Spy photographers have just caught the first glimpse of the BMW M6 GranCoupe undergoing some rigorous testing. The high-performance version of the big sedan will come twin-turbocharged and ready to rock the luxury sedan market. The new M6 GranCoupe will rival the Mercedes-Benz CLS63 AMG and it will provide it a bit of a challenge, though it isn't quite as powerful. The BMW sedan will run on a twin-turbocharged 4.4-liter V8 engine, capable of putting out 552hp and 501 lb-ft of torque.

Power will be transferred to the back wheels via a seven-speed dual-clutch transmission. In comparison, the Mercedes-Benz CLS63 AMG puts out 518hp and 516 lb-ft of torque from its biturbo 5.5-liter V8. The muscular M6 GranCoupe can run from rest to 62mph in just under 4.5 seconds and reach a limited top speed of 155mph. It runs those times looking good, with its M-style quad exhausts and body kit. The new BMW M6 GranCoupe should debut later this year and will be launched in 2013.