Despite its age, the classic styling of Mercedes SL range still manages to turn heads and is hardly in need of a facelift. However, engineers are hard at work developing an update for the car ahead of the launch of an all new model in 2011. Prototypes of the facelifted model have been spotted testing at the Nurburgring including a few test-mules for the potent AMG prepped versions.

The new car is likely to pick up the handbuilt AMG V8 engine to form a new SL63 and should develop up to 525hp, giving it a slight edge over the E63. Further down the track, engineers will also add the sledge hammer 612hp twin-turbo V12 engine for the new SL65. According to some reports, there’s even a rumor Mercedes could add a new 4.2L CDI diesel V8 engine to expand the SL range.

The facelifted SL will debut towards the end of next year, while the AMG version should arrive less than six months after launch of the standard model. Click through to AutoGespot to see more images of the prototype or click here to see a rendering of the new car.