DRIVEN: Roadster Roundup: Porsche Boxster S vs. Mercedes-Benz SLK55 AMG vs. BMW Z4 sDrive35is

July 2, 2012
Roadster Roundup Front View
Forty-five degrees with a cold wind blowing and showers in the forecast -- bad timing for the new Porsche Boxster S to meet its two closest rivals? Far from it. For a start, there's no weather you can't beat by pulling out an appropriate set of clothes. Second, putting the roof down is not mandatory -- we would not have seen an indicated 287 kph (178 mph) in the Porsche and 291 kph (181 mph) in the SLK with the wind in our hair and tears in our eyes. And a bit of precipitation is actually not so bad; wet tarmac lowers the limit of adhesion and lifts the spirits of those who like a bit of attitude when tackling a set of twisties. So we ignored the puffy gray clouds that seemed to almost touch the sun visors, cranked up the seat heaters, and pretended that a couple of blue patches in the sky were a sign that summer was just around the corner.
Roadster Roundup Rear Right View
Of our topless trio, two are new. The SLK55 AMG is the top-of-the-line version of the third-generation baby SL, introduced in early 2011. Why did we pick the much more expensive, 415-hp AMG version over the 302-hp SLK350? Because the V-6-powered SLK can't hold a candle to the 315-hp Boxster S, because price is not a prime buying motivation in this segment, and, quite frankly, because the idea of 398 lb-ft of torque making those fat rear tires spin appeals to our childish nature. For similar reasons, the Z4 featured in this threesome is not the 300-hp 35i but the more powerful 35is variant that musters 335 hp and is mated to a standard seven-speed dual-clutch automatic transmission.
A roadster is an undeniably emotional purchase, and falling in love with a car is a complex affair. Although the specs and stats are only of fringe importance, looks are absolutely pivotal. That complicates the decision-making process in this case, because all three cars make arresting visual statements. The BMW is a perfectly proportioned centerfold on wheels. Wide, low, and neatly sculptured, it's small enough to be chuckable and chic enough to score on the street-cred chart. The downside concerns the retractable hard top. It may be practical, but the cutlines it necessitates aren't exactly pretty, and its bulk compromises the passenger compartment as well as the luggage bay. The SLK, which also features a retractable hard top, has similar drawbacks. Unlike lesser SLKs, the butch AMG version is highlighted by available black ten-spoke wheels and four massive tailpipes, its body littered with drag-cutting and attention-grabbing details. In contrast, the new Boxster S looks clean, subtle, and totally unaggressive, with the exception of the bright yellow paint job featured here. The only obvious concession to fashion is the design of the motorized rear spoiler and the lateral extensions that peter out in the taillights. The Porsche has the fastest-lowering roof (nine seconds!), the roomiest cabin (by quite some margin), and two luggage bays that are undiminished by the stowed top. Such practicality can be a big bonus when it comes to making love last.
Launched in 2009, the Z4 is the oldest model here. At 3549 pounds, the BMW feels pudgy, and indeed it weighs as much as the V-8-engined Benz; both are much heavier than the Boxster S, which tips the scales at only 2976 pounds with the PDK gearbox. As soon as the serious driving begins, however, the BMW looms in the mirror of whoever is leading the pack. The twin-turbocharged straight six is a compelling engine. Cranking up the boost pressure squeezes out an extra 35 hp over the mainstream version, but even more significant is the bulge in the torque curve that now peaks at 1500 rpm, where we find 332 lb-ft (369 lb-ft during short spurts of "overboost"). Although the redline promises a lofty 7000 rpm, the Z4 is more about low-rpm grunt. In combination with the seven-speed DCT, the six-cylinder performs like a mix of turbine and afterburner. Part-throttle upshifts are particularly impressive and become whiplash-punchy the harder you depress the accelerator.
Porsche Boxster S Wheel
It even sounds pretty good, that BMW engine. The underlying deep growl turns into a muffled roar as the tachometer needle swings across the dial, but it's also hard not to be smitten by the overrun burble, the angry blat-blat that accompanies every downshift, and the throaty hiss that plays an impatient background bass at idle speed. The real virtuoso among these three is, of course, the SLK55 AMG, which fields two extra lungs and a mighty breathing volume of 5461 cubic centimeters. Although it is now equipped with high-tech items like cylinder deactivation, auto start/stop, and alternator energy recuperation, the V-8 still plays all our favorite heavy-metal tunes. Below 2000 rpm, the exhaust makes sure the bypass valves are closed to keep on good terms with the neighbors, but as soon as you bury the accelerator, windowpanes are guaranteed to rattle in their frames. Nice! But nicer than Porsche's boxer engine? Certainly different. The latest 315-hp flat six has the same tone of voice as all flat sixes conceived in Stuttgart since 1963, no matter whether they are cooled by air or water. It's a great soundtrack, kind of rock meets classical, very powerful and yet melodious. Although Porsche keeps modifying the boxer and making it more efficient, evolution has mercifully had no effect on the engine's special character and charisma.
The Boxster S is an all-new car, but its 3.4-liter flat six is only a variation of the previous engine. Maximum power is up a modest 5 hp to 315 hp at 6700 rpm; max torque remains an unchanged 266 lb-ft available at a slightly higher 4500-to-5800 rpm. Bolstered by new technologies such as auto start/stop and an engine-decoupling coasting mode, the new Boxster S earns an EPA fuel economy rating of 21/30 mpg city/highway (20/28 mpg with the stick shift). Having said that, we recorded a not-so-impressive 18 mpg. Still, that's better than the 16 mpg we saw with the Z4 (EPA rated at 17/24 mpg) and the SLK (EPA rated at 19/28 mpg).
The Porsche is the only car here that still offers a choice between manual and automatic transmissions. (The Z4 and the SLK also offer manuals but only in lesser versions.) The stick shift may be a little more involving, but you need the PDK automatic plus the wizardries of the Sport Chrono package (such as launch control and faster shift timing) to accelerate in 4.5 instead of 4.8 seconds from 0 to 60 mph. When you tick the box marked PDK, be sure to also specify the SportDesign steering wheel, which incorporates proper shift paddles rather than fiddly thumb switches.
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.
Mercedes Benz SLK55 AMG Taillight
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.
BMW Z4 SDrive35is Side Badge
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.

Porsche Boxster S

BASE PRICE $61,850
Powertrain
ENGINE
24-valve DOHC flat-6
DISPLACEMENT 3.4 liters (210 cu in)
POWER 315 hp @ 6700 rpm
TORQUE 266 lb-ft @ 4500-5800 rpm
TRANSMISSION 7-speed automatic
DRIVE Rear-wheel
Chassis
STEERING
Electrically assisted
SUSPENSION, FRONT Strut-type, coil springs
SUSPENSION, REAR Multilink, coil springs
BRAKES Vented discs, ABS
TIRES Pirelli PZero
TIRE SIZE F, R 235/35YR-20, 265/35YR-20
Measurements
L x W x H
172.2 x 77.9 x 50.0 in
WHEELBASE 97.4 in
TRACK F/R 60.1/60.6 in
WEIGHT 2976 lb
WEIGHT DIST. F/R 46.0/54.0%
CARGO CAPACITY 5.3/4.6 cu ft (front trunk/rear)
EPA MILEAGE 21/30 mpg
0-60 MPH 4.5 sec
TOP SPEED 172 mph

BMW Z4 sDrive35is

BASE PRICE $65,095
Powertrain
ENGINE
24-valve DOHC twin-turbo I-6
DISPLACEMENT 3.0 liters (182 cu in)
POWER 335 hp @ 5900 rpm
TORQUE 332 lb-ft @ 1500 rpm (369 lb-ft with overboost)
TRANSMISSION 7-speed automatic
DRIVE Rear-wheel
Chassis
STEERING
Electrically assisted
SUSPENSION, FRONT Control arms, coil springs
SUSPENSION, REAR Multilink, coil springs
BRAKES Vented discs, ABS
TIRES Bridgestone Potenza RE050A
TIRE SIZE F, R 225/35YR-19, 255/30YR-19
Measurements
L x W x H
167.0 x 70.5* x 50.6 in
WHEELBASE 98.3 in
TRACK F/R 59.5/60.5 in
WEIGHT 3549 lb
WEIGHT DIST. F/R 49.3/50.7%
CARGO CAPACITY 10.9/6.4 cu ft (top up/down)
EPA MILEAGE 17/24 mpg
0-60 MPH 4.7 sec
TOP SPEED 155 mph
*width without sideview mirrors

Mercedes-Benz SLK55 AMG

BASE PRICE $68,375
Powertrain
ENGINE
32-valve DOHC V-8
DISPLACEMENT 5.5 liters (333 cu in)
POWER 415 hp @ 6800 rpm
TORQUE 398 lb-ft @ 4500 rpm
TRANSMISSION 7-speed automatic
DRIVE Rear-wheel
Chassis
STEERING
Hydraulically assisted
SUSPENSION, FRONT Strut-type, coil springs
SUSPENSION, REAR Multilink, coil springs
BRAKES F/R Vented discs/discs, ABS
TIRES Continental ContiSportContact
TIRE SIZE F, R 235/40YR-18, 255/35YR-18
Measurements
L x W x H
163.2 x 79.0 x 51.2 in
WHEELBASE 95.7 in
TRACK F/R 61.0/61.7 in
WEIGHT 3549 lb
WEIGHT DIST. F/R N/A
CARGO CAPACITY 10.1/6.4 cu ft (top up/down)
EPA MILEAGE 19/28 mpg
0-60 MPH 4.5 sec
TOP SPEED 155 mph
All measurements per manufacturers

Comments

Photo Gallery

Research A Vehicle

Select a vehicle to research pricing, see photos, compare similar vehicles and much more.
Research Now

Top Lists

Find Local Car Deals