PUNCHING IT UP; Bigger engine makes top-of-the-line C-Class a real gas.(Mercedes-Benz C350)(Product/Service Evaluation)

Byline: ROGER HART

The C-Class is the smallest sedan Mercedes-Benz sells in the United States (for now) but it's also the biggest seller, so an argument could be made that it's the most important vehicle in the portfolio. Mercedes sold 69,251 C-Class models last year in the States, accounting for 31 percent of the 221,610 vehicles sold here wearing the three-pointed star.

If you need more evidence, consider that the C-Class was all-new just four years ago, received an exterior freshening along with changes in suspension and steering last year, and this year gets a serious powertrain upgrade. The top-of-the-line models are now fitted with the company's new 3.5-liter V6 and a new seven-speed automatic transmission.

The C350 will replace the C320 and will be available in Sport and Luxury trims. A standard seven-speed automatic transmission comes in the Luxury model while the 4Matic awd version comes with a five-speed auto. The Sport package comes standard with a six-speed manual while the seven-speed auto is optional.

The 3.5-liter V6 is the same engine we first saw in the SLK launched last year (AW, March 29, 2004). The engine uses variable cam timing for the 24 intake and exhaust valves, improving efficiency significantly from the 3.2-liter V6 it replaces. While no official EPA figures are yet available, M-B says fuel efficiency is 12 percent better despite the bigger displacement and improved output. The engine uses the latest in metallurgy, with the crank-case and cylinder heads made of aluminum; the pistons are iron-coated aluminum with steel connecting rods; and the cylinder liners are aluminum-silicone. The weight saving from conventional cast-iron liners is more than one pound per cylinder.

From this springs an engine producing 268 hp at 6000 rpm and 258 lb-ft of torque from 2400 to 5000 rpm. This output is up from 215 hp at 5700 rpm and 229 lb-ft at 3000 to ...