BMW $50,000 four-cylinder roadster

Like cops shutting down a raucous frat night, federal gas-mileage restrictions are busting up the binge that’s led sports-car companies to make ever more powerful autos. The party’s over.With corporate average fuel economy set at 34.1 miles per gallon by 2016, 500-hp supercars that chug petrol by the kegful are going to need a dose of aspirin.Which brings us to the latest BMW Z4 convertible, with a new, less-powerful engine. Available this autumn, it’s the first four-cylinder from the company in a dozen years.To sports-car enthusiasts, that probably sounds like a big uh-oh.The change is making my head throb all right, but in a good way. I’m slinging the roadster along a mountain road shaped like an EKG readout. While the abstrusely named Z4 sDrive28i is less powerful than its six-cylinder predecessor, I’m not missing those two extra chambers one bit.The two-seat, hardtop roadster starts at $49,525 and is better-balanced than the naturally aspirated sDrive30i it replaces.Horsepower may have dropped to 240, from 255, but torque has increased to 260 pound feet from 220. Which means it has more grunt from a standstill and ample power in lower gears, perfect for scrambling through mountain passes -- the kind of thing buyers are actually looking for in a roadster.BMWs are typically electronically limited to 130 mph or 150 mph. When was the last time you saw a U.S. road where those speeds were legal? I can’t really see any downsides to the new engine. With less weight in the nose, the Z4 feels like it just kicked a bad habit and has a new spring in its step.BMW says gas mileage is improved about 20 percent in the model with the new eight-speed automatic. EPA numbers aren’t released yet, but I saw approximately 21 mpg in the city and 33 on the highway in my test car with a six-speed manual, versus the previous model’s 18 and 28. (A turbocharged 3.0-liter six- cylinder is also available on the sDrive35i and even more powerful sDrive35is. They start at $56,025 and $65,075 respectively, with 300 hp and 335 hp.)I realize that driving enthusiasts are supposed to worship at the altar of 12-cylinder engines. And no question, the monumental wall of sound emanating from the hood of an Aston Martin DBS or Ferrari 599 gives me the happy shivers.Yet the idea that performance comes only from a high- displacement V-12 or W-12 is kind of like saying that Big Ben is inherently a better time keeper than your Casio digital watch. The former is impressive, but the latter is a heck of a lot more efficient.BMW is justifiably proud of its new 2.0-liter engine, which has direct injection, a twin-scroll turbocharger and variable valve management.“I think we made a lot of people nervous when we announced that BMW was bringing four-cylinder engines back to the U.S. market,” said Paul Ferraiolo, head of product planning and strategy in the U.S. “But the new engine has the power of a six-cylinder with the efficiency of a four.”The last Z4 I had on the racetrack swiveled neatly on rolling turns, yet felt a tad dull. It just wasn’t as hungry as I’d like in a small two-seater. Like the latest Mercedes-Benz SLK convertible which I recently reviewed, the Z4 makes no claim to a practical nature, so it better be fun.The new engine achieves peak torque at a low 1,250 rpm, so it pulls hard very quickly. Every time I slap down on the gas pedal the car suddenly comes alive, vibrating and making a nice noise, visceral and vital.Of course, prospective buyers and onlookers won’t actually see the motor. Rather they’ll notice things like the nuclear yellow the car is available in, and the fact it looks athletic and fun with the top both up and down.The 1990s Z3 from which the Z4 evolved was a fragile- looking, dorky thing. This Z4 is anything but.BMW’s roadster is a niche car, however, hardly one of the company’s biggest sellers. To prove that the Munich-based automaker is serious about its new four-cylinder, the engine will also make its way into the 5 Series sedan. Yep, the 2012 528i will be available this autumn as a four-banger. I never thought I’d see the day.