BMW's new four-banger, the 2012 Z4 sDrive28i (photos)

Since it phased out the four-cylinder version of the Z3 in 1999, all of BMW's cars have had engines with six cylinders or more in the U.S. But BMW is reintroducing the four-cylinder, using its latest direct injection, turbocharging, and Valvetronic technologies.

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This version of the Z4 will launch as a 2012 model, called the Z4 sDrive28i. BMW aficionados know that current BMW models using the 28i number have naturally aspirated 3-liter straight six-cylinder engines. But this new Z4 heralds BMW's move to phase out all naturally aspirated engines, so more 28i cars should see this four-cylinder engine.

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Although it gets a smaller displacement engine, BMW does not stint on this new Z4, equipping it with all the performance technologies, including active damping suspension, as the top trim Z4.

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BMW took pains to counteract extra vibration from the new engine, such as using damping shafts in the crankcase for versions of the car equipped with the manual transmission.

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The Z4 sDrive28i can be had with either a six-speed manual or an eight-speed automatic transmission. The new automatic transmission is actually lighter than the previous six-speed automatic, and its wide gear range helps fuel economy.

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Although short two cylinders of BMW's 3-liter six-cylinder, this new engine uses a twin-scroll turbo to produce 240 horsepower, 15 less than the six cylinder engine, and 260 pound-feet of torque, which is about 30 more than the six-cylinder. BMW has not released fuel economy numbers yet, but this new Z4 is expected to do better than the outgoing version.

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Part of this engine's efficiency comes from this twin scroll turbocharger, mounted to the lower side of the engine. It separates the exhaust flow into two sets of two cylinders, using it to turn the turbocharger. That arrangement helps eliminates dips in the turbo's speed, leading to smoother acceleration.

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BMW also had this demonstration version of the engine on display at the 2011 New York auto show. Doing away with the engine block, it exposes the pistons, turbocharger, and camshafts, making BMW's double Vanos system visible.

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