Set to makes its world debut in Detroit next month at the North American International Auto Show, BMW's latest take on the Z4 Roadster promises to be one of the quickest and best handling sports cars ever produced by the German automaker.

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On sale here in the spring, the new range-topping sDrive 35is may not be a full "M-spec" alternative, but it does raise the Z4's total performance potential by adding a serious dollop of power and an extra measure of agility to the existing sDrive 35i mix. A higher-capacity induction system and additional boost for its twin-turbochargers raises the output of BMW's 3.5-liter direct-injected inline-six from 300 to 337 horsepower. The engine's impressive torque number gets a similar bump, going from 300 lb-ft at 1,400-5,000 rpm to 332 lb-ft between 1,500-4,500 revs and adding a computer-controlled overboost function that allows it to develop 369 lb-ft of peak twist for a short period of time under full-throttle acceleration. Even the exhaust system has been modified to give it a uniquely throaty low-range rumble.

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The hotter engine is teamed with a specially programmed version of BMW's paddle-shifted seven-speed Double-Clutch Transmission (DCT) that sends power to the rear wheels. Together, that combo will reportedly drop the 0-60 mph time from 5.0 seconds in an sDrive 35i to just 4.7 ticks in the 35is and raise its electronically-limited top speed from 150 to 155 mph. To complement the extra underhood firepower, the sDrive 35is also comes standard with a suitably tweaked Adaptive M Suspension package that lowers ride height by 10mm without impacting ride compliance.

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In addition to its special five-spoke 18-inch alloy wheels -- and 19-inchers are optional -- the exterior of the new Z4 sDrive 35is is distinguished by matte-aluminum trim accents in each front outer air intake and a functional body color-and-black rear diffuser. Inside, deeply bolstered sport seats are covered in new Kansas Walnut leather and it gets a thick-rimmed M Sport steering wheel, special grey-faced instruments, a driver's footrest and a host of other M-style trim elements that befit its primo status in the Z4 hierarchy. No word as yet on pricing, but a $55,000-$60,000 window seems likely.

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