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BMW (on Dec 13 08) released photos and details of its new Z4 roadster , which will be unveiled during the Detroit auto show next month. There are two major developments: The Z4 will be the first BMW roadster to come with a lightweight retractable hardtop , and it will come turbocharged . The retractable hardtop is a two-piece aluminum shell . At the touch of a button, the roof opens and closes within 20 seconds. BMW says that the side windows are 40 percent larger than the current model, and all-round visibility has been increased by 14 percent. The other big piece of news is that the top trim level, the Z4 sDrive35i, is equipped with a twin-turbocharged 3.0-liter in-line 6 . In the new Z4, the engine makes 300 horsepower and 300 pound-feet of torque. The lower trim, Z4 sDrive30i, is equipped with a naturally aspirated 3.0-liter in-line 6 that produces 255 horsepower. There are three transmissions available . A manual 6-speed gearbox is available with both models. The Z4 sDrive35...