Like the Boxster, the Z4 uses a six-cylinder engine mated to a seven-speed dual-clutch automatic. Limited to 155 mph, the Z4 35is nearly matches the Boxster S in acceleration to 60 mph, but its base price is $3245 more. Only $6525 separates the Boxster S from the SLK55 AMG, which is a fair deal considering the Merc's power and torque bonus. But once again, performance is not the decider. The AMG car is a mere two-tenths of a second quicker to 60 mph than the BMW and equal to the Sport Chrono-equipped Boxster (according to the carmakers' estimates), and it needs the optional AMG Handling Package to lift the maximum U.S.-market speed from 155 to 174 mph, thereby eclipsing the Boxster by a token 2 mph. Throughout the entire mph range, the three contenders shadow each other like a pack of NASCAR racers. From 0 to 125 mph, the Boxster S edges the SLK55, which in turn opens a gap on the Z4. On the autobahn, there is again very little separating these three racy roadsters. The driver with the steeliest nerves will emerge as the winner -- he who lifts early loses.
Pushing a car to the limit is all about confidence, and confidence has a lot to do with stability -- or rather with the controllability of instability. On a racetrack, with room to spare, the SLK55 AMG is a hilarious plaything that permits silly drift angles and is willing to hold them. On public roads, however, it's a different story altogether. Here, the Mercedes seems to run an in-car gyro that makes its vertical axis spin whenever steering angle and torque flow reach a critical level -- which happens early, often, and with vigor, especially on undulating and slippery turf. Through second-gear kinks this attitude can be fun for a while, but in taxing fourth-gear sweepers, sudden tail-out episodes are not exactly friendship-forging behavior. This is a very loose car, no doubt handicapped by a short wheelbase and nose-heavy weight distribution. The fat tires help but only to an extent. In the rain, the Continental Conti-SportContacts reach their limit of adhesion with an angst-inflicting abruptness, tramline like snakes racing each other, and are partly responsible for the stiff ride, which adds a dash of indifference to the handling equation. In the dry, their gumlike grip helps reestablish a comfort zone, but the commendably quick steering still feels overly light as well as somewhat artificial, and despite the very firm suspension, the chassis displays a latent wobbliness that makes relaxing difficult.
In the BMW, we see less space than in the SLK, and the driver sits far back if not quite as low as in the Porsche. The steering-wheel shifters are about as intuitive as an evil sudoku, and at low speeds the nineteen-inch Bridgestones are noisy and feel brittle. Unlike the SLK, which has a fixed suspension, the Z4 35is is fitted with adaptive dampers as standard equipment. (The Boxster S can be ordered with a similar system, known as PASM.) Although the BMW's 335-hp, 3.0-liter six briefly unleashes as much as 369 lb-ft, the BMW is more two-seat convertible than hard-core roadster. The engineers did what they could to make the Z4 look, sound, and drive like a sports car, but heavy steering, heavy brakes, and relatively ponderous handling create the impression of an open-air GT.
Our loaded Boxster S wasn't exactly what you'd call a bargain, but the mid-engine roadster would still cost some $36K less than a comparably equipped (and also brand-new) 911 cabriolet. True, the 911 is the more iconic sports car, but the difference in performance between the 315-hp Boxster and the 350-hp Carrera is marginal, not to mention the fact that the cheaper car actually offers a few advantages -- and we are not talking only luggage capacity here. For a start, the engines make the same beautiful noise and combine telepathic throttle response with linear power delivery. Both can be mated to one of the best automatic gearboxes extant -- one that gets even better with a push of the sport button. The 911 feels like no other car on the planet, but it doesn't turn in quite as rapidly as the Boxster, is trickier when you choose to deactivate all the electronic safety aids, and is not quite as firmly planted in a straight line. In other words, the Boxster S is the more grown up, more complete, more homogenous car.
At high speeds, the Porsche's strong aerodynamic stability backs up the remarkably unperturbed suspension and the rock solid, almost lean-free body. Through second- and third-gear corners, the Boxster invites you to modulate the handling in a way the Z4 and the SLK simply cannot match. Throttle and steering plot the course, engine and transmission set the pace, the Pirellis and the strut-type suspension keep it grounded. There is a fluidity and a transparency to this car that makes even a 911 feel a little edgy in comparison. Where the Mercedes is two-dimensional in its oversteer-or-not persona, the Porsche is multifaceted in the way it is always ready to explore the full handling spectrum, from mild understeer to lurid oversteer. Like the SLK, the Boxster responds really well when stability control is set in the mid-position, which encourages waltzing steps without dancing you dizzy. And while we do not prefer the new electric power steering over the previous hydraulic setup, it was easy to get used to its fresh talents, such as the more pronounced self-centering action and the mild correcting tug under braking on split-friction surfaces. However, the optional, speed-sensitive Power Steering Plus (which dramatically boosts assist at parking-lot speeds) is more of an acquired taste.
The rev-happy flat six trumpets, howls, and barks the Boxster S to victory in this company. There is very little to fault with the car. The ergonomics are a bit of a mess and the choice of available driver-assistance systems is limited at this point. But there is no doubt that the Porsche has the best (non-carbon-ceramic) brakes, the most attentive steering, the fastest-shifting gearbox, the most riveting grip, and the least compromised ride, even when fitted with twenty-inch footwear. It is a seamless performer, athletic yet totally compliant, sharp-edged yet nicely balanced, absolutely focused yet very relaxed. The Z4 is a strong all-arounder with a lovely engine but is let down by that space-wasting roof and the neither-here-nor-there positioning in the marketplace. The brawny SLK55 AMG is all engine and not enough chassis. It looks and drives like a muscle car, falling short in terms of composure, refinement, and, dare we say it, style. So the Boxster wins. After one exciting week and more than 600 fast miles, it not only beat its two sparring partners, it also comes unashamedly close to the 911.