New engines were added to BMW's Z4 sports roadster in 2006. The new 3.0-litre straight-six in the BMW Z4 Roadster 3.0si was called "the most innovative engine in its class." In the likely event you get carried away with the thrill of driving this car, consumer models of the Z4 are electronically blocked from traveling at speeds over 155 miles per hour. There were only subtle style changes in 2006, but the biggest addition after the new engines was the introduction of a new, high-performance M version. The M Roadster features the same engine as the M3 coupe, a 3.2-liter in-line six-cylinder generating 330 horsepower.