Chevy puts a small engine in its big sedan, and it works.

The first batch of Chevrolet's new Impalas arrived in showrooms with a 305-horsepower 3.6-liter V6, a six-speed automatic transmission, and EPA fuel economy ratings of 19 mpg city, 29 highway. If customers wanted better fuel numbers and didn't really need big horsepower, they were advised that a pair of four-cylinder engines were in the pipeline.

This is the first of them. GM offers a 2.5-liter four in other applications -- the Cadillac ATS, for example -- but the Impala version includes a valvetrain design update that will probably extend to other members of this engine family.

Basics: Like the ATS engine, the Impala 2.5 is a long-stroke, undersquare DOHC 16-valve design with aluminum block and head, variable valve timing, and direct fuel injection. But the Impala 2.5 includes a new Intake Valve Lift Control (IVLC). When the driver demands max power, the system increases valve lift on the intake side, improving breathing and of course power.

Particularly in contrast to the 305-horsepower V6, the 4-cylinder's max output -- 196 horsepower at 6300 rpm, 186 pound-feet of torque at 4400 rpm -- may seem a little tame in a large sedan with a curb weight close to 3700 pounds (Chevrolet has not yet furnished an official curb weight for the four-cylinder Impala).

But if the 2.5 Impala isn't particularly quick out of the starting blocks, its passing performance is surprisingly brisk, thanks to revised throttle and transmission mapping. Beyond that, the lower weight of the four-cylinder engine gives the Impala better front-to-rear weight balance, making it a little quicker on its feet than the V6 version.

The new electric power steering could convey more road feel, but a little time behind the wheel gives the driver a sense of system accuracy and respect for how easy it is to place this big sedan precisely as desired on twisty roads.

Other dynamic virtues of the Impala 2.5 are first rate ride quality -- supple without the old-time mushiness that once afflicted American sedans -- and exceptionally low noise levels.

And as you'd expect, there are benefits to the wallet. With a base price of $29,875, the 2.5 is about $1000 less than the V6 Impala; think of it as saving $500 per cylinder. More important, the 4-cylinder should save at the pump, with EPA fuel economy ratings of 21 mpg city, 31 mpg highway.

This is the only naturally aspirated (i.e., non-turbo) 4-cylinder engine offered in the premium large sedan class. For the driver who values fuel economy and quiet operation over 0-to-60, it may emerge as a favorite in this category.

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