The CLS550 is fitted with a twin-turbocharged

   The CLS550 is fitted with a twin-turbocharged 4.6-liter V8 rated at 402 horsepower and 443 pound-feet of torque. The four-door features a seven-speed automatic transmission sending power to the two rear wheels (the automaker's 4Matic all-wheel-drive system is offered as an option). Hardly a slouch, the standard CLS will sprint to 60 miles per hour in just 5.1 seconds with a top speed governed at 130 mph. However, and despite its valid reputation as a true driver's car, the CLS550 lost out to an Audi A7 3.0T in our comparison last year – blame its age, despite its refresh.The CLS63 AMG turns things up significantly. Under the hood is a twin-turbocharged 5.5-liter V8 rated at 518 horsepower and 519 pound-feet of torque in standard trim, or 550 horsepower and 590 pound-feet of torque when optioned with the AMG Performance Package (as our test car was). To handle the additional torque, and deliver a sporty driving feel, the traditional automatic is replaced by an AMG Speedshift seven-speed MCT gearbox with launch control. The standard CLS63 AMG will hit 60 mph in 4.4 seconds with a governed top speed of 155 mph. With the Performance Package, the factory claims that 60 mph falls in 4.3 seconds and the top speed governor is raised to 186 mph. (Consider those numbers conservative as Car and Driverrecently tested the 2012 CLS63 AMG at 3.8 seconds with a quarter mile of 12.0 seconds at 121 mph. Motor Trend got 3.9 seconds.)