When the next generation Audi S4 makes its expected debut sometime in 2009 as a 2010 model, it will lose its snarling V8 for a more fuel-efficient forced induction V6, like the previous-generation, B7-chassis car. While the all-new V6-powered Audi S4 has only lived in our imaginations until now, spy photographers have finally captured the first images of the next-generation car while out testing.

While the move to a V6 was foreseen, at the Vienna Motor Symposium, Audi revealed some details on the engine. A supercharger will sit under the hood of the European and North American S4, which is due out this fall. Power is rated at 350 horsepower, but fuel economy will be much improved over the outgoing S4's 340hp 4.2-liter V8.

The engine will make its debut in the A6, although in slightly detuned form, producing 290 hp and 310 lb-ft of torque. The A6 is due out this fall, with the S4 not far behind, according to Germany's Auto Motor und Sport.

The new S4 will be lighter as well, as the B9 A4 body is about 100lbs lighter than the outgoing model and the two will share body panels. An active differential sourced from Magna will be able to distribute power on demand between wheels on the same axle is also expected to be part of the package to help the all-wheel driver handle like a rear-wheel drive vehicle. The same differential will make its way into the company's Q5 SUV and S5 GT coupe as well.

Like previous S4 models, the new car will be visually similar to the standard A4, but will have some design cues to separate it from lesser cars — including a lower stance and quad-exhaust.

Audi's move back to forced induction is in tune with a recent prediction turbocharged gasoline engines will double by 2013 on account of the gas savings they offer.