REVIEWS: 2007 BMW Z4 M Roadster vs 2006 Porsche Boxster S

May 1, 2006
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The Porsche Boxster versus BMW roadster rivalry is certainly not a new one, as two different iterations of Z3-based M roadsters have been offered to Americans during the Boxster's tenure. The first one bowed in 1998 with the 240-hp, 3.2-liter six from the E36-based M3. Then, after the E46 M3 debuted for 2001, BMW briefly offered a 315-hp version of its 3.2-liter in the same Z3-based body. (That six also was installed in the often-overlooked 2001 M coupe.) Like those Z3-derived ragtops, the new Z4 M hews to BMW's classic roadster recipe: long-nose, short-deck styling, a lightweight, rear-wheel-drive chassis, and a rev-happy, front-mounted engine. It's a formula that dates back decades, to the 1928 Wartburg Sport and the 1935 BMW 315, and one that has produced some very handsome and desirable automobiles.
But the shark-nosed Z4 has not been as well received as the Z3, even though it offers crisper handling, an avant-garde interior, and a stronger brace of six-cylinder engines. We were anxious, then, to see what a full-body treatment in the M division's powertrain, suspension, and aesthetics spa could do for the South Carolina-built roadster. Could it rescue the Z4 from also-ran status? And could it topple the revered Boxster S from its pedestal?
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One of the best views in modern motoring is framed by your rearview mirror as a Z4 approaches from behind. From that perspective, the hunkered-down BMW looks sinister and menacing. The Z4 M amplifies the effect with a deeper and wider front fascia, a blacked-out grille, and what BMW refers to as "corona rings"-but are commonly known as "angel eyes"-around the headlamps. The rear bumper and air diffuser also differ from the Z4's. The standard-equipment M-style seats are highly bolstered, comfortable, and supportive, and they provide occupants the increasingly rare ability to move the rear portion of the seat bottom down, which provides better support for one's thighs. (Electric adjustment and memory, however, are optional.) The thick, red-and-blue-stitched M-style steering wheel looks cool, is just the right size, and has bump outs at ten and two o'clock so that you can place your hands properly.
Our test car's instrument panel and center console also were lined with BMW's optional new carbon-fiber-pattern leather. It's too bad this snazzy material was marred by an optional, pop-up navigation screen that looked like an aftermarket installment. The Bimmer has decent cabin storage, with a low, wide bin behind the seats, a cell-phone slot aft of the handbrake, and pockets in the doors. The trunk capacity is 7.1 cubic feet with the top down, enough for our photographer's assortment of camera bags, tripods, and lighting gear, but the battery and flat-tire repair equipment hog some of the space. When the fully automatic soft top is raised, trunk space expands to 7.8 cubic feet. In contrast, the Boxster's top isn't completely automatic-you have to loosen a center clamp at the windshield header before the electric motors take over-but the Porsche beats the BMW in cargo capacity, with 4.9 cubic feet in the front trunk plus 4.6 cubic feet behind the mid-mounted engine.
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For the Z4 M, BMW plucked plenty of chassis technology from the M3, including stiffer springs, dampers, and bushings as well as forged-aluminum front control arms. The rear suspension's wheel bearings, longitudinal links, and antiroll bar also follow the M3 recipe. The Z4's electrically assisted steering is ditched in favor of hydraulic assistance, which offers better high-speed performance. The base Z4's front track has been widened by half an inch, to 58.5 inches, and the ride height has been lowered by another half an inch. The aluminum-hubbed, cross-drilled brakes, originally developed for the M3 CSL, measure 13.7 inches in diameter at the front and 12.9 inches at the rear. The M roadster also gets the same limited-slip differential, which BMW calls M Variable Differential Lock, as the M3 and the M5.
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Most important, the Z4 M gets the M3's 330-hp in-line six, which is, as Sherman notes, "an extraordinary engine." It displaces the same 3.2 liters as the Boxster S's flat-six, but BMW extracts an extra 50 horsepower, in part by revving to 8000 rpm versus the Porsche's 7200 rpm. Since the BMW weighs only about 100 pounds more than the 3100-pound Porsche, it has the better power-to-weight ratio.
0605 X+2007 Bmw Z4 M+interior Driving
In the M roadster, the six is mated only to a six-speed manual transmission. "The lack of SMG [sequential manual gearbox]," opines Sherman, "shows that BMW is responding to growing customer demand for fun-to-drive cars rather than crude speed." You get into the M roadster and sink your left foot into the clutch, and the pedal has the same smooth fluidity and lightness as the M3. Final engagement at the top of its travel can be abrupt, though, so engaging first and second gears flawlessly takes practice, and the two-three throw is a bit long. Turn the key, and the in-line six's familiar metallic energy fills your eardrums, overlaid with farts and pops from the exhaust overrun.
With such an eager powertrain and a relatively modest curb weight, the M roadster's raison d'etre is straight-line acceleration. We measured 4.8 seconds from 0 to 60 mph and did the quarter-mile sprint in 13.4 seconds at 108 mph. The less-well-endowed Boxster S needs 5.4 seconds to reach 60 mph and does the quarter-mile in 13.9 seconds. On the autovaa leading toward the city of Jerez in south-western Spain, the Z4 M dove among ratty Renaults, Citroens, Seats, and Ford panel vans, gobbling up tarmac while maintaining excellent directional stability. It feels small and nimble when it's loping along at 125 mph. "The excellent damping keeps the car from squatting, pitching, and rolling excessively," says Sherman, "without damaging the pleasant ride motions." The steering has heft and becomes more communicative at speed, whereas on two-lane roads it requires more attention. The Boxster S, for its part, is no slouch on the freeway, but it cannot match the M roadster for sheer speed. The mid-engined Porsche's front end sometimes goes light at high speeds, a trait it shares with the 911, but the overall stability of its chassis more than compensates.
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At the Circuito de Jerez, the M roadster's propensity to understeer faded as speeds rose. Unlike the Boxster S, which lives for corners, the BMW doesn't rotate as naturally on its axis. "The technique that worked best for me was to drive to and through the understeer limit," Sherman recalls, "then back off slightly to let the front rubber secure a tight grip in the first half of the bend. Stepping down on the throttle at the apex would then cause the tail to drift wide a few useful degrees. Being patient with the steering-not dialing in too much countersteer too soon-and steady with the throttle yielded very efficient drifts exiting the turn. The M roadster does this consistently, with no drama or threat of spinning or wobbling. It's BMW tuning at its finest." Unfortunately, hot laps cause the BMW's brakes to fade quickly.
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The Boxster's basic shape has been around for a decade, but apparently that was news to the citizens of Jerez. Old men, matrons, stylish mothers pushing strollers, giddy school girls, and jaded teenage boys all turned their attention to el coche amarillo as we rumbled down the narrow cobblestone streets and alleyways of the old town center, where nearly every street corner has a stack of wine casks on the curb, commemorating Jerez's longtime role as the world capital of sherry production. When the Porsche was joined by the BMW, the Spaniards were even more enthused, with construction workers calling out demands to hear the engines rev. Naturally, we obliged, and the distinct notes of the Boxster's flat-six and the M roadster's in-line six reverberated against Jerez's ancient stone walls.
An hour northeast of Jerez, the two-lane A-373 spirals up into the Sierra de Grazalema as it leaves the pueblo blanco of Ubrique. The town's cluster of white buildings recedes as A-373 zigzags through meadows jutting with rocky outgrowths and dotted with spring crocuses. We charge up a few kilometers in the BMW, and then drive back down. The M roadster grips well and rockets along the brief straights, but it understeers in the tightest corners and requires more steering input and correction than we'd like. Still, it's an enthusiastic partner.
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Then we jump into the Boxster S and repeat the route. The Porsche feels like a glove around us, and all the primary interfaces with your feet and your hands are ideal. The clutch pedal provides just the right resistance. Gearshifting is great, with short throws and precise engagement. The steering feels telepathic compared with the BMW's. The wheel requires minimal input, and it's easy to place the car perfectly in a corner. Everything about driving the Boxster is fluid and predictable.
0605 X+2007 Porsche Boxster S+interior
After the last glimmer of sunlight disappears and the cameras are stowed in the BMW's trunk, we set off in our roadster duo on A-375, which wends, bends, and sweeps through thirty deserted miles of national forest. This narrow, newly paved Andalusian byway is about as fine a road to test a sports car's mettle as any we've ever driven. It has constant camber changes, dips, and turns and nary a house in sight, although our headlights do pick up a few grazing cows at the unfenced roadside. Dozens of minor bridges are lined with white-painted blocks of concrete, the better to interrupt your four-wheel-drifting path into a ravine if you overestimate your driving abilities. The few oncoming cars announce their presence via headlights stabbing through the darkness, so it's easy for us to use the whole road. We hustle the BMW from corner to corner, braking hard, slamming the shifter between second and third gears, and mashing the accelerator. The BMW's wind blocker, robust heater, and warmed seats keep the nighttime chill at bay. The rear brakes begin to squeal, but their performance is not compromised.
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If the Z4 M roadster were the only car we drove on this magnificent road, we'd walk away perfectly happy-thrilled, even-with its performance. But then we trade the BMW for the Porsche, and the Boxster demonstrates once again that it is the more natural performer. It seems custom-built for this road as it shrugs its shoulders at hairpin after hairpin. Its brakes never protest the harsh treatment we subject them to, and its wailing flat-six provides plenty of power. On this road, the Porsche makes us feel like we couldn't possibly cover ground faster or more easily in any other car.
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At the end of this fantastic drive, A-375 deposits us in the village of Alcal de los Gazules. As we trundle through the little town's main drag toward the autovia, the booming cadences of a marching band drift into our open cockpits. Surprisingly enough, on this dark Wednesday evening in February, a large, well-tuned student ensemble is stepping up a side street toward us, brass instruments blaring, drums pounding. Is this a celebration of some obscure Spanish holiday? Not that we know of, and Ash Wednesday is still two weeks away. We conclude, then, that the lively music is a salute to our two roadsters and the incredibly invigorating drive we have just finished in them. But the high note undoubtedly remains the Boxster S, because the two days we've spent in these two highly desirable sports cars has confirmed that the Porsche is still the one to beat.

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