox can be used as an automatic or operated manually via the steering wheel paddles. Because the gearbox is mechanical, as opposed to the fluid coupling of an automatic, gear changes are crisp and instantaneous, taking less than 0.2 seconds from cog to cog whether in automatic or manual mode. When driving hard, downshifts are accompanied by a satisfying blip of the throttle adding to the smoothness and sportiness of the experience. Audi has been at the forefront of four-wheel drive development since the introduction of its Quattro model in 1980. The S4 Avant uses a Torsen centre differential to allocate the optimum amount of power to each wheel for all conditions. The car feels like it corners on rails. Even full throttle acceleration when rounding a corner fails to upset its equilibrium and poise - it takes bends like a supercar. The ride is good and the feel through the steering wheel puts the driver in touch with the road conditions. The optional Audi Drive Select is worth the outlay. With its comfort, auto and dynamic settings, the driver can choose between the comfort of a long-distance tourer or the excitement of an out-and-out racer. An individual mode allows the owner to choose a tailored set-up from the on-board computer menu. The interior is first class; the materials used and the fit and finish are of the highest order. The optional sports seat in two-tone leather is exceptionally comfortable and provides excellent support while cornering. The multifunction steering wheel allows the driver to control all major comfort and entertainment features without lifting a hand. Boot space is good at 490 litres, and this can be increased to 1,430 litres with the rear seats folded down. There is even a 12-volt socket for equipment such as a refrigerated box. The only niggle is that space in the driver footwell has been compromised by the transmission tunnel. The lack of a left foot rest could be uncomfortable on longer journeys. HK$734,000 (HK$807,475 as tested) may seem like a lot of money for what looks like a family estate car, but the S4 Avant offers terrific bang for your buck.