The BMW M4 CSL is the Bavarian automaker's worst-kept secret right now. BMW hasn't officially announced the new hardcore performance coupe yet, but spy shots have shown a hotter M4 is in development. Rumors suggest it will revive the legendary CSL nameplate last used for the E46 M3 in 2004, indicating it will be significantly lighter and more agile than the standard car.

A few weeks ago, our spies caught a prototype of the M4 CSL cruising on a highway in Germany, but now track testing has begun at the Nurburgring to give the performance car a proper workout.

There are several design changes that indicate this is no standard M4. The prototype retains the standard car's controversial grille, but it's been redesigned with Y-shaped horizontal slats. There's also a new splitter fitted to the front and the headlights are covered up, which suggests they will sport a new look.

At the back, the taillights have been tweaked with new graphics featuring a straighter design than the current car's curved graphics and there is a more prominent trunk lid spoiler. The rear side windows are also covered up, which suggests the rear seats have been removed to save weight.

Under the hood, the M4 Competition will share the same 3.0-liter twin-turbocharged inline-six engine as its standard sibling, which generates 505 hp and 479 lb-ft in the M4 Competition. BMW will reportedly increase the output to around 540 hp in the hardcore M4 CSL, which is a massive boost. Power will only be sent to the rear wheels as the M4 CSL allegedly won't be offered with all-wheel drive. It will also only be available with an eight-speed automatic transmission according to reports.

Since testing appears to be at an advanced stage, there's a good chance the BMW M4 CSL will debut early next year to celebrate BMW M's 50th anniversary.