BMW Z4

BMW cranks even more power out of the twin turbo-charged engine in its Z4.

The BMW Z4 sDrive35i is an excellent sports car, but BMW's engineers, who must experience self-critical dreams every night about how much smoother a valve could be or whether changing the timing of a spark by a millisecond might improve combustion, couldn't leave it as is, coming up with the 2011 BMW Z4 sDrive35is, a higher performance version of BMW's roadster.

Like the sDrive35i, the new roadster uses a twin turbo 3-liter straight six, but BMW fitted it with a new twin turbo unit to crank up the horsepower from 306 to 335, with torque up from 295 pound-feet to 332. This engine tweaking gives the sDrive35is acceleration to 60 mph in 4.7 seconds.

Beyond the engine work, BMW tuned the exhaust to give a more pleasing, sports car sound. Baffles and mufflers combine to give the car what BMW describes as a "deep rumble with a focus on the low frequency sound range." People will be surprised when they see a little roadster following that exhaust note as it works its way up the street.

The sDrive35is also gets BMW's seven speed Double Clutch Transmission, a manual with two computer-controlled clutches. The driver can choose an automatic mode or shift manually, but doesn't have to push a clutch pedal. The result is no power loss at the wheels during shifts.