The top of the line variant for the E85 Z4 was the Z4 M Roadster. Powered by the highly regarded BMW S54 inline 6 engine, the Z4 M Roadster had 330hp going to the rear wheels through a 6 speed manual transmission. A notable difference between Z4 M models and the standard Z4 is that the M models use hydraulic power steering as opposed to the drive-by-wire systems fitted to standard models. A short lived variant, the Z4 M Roadster was produced until 2008.