motor

We know that AMG, the high-performance wing at Mercedes-Benz, has a pretty good handle on engine tweaking—just look at its spring catalog. AMG offers up 14 models, each packing no fewer than eight cylinders and outputs ranging from 355 horsepower to a mind-boggling 604 horsepower and 738 pound-feet of torque.

And although we've rarely knocked an AMG vehicle for lack of straight-line juice, neither have we been known to heap praise on the entire package, as it's been our experience that these souped-up cars are sometimes overweight and underperforming on the skidpad and during back-road boogies.

Now, lest you think we're making excuses for this new SLK55, AMG's least-powerful car, let us introduce a previously unused AMG performance trick: Mercedes' seven-speed automatic. The German automaker introduced this transmission for 2004, but until now the most powerful engine it was bolted to was the company's 302-hp, 5.0-liter V-8, so all AMG models had to rely on the five-speed automatic. So what are a couple more gear ratios going to do? Let us explain.

The previous-generation car, the SLK32 AMG, had a supercharged and intercooled 3.2-liter V-6 putting down 349 horsepower and 322 pound-feet of torque. The last one we tested ["Topless Toys," C/D, August 2001] weighed 3265 pounds, scampered to 60 mph in 4.5 seconds, and crossed the quarter-mile in 13.0 seconds at 110 mph. Now, the SLK55's 24-valve, 5.4-liter naturally aspirated

Despite this fact, the SLK55 outaccelerates its predecessor, ripping to 60 mph in 4.3 seconds and clearing the quarter in the 12s—precisely, 12.7 seconds at 111 mph—largely because of improved gearing, not just power. Still not convinced? How about this: The 400-hp Corvette in a recent comparo ["It's-All-About-Me Roadsters," C/D, March 2005] outguns this SLK55 by 45 horsepower and 24 pound-feet of torque, and weighs 155 fewer pounds.

Yet the Vette ties the SLK at 4.3 seconds to 60 mph and loses the quarter-mile race by a 10th, 12.8 versus 12.7 seconds. And that was a six-speed manual Corvette, not the wimpy four-speed automatic. Or consider this: The SLK55 matches its big brother E55 from 0 to 60 mph—despite the E55's 469 supercharged horsepower propelling fewer pounds per pony—and is just 0.2 second slower through the quarter-mile.

Besides the overachieving numbers, acceleration runs are just so darn pleasing in the SLK55. Modulate the throttle for launch to produce an appropriate amount of wheelspin, and then hammer it—the rest is taken care of for you. With the closely spaced cogs, the engine is always kept in a sweet spot between 5000 rpm and the 6700-rpm redline; thus, acceleration is never peaky, just strong and constant, with upshifts executed nearly manual-tranny quick. Mercedes says AMG's modifications to the standard seven-speed netted 35-percent-faster shifts, and the tranny must swap ratios quickly since it has to shift twice before reaching 60 mph in the SLK55. Delayed reactions would spoil any increased performance from the enhanced gearing.