BMW's latest M Roadster succeeds in two related ways: It grows some, ahem, "whiskers" on the otherwise masculinity-challenged Z4 with butch styling; and secondly, the implicit promise that the body makes is kept with honest-to-M3 muscle. More than a mere dress-up kit or sport pack, the 2006 M Roadster takes the finesse inherent in the Z4's already competent, baseline chassis and infuses it with heart-pounding performance derived from the M division's vast knowledge — with a little help from parts-bin raiding.

Cute goes bad
The M Roadster's more chiseled M-spec styling forecasts its speed and agility with a manly bravado the Z4 donor car only insinuates. Where the Z4 looks like an overreaching design student's idea of a roadster, the M Roadster's unique sculpting identifies it as the epitome of a real one.

At the front, its unique grille and aggressive fascia allow for better engine and brake cooling. Standard self-leveling xenon headlamps (with BMW's "corona" rings) light the way. More than just a fashion statement, the 18-inch M double-five-spoke alloy wheels are surrounded by serious Continental ContiSport Contact tires that are not (thankfully) heavy run-flats. Instead, in the trunk you get a compressor and a can of goo to get you to the nearest shop for tire repair/replacement. The otherwise amorphous aluminum hood features longitudinal creases that must've been painstakingly located to precisely align with the driver's sight lines.

Out back, there's an M-specific bumper ensemble with cutouts for the telltale quad-exhaust tips and a real air diffuser. The '06 M Roadster showcases new, adaptive LED taillamps (also on '06 Z4s) that light more brightly the harder the brake pedal is pushed. The overall effect of the M treatment to the exterior is supremely satisfying. Several times while driving the M Roadster, I saw lesser Z4s and my chest swelled with the satisfaction that comes from the exclusivity that all M variants offer their buyers. Riffing on the Z4's flame-surfacing (on a bar of soap) design netted a shape that looks like the car the Z4 should've been from the start.

Beyond kit
While it's apparent from the outside the M Roadster is more dedicated and antagonistic than the Z4, there's more — a lot more — going on under its taut skin to give it a bite as serious as its bark. The M Roadster's brake system is part of the M3's Competition Package, and standard on the M5. The trick hardware reduces unsprung weight, enhances cooling and resists fading and warping when pushed to the limits this car is capable of producing. The 110-foot stop from 60 mph doesn't begin to indicate how well the brakes communicate and work, repeatedly and consistently.

The widely celebrated cast-iron "S54" DOHC inline-six engine, replete with BMW's steplessly variable valve timing, is transplanted from a current M3. It produces three fewer horsepower than in the M3 (Roadster exhaust particulars), and makes an ample 330 hp to propel our measured 3,277-pound roadster to 60 mph in 5 seconds flat. The 3.2-liter engine has a linear, almost electric quality to it so that any rev range feels as capable as any other right up to the 8,000-rpm fuel cutoff. With its low reciprocating mass, immediate throttle response, high compression ratio (11.5:1) and reedy exhaust note, there's no mistaking the M motorsport connection.

In raw terms, however, compared to the previous Z3-based 315-hp M Roadster, the new car shows only a negligible improvement in outright acceleration. The likely culprit is the Z4-based car's added weight and slightly different gearing. Depending on whose "curb weight" is referenced, the Z4 M is about 150 pounds heavier than the Z3 M. That difference cancels out the additional 15 hp for a no-sum gain. Don't worry. Nobody will ever say the car is slow. It'll convince V8-minded muscleheads and turbo four-bangers of the merits of a BMW inline six with one blast through the gears. The M Roadster still propels about 9.8 pounds with each horse, but how those extra pounds are carried around corners is a different story.

Handled
When our tests focused on handling, the script changed dramatically. Unlike the twitchy, previous M Roadster, the new one is chock full o' costly bits from the M3's suspension and driveline. The effect is, to say the least, pleasingly capable.

To accommodate the M3's burly rear differential, a new subframe was fitted to the Roadster. That differential (fitted to the M5 as well) features a variable locking mechanism that fluctuates the amount of lock-up between the two rear wheels based on their relative speeds. The more one wheel spins, the more locked together they become. While this system's wet-weather benefits are obvious, there's also a noticeable improvement in how the car behaves when driven hard out of corners at wide-open throttle. Combined with its M3-derived rack and pinion steering plus front and rear suspension, the M Roadster's at-the-limit control is sharper than an M3's (that always feels like it's on its tippy-toes), head and shoulders above its M predecessor (which comes off as dangerously edgy) and simply in a different world from the commuting-friendly Z4.

A 69.6-mph slalom speed is only a numerical indicator of how confident the M is. Regarding the car's balance, test-driver Josh Jacquot emoted after his slalom runs, "spectacular limit-balance in the slalom test's high-speed transitions," but he continued, "communicative steering and a meaty steering wheel that should be standard-issue on every sports car." Like a circular black banana, the "girthsome" leather-wrapped wheel forces a relaxed grip and supplies volumes of useful tire/surface information to the driver.

Conspicuously absent
We find it exceptionally interesting that this M car features a back-to-basics hydraulic steering rack (no electric assist, no variable ratio, no over-engineered "active steering") and that the company's self-touted auto-clutch manual "SMG" paddle-shifter transmission is not even an option. An evolved version of a ZF Type H six-speed manual transmission completes the man-machine symbiotic relationship. While an SMG might represent impressive "because-we-can" technology, there's nothing rewarding in pulling a paddle for an upshift; no self-congratulations when the car executes one of a thousand perfectly matched-rev downshifts, and we've never driven a BMW SMG that was as smooth as we were from a dead stop in traffic. It seems that when BMW really wants to get it right, old school is still the best way to go. It's redeeming to witness that BMW hasn't forgotten how to "do" a proper steering system and manual transmission.

An excuse to just take a drive
Put all this trick hardware under your butt, lower the fully lined one-touch convertible top, find yourself a winding farm road or mountain pass, and you'll discover why the M Roadster commands an entry-level price $10K over a 2006 Z4 3.0si. The M Roadster inspires a gratifying three-way conversation between the driver, the car and the road that's about as perfect as it gets. Every bend becomes a really good one. Each approaching corner becomes an excuse to heel-toe downshift. Learn to trust it, and the M Roadster becomes an invisible force that carries your consciousness down the road as rapidly as you dare. If you can both afford and appreciate the differences, the M Roadster is worth the extra M-money over a Z4.

Twitchy or responsive?
Even more than the previous-generation M Roadster, the newest Z4-based performance car reveals to its driver all the mindless sloppiness that has crept into his non-rigorous commuting chores. This car feels like the Z4 the BMW engineers and test-drivers originally intended to build before the focus groups electrified the steering, softened its edges and added slop to make it less taxing to drive. The M Roadster's theme is immediacy: quick steering, instant brakes, direct throttle and caffeinated ride.

The M Roadster teaches its driver to tighten up his game. Don't downshift without a proper throttle blip. Don't glaze over in traffic and in a false emergency stand on the brake pedal unless you really mean to stop. Don't even sneeze if you're the type who sometimes uses his knee to steer while you remove the top from your café latte. Driver laziness and ham-fistedness are not only discouraged, they're punished with an immediate response from the car…and we love that. As the enthusiast arm of Edmunds.com, Inside Line has great respect for car companies with the guts to build a car for those who love to drive cars.

Clubbing
With all its intuitive athleticism, the M Roadster gains membership to an exclusive club of sports cars whose interview process weeds out wannabes, poseurs and cars that almost but never quite achieve the complete package.

Oftentimes, sporty cars offer great handling, but lack motivation (Mazda MX-5 Miata). Others provide ample firepower, but can't find their way around corners (any AMG product). Unlimited membership declined to both applicants. The M Roadster is granted true weapons-grade sports-car status with its masterful blend of engine, chassis and design. The whole package works in harmony. The M Roadster is so good that we'd put it in the same club, albeit in a different division, with the Porsche 911 Carrera 4, Ferrari F430 and Corvette Z06.

Respectively, each one of those cars represents the best that that configuration can be. For a sub-100-inch wheelbase, front-engine/rear-drive convertible, the 2006 BMW M Roadster is about as good as it gets. The M division turns the Z4 from a question mark into an exclamation point.