Our spy photographers have caught Audi testing a prototype mule of the upcoming Audi S7 Sportback sports sedan. The car fits into the Audi 'S' model range as the largest and most luxurious in the showroom.

The S7 will feature revised front and rear bumpers over the standard A7 Sportback, showcasing more sporty highlights including a deeper and lower front spoiler, and side intake vents.

The Audi S7 will feature various performance-oriented enhancements including larger disc brakes with multi-piston calipers, lower and sportier suspension settings, sportier and larger alloy wheels and a high-output engine.

Details on the engine are kind of sketchy at this stage but reports say the new Audi S7 will get the 4.2-litre V8 also seen in the Audi R8 supercar. It is expected to produce around 320kW of power. The V10 from the R8 would then be reserved for the Audi RS7, producing upwards of 380kW.

The S7 will be equipped with Audi's quattro all-wheel drive system and use a seven-speed dual-clutch automatic transmission with paddle shifters. Acceleration from 0-100km/h is expected be in the four-second range.

From the back it's easy to see this test mule is indeed the S7, showing off the traditional 'S' quad exhaust and slightly more aggressive rear bumper bar layout.

Although it hasn't been officially announced, the Audi S7 is expected to make an international debut at this year's Frankfurt Motor Show in September. It will then go on sale sometime in the first half of 2012.