The Porsche Boxster and the BMW Z4 are about to get their comeuppance. Later this year, at some multilane intersection somewhere in these United States, an unsuspecting Boxster S driver is going to glance into the next lane and see a 2005 Mercedes-Benz SLK, its new, 3.5-liter, 24-valve V-6 burbling under the newly sculpted hood. The Porsche owner, accustomed to treating SLKs with the contempt usually reserved for a buzzing fly, will regard the new Benz with only casual and condescending interest. But then he will notice the SLK driver's look of keen anticipation, and suddenly it will dawn on him that, laughable as it may seem, his precious Porsche is being invited to participate in the latest round of the world's longest race, the stoplight grand prix. With an SLK! A car that for years has been the wimpiest, girliest little German roadster this side of an Audi TT. Wouldn't it be cool to be at that intersection and see that Porsche driver's face the first time a 2005 Mercedes-Benz SLK350 smokes his butt into yesterday?

OK, we might be stretching a bit. Mercedes estimates a 0-to-60-mph time of 5.5 seconds for the new SLK350, which is at least fully competitive with the Boxster S and the Z4, if not literally fast enough to beat them. The little Merc is no longer an effete, poseur mall crawler; and no longer is its retractable hard top its principal virtue. It's now a real, honest-to-Hans sports car, and it finally looks like one, too.

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The SLK's transformation started with its body and chassis, seriously weak and flabby in the outgoing car. On paper, the improvements are expressed as 46 percent greater torsional rigidity and a nearly 20 percent increase in bending stiffness, partly the results of a body shell made from a new cocktail of high-tensile steel, aluminum, and magnesium, with a few garnishes of fiber-reinforced plastic. On the road, progress is measured by a chatty chassis that incessantly tells the driver what's going on under those four Pirelli P Zero contact patches. For anyone who has spent time in the first SLK, pushing the new car over a challenging road will be a revelation. It feels, sounds, and acts like a thoroughbred.

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The SLK's stiffer body cradles improved suspension, steering, and brakes, as well as a new powertrain. The previous control-arm front suspension has been replaced by a strut-type design, which provides well-snubbed damping and good ride comfort. Rack-and-pinion steering replaces the recirculating-ball format. The SLK doesn't match the Boxster's steering feel and feedback, but it's on par with the Z4 and far surpasses the TT. In the mountain switchbacks of Mallorca, where we had to keep decelerating for hundreds of professional cyclists grinding up hillsides in preparation for the Tour de France, the powerful brakes never faded.

We have few fond memories of the original SLK's supercharged, 185-horsepower, 2.3-liter four-cylinder engine, which delivered reasonable performance but always sounded as if it had a bad cold. That problem was addressed originally by the introduction in 2001 of the optional 215-horsepower, 3.2-liter V-6 and then in 2002 with the SLK32 AMG's supercharged, 349-horsepower V-6. Yet no matter how much power was speaking up from under the hood, the SLK's complaisant chassis was, unfortunately, dominating the conversation. For 2005, Mercedes ditches the raspy blown four altogether, and we doubt any tears will be shed over the loss.

Cheers, then, for the new, standard, 268-horsepower, 3.5-liter V-6, which is our first taste of a new family of four-valve V-6 and V-8 engines supplanting the current powertrain lineup over the next few years. (The CLS four-door coupe coming next year will be the first to have the next-generation V-8.) A DOHC V-6 might not seem very exotic in light of the midships-mounted boxer six in the Porsche or even the free-revving in-line six in the BMW, but we predict that the new 3.5-liter Mercedes unit soon will be regarded as one of the best V-6s in the world. Compared with the three-valve-per-cylinder, 3.2-liter V-6 it replaces, it employs variable valve timing to help provide 53 more horsepower and an additional 29 pound-feet of torque, for totals of 268 horsepower and 258 pound-feet, figures that handily beat those of the Boxster S, the Z4 3.0, and the V-6-engined TT.

Best of all, the new V-6 has an amazing set of pipes. In concert with dual oval exhausts, it sounds fantastic, especially when its throaty growl is reverberating against the sand-colored stone walls that line many of Mallorca's roads. The Mercedes engineers assessed the acoustic characteristics of most of the engine's 210 separate parts, paying special attention to the air intake, in order to arrive at an overall sound that's pleasingly sporty but never gratingly noisy.

The V-6's superb midrange torque gives the SLK350 considerable cojones for passing on two-lanes and for squirting in and out of holes in freeway traffic. Engage third gear, mash the accelerator, and you're gone-it's as simple as that.

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The engine is mated to a standard six-speed manual transmission or to Mercedes-Benz's new seven-speed automatic. The automatic transmission, which can be shifted via buttons on the steering wheel, works extremely well at all speeds, especially around town, exhibiting none of the herky-jerky upshifting that characterized the first-generation SLK230's powertrain. The six-speed, although it is no Honda/Acura precision instrument, is a vast improvement over the tractorlike manual gearshifters that Mercedes has been sending our way since 1999. The shifter now feels about ten times better in your right hand, because the gears are engaged by means of one rod instead of the two that were used in the past. The same rod selects reverse gear as well, whereas the old transmission used a cable, such that you had to pull up on the gearshift lever to select reverse, a quaint maneuver twenty-five years ago but one that is woefully out of touch in the new millennium.

The wheelbase of the new SLK is 1.2 inches longer than that of its predecessor, and overall the car is about three inches longer and wider and about an inch taller, just enough to give occupants a little more breathing room. Curb weight rises by 110 pounds, to 3231 pounds. With a 22-second retraction time, the fully automatic hard top now puts you three seconds closer to a sunburn. Taking a cue from the SL, the rear window rotates forward 150 degrees on its own axis, which allows it to snuggle up more closely with the folded roof pieces, helping to increase top-down trunk capacity by 1.8 cubic feet, to 6.6 cubic feet. Unfortunately, the SLK lacks the SL's feature whereby the entire folded-roof stack can be lifted up inside the trunk for easier access to the cargo space.

Top-down driving is enhanced considerably by a new technology that has been in development for several years. Amusingly called Airscarf in Europe and boringly renamed Active Vented Headrest for America, it consists of a heating element and a fan hidden inside the seatback, connected to a duct leading to the headrest. Press a three-level switch on the center console, and a steady stream of warm air is pumped through small vents in the headrest onto your head and neck. In combination with the conventional heated seats, a wind net over the roll hoops, and the overall aerodynamic design of the car, with air directed off the windshield and over the occupants' heads, it makes for a very cozy cocoon and expands top-down driving opportunities in chillier weather. Bald men will still need a hat, however.

Design editor Robert Cumberford weighs in with his professional opinion on the SLK's styling in the accompanying design analysis, but it is obvious even to the most untrained eye that Mercedes endeavored to increase the SLK's butch quotient considerably. There is no easier way to do that than to mimic the SLR McLaren supercoupe's styling, replete with a somewhat phallic, Formula 1-inspired nose cone overlying the hood, punctuated at the grille by a three-pointed star the size of a soup bowl.

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For a muy macho Mercedes, you'll have to wait only a couple of months after the SLK350's debut for the SLK55 AMG, which we have not driven yet. It will feature the first V-8 engine in its class, a 5.4-liter unit making 362 horsepower and 376 pound-feet of torque, mated exclusively to the seven-speed automatic. "The SLK32 AMG was fun, like a go-kart," says AMG head of product development Hendrik Hummel, "but the engine was too powerful for the car. For the new SLK, we knew from the start that we were going to do a V-8, so the SLK55 will be much more dynamic."

The SLK350's base price will be in the mid-$40,000s when it hits U.S. dealerships in September. Mercedes-Benz USA hopes to sell about 10,000 a year and to increase male ownership from the current 48 percent to 60 percent. We predict there are a lot of guys out there-Boxster and Z4 owners among them-who will for the first time lust after the SLK as much as their wives.