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

Greg Pajo

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.

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.

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.

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JimInSF
Ok, so I've now driven both the SLK55 and Boxster, and while M-B's behavior on the DISTRONIC package is deceptive IMHO (and hopefully they will change this), the car does have some things to its credit that I don't think are adequately accounted for in the article. First off, the SLK55's chassis is great fun & fine on public roads if you're not driving at the point of traction loss on fairly grippy tires - methinks the authors are really pushing it here trying to describe this as not enough chassis for 99% of the potential customer base. In fact, it's irresponsible & misleading IMHO for them to describe driving on public roads at the limits of traction on "taxing fourth gear sweepers" - so you're expecting people to be sliding the car around at 80mph on public streets? Secondly, the Boxster's engine is ok but needs to be SCREAMED to get decent power comparatively. The AMG's V8 makes fully 50% more torque and would be much more satisfying in traffic as a DD if you don't want to drive around at 7000rpm.
mjoen
If you want to compare base prices, then the cars you test should be equipped that way. When I was comparing the Z4 to the Boxster, (before I bought the 35is), and started adding options to the Porsche, (to match what came with the 35is), I was sticker shocked. Similarly equipped, the Boxster was over $10,000 more than my Z. Porsche gouges the buyer unless they purchase a bare-bones car.Overall, this article seemed written and tweaked where necessary, to validate the authors biased opinions.
JimInSF
You mention with respect to the Boxster that "the choice of available driver-assistance systems is limited at this point" by comparison, but actually, while M-B claims to have PRE-SAFE BRAKE and various other assistance features like lane monitoring, etc. available on the SLK55 (part of the DISTRONIC PLUS package, package 997), they will refuse to actually sell you a car with those features if you try to buy one.In a bizarre bit of what feels to me like deceptive marketing, it's an official option package that they refuse to take actual orders for, it showed up in the press release for the model and is in the Dealer Ordering Guide but they have disabled the ability to actually order it in the computer and I've been told there are no plans to actually make it available. Your friendly M-B salesman will likely point out that they can sell you an SL550 with those features... bait and switch, anyone?
tinlizzy50
I used to think that roadsters were God's gift to driving enthusiasts: light, tossable, fun little toys, with manual transmissions and soft tops. Yes, ABS is good, as are no more side curtains, but heavy, hard-topped automatics?? I don't know what these cars are, but they sure don't meet my definition of "roadsters." Expensive two-seaters, yes; roadsters, no.

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