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.
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.
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.