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?

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.

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.

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.