Our photographers caught on camera a BMW 6-Series Gran Coupe developed by Alpina while undergoing testing.

The prototype was spotted riding on Alpina's elegant multi-spoke alloy wheels and was hiding some changes to the front and rear bumpers, as well as the rear spoiler. Aside from visual tweaks, Alpina's 6-Series Gran Coupe will also come with some upgrades in terms of suspension and brakes.

Our sources have informed us Alpina is tuning the 650i model which in its standard configuration offers 450 HP (330 kW) and 479 lb-ft (650 Nm) of torque from its twin-turbo V8 4.4-liter engine. A power boost of 100 HP (73 kW) is rumored, bringing total output to an impressive 550 HP (404 kW) and close to the 2014 M6.

The tuner could decide to unveil the car in September at the Frankfurt Motor Show.

Gallery: Alpina 6-Series Gran Coupe spied testing

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