2006 BMW M
By borrowing parts and technology from the M3 coupe, BMW has turned the Z4 M roadster into the best version of BMW's small convertible. Previously known simply as the M roadster, the new car has substantially more balanced performance than the base model. While BMW is better than most at employing technology in the service of performance, there's something to be said for getting back to basics, to a car without electric steering, adaptive suspension, active stabilizer bars or that maddening BMW iDrive control system. The Z4 M is in a good position to challenge, though not necessarily beat, the uncomplicated Porsche Boxster S, which is the only model in its price and performance range.
Exterior & Styling
When the Z4's predecessor, the BMW Z3 roadster, came out, I wasn't crazy about it. I thought it looked like a nose, but in time it grew on me. (Unfortunately, so did my nose.) The Z4 was one of the controversial early designs in BMW's current styling direction, headed by American Chris Bangle. I actually think it looks pretty good — better than the Z3 — when the top is down. When it's up, the profile is as awkward as that of most ragtops.
The standard top is fully automatic, which means you don't even have to detach it from the windshield frame. It goes all the way down/up in about 10 seconds. Nice.
In the BMW M tradition, the Z4 M roadster is something of a sleeper: To the uninitiated, it looks like a regular Z4, albeit a quick one. Many characteristics do distinguish it from the plain roadster, aside from the obvious M badges. The nose is custom from the hoodline downward. Most obvious are the larger openings below the bumper for engine and brake cooling. In addition, the main grille is blacked out and deeper set than the regular Z4's. The hood is divided into thirds by two creases that give it a power-bulge look the base model lacks.
The rear bumper is cut out for modest chrome quad tailpipes. The small slot between them is a functional air diffuser. To that end, the upturned trunklid serves as a spoiler, but BMW offers an optional supplemental piece for this model. My test car didn't have it, and I managed not to fly off the road.
Ride & Handling
BMW didn't hold back in upgrading the M version's suspension bits. The front track is wider, and the lower control arms and steering knuckles are exclusive. Around back, the whole subframe is reinforced, and the wheel bearings, stabilizer bar and some of the suspension links are borrowed from the BMW M3 coupe. The alloy wheels are 18 inches in diameter; mine wore Continental ContiSportContact summer performance tires rated P225/45ZR18 in front and P255/40ZR18 in back. The result of all this hardware is a lower, firmer-riding Z4 with more tenacious handling.
The ride quality is reasonably comfortable; anyone who would even consider this car is unlikely to think twice about it. The handling is definitely the best of any Z4, which may seem a foregone conclusion, but I'm not talking about simple grip or reflexes. It just feels more natural. There's something odd about the Z4 that's been tough for me to sort out. Since its debut it has had the "perfect" front/rear weight distribution of 50/50 or thereabouts, yet it always seemed to understeer too much, and the original base 2.5-liter inline-6 didn't have the guts to overcome it though application of power to the rear wheels. The best theory I've developed is that the driver's seat is closer to the rear axle than in most cars, which puts the car's yaw axis in front of you. In most cars, the driver seems to sit on or closer to the axis around which the body naturally wants to spin.
The M version's engine has more than enough power to keep the car balanced in turns, but the overall dynamics are a bit alien to me. Though it's not to my liking, I wouldn't say there's anything inherently wrong about it. If you have any experience with this model in any form, I'd love to hear from you (use the email link at the end of this page).
At its limits, the M roadster is predictable and reasonably controllable. A standard limited-slip differential keeps the tail from getting out of hand. (BMW calls it the M Variable Differential Lock and gives a detailed description, but it sounds like a simple viscous coupling to me.) Also standard is an electronic stability system that's calibrated for sportier driving. In other words, it demands more provocation, intrudes less often and does so less abruptly. It seldom kicked in on dry streets, and was so subtle as to be inoffensive.
Electric power steering has met with mixed reviews. Some automakers have executed it well, and others not so well. The regular Z4 is the only BMW model equipped with it, but this M version employs a conventional hydraulic rack-and-pinion design borrowed from the M3. BMW would probably deny that this is an indictment of the electric system, but it has to mean something, right? The steering is precise, and perhaps a little underassisted at high speeds — something I don't think I've ever said. In this car, the Sport button next to the shifter doesn't change the amount of steering boost as it does with the regular Z4.
The downside of this beefed-up hardware is an additional 2 feet of turning radius, now 34.3 feet, which wipes out one of the Z4's advantages over its competition.
Going & Stopping
In its continued cannibalism of the M3, the M roadster uses its engine, a 3.2-liter inline-six-cylinder, which has a larger cylinder bore, longer piston stroke and a higher compression ratio than the 3.0-liter of the regular Z4. In all, it has 330 horsepower versus the Z4 3.0si's 255 hp.
|BMW Z4 Roadster Engines|
|Model||Z4 3.0i||Z4 3.0si||Z4 M|
|Type||3.0-liter inline-6||3.0-liter inline-6||3.2-liter inline-6|
|Induction||normally aspirated||normally aspirated||normally aspirated|
(lbs.-ft. @ rpm)
|EPA gas mileage* (city/highway, mpg)||20/30||20/30||16/24|
|* With manual transmission|
Source: Manufacturer data
The engine features continuously variable valve timing and a throttle for each cylinder. This design is the next step in metering precisely the right air-to-fuel mixture. Be it because of the close proximity of these valves to the cylinders or some other provision, the engine's accelerator response is very quick. The same could not be said of BMW's early-generation by-wire throttle, whose lag would have been unacceptable in an econocar. A Sport button changes the sensitivity of the accelerator: When it's activated, you get more throttle for less pressure on the accelerator. Drivers acclimate to a specific response, and changing it back and forth seems pointless. Given that a driver is likely to pick one and stay with it, the selection should remain when the car is turned off. As is, it defaults to the non-Sport mode.
BMW says the M does 0 - 60 mph in 4.9 seconds versus the Z4 3.0si's 5.6 seconds. As shown in the table, the torque peak is low relative to the horsepower, and it comes at a high 4,900 rpm. The car isn't anemic at low speeds, but the power definitely swells with speed, all the way up to the 7,900 rpm redline. The sound is genteel in normal driving and nice and authoritative under heavy acceleration, though quieter than I expected.
What does this power cost you? Gas mileage of 16/24 mpg in city/highway driving according to the EPA. It also costs you a $1,000 gas guzzler tax.
The antilock brakes are highly effective, linear and consistent. Only the calipers surprised me. I'm not bothered by their small size, because performance is what matters, but when you have big, expensive wheels and cross-drilled brake rotors drawing attention, do you want lackluster black factory-looking calipers? Smooth, colored, insignia-garnished calipers do wonders for the look of lesser cars than this.
The relatively unadorned interior is typical BMW. Though I liked my test car's aluminum trim, it's not exclusive to the M version. Distinguishing attributes include upgraded leather and the option of "extended leather trim" on the doors, pillars, roll bars and center console, as well as exclusive trim packages: Carbon Leather or Walnut Medeira wood. (Carbon Leather is a no-cost option.) The manual tilt/telescoping steering wheel does away with the Z4's silver trim and adds an M logo and matching blue and red stitching. Logos also appear on the head restraints, instrument panel, doorsills and the shifter knob. The shift pattern is backlit at night — a very nice touch.
You have to fold yourself a bit to get into the M when the top's up, and look out for that high doorsill. Accommodations are good for someone 6 feet tall, but for anyone taller, comfort and visibility are questionable. Close to the driver's head, the glass rear window provides a decent view, but the roll bars and head restraint are obstructive, as is the pillar formed by the raised top. (The fixed roll bars illustrate why some automakers have switched to those that only pop up when a rollover occurs.) Parking can be tricky due to the obstructed view and difficulty judging over the trunk and hood exactly where the rounded body ends.
The dual-layer soft-top helps cut down on noise, but this is clearly a convertible. When the top's down, turbulence in the cabin is reasonable at highway speeds. Interior storage is so-so, with narrow door pockets and a tiny glove compartment that has no lock, leaving it accessible if you park the car with its top down. On the other hand, the center storage behind the driver's right shoulder locks automatically along with the doors. Though it's hard to reach, it has a respectable amount of space divided into two stacked compartments — each with its own light.
I'm surprised by the plain, black pedals. Stainless steel is a common upgrade in performance models.
Active safety features include the antilock brakes and stability system. Passive features include front, knee and side-impact airbags. The latter deploy from the backrests. There are no curtain airbags, a definite downside to most convertibles, especially because they sit low and have no B- or C-pillars to serve as barriers. (Porsche and Volvo have introduced curtains that deploy upward from convertible doors.)
Being a low-volume convertible, the Z4 hasn't been crash tested by the Insurance Institute for Highway Safety, our preferred source, and it probably never will be. The National Highway Traffic Safety Administration's less-real-world tests give it four stars out of five for both occupants in a frontal crash. The side-impact rating is three stars; considering that they didn't measure head trauma — the most common and life-threatening injury in a side impact — the car's side-impact protection is a mystery and, thus, a concern.
Cargo & Towing
As small roadsters go, the Z4 has decent trunk space. The Z4 M is rated at 7.1 cubic feet, though it can be expanded to 8.5 cubic feet when the top is up by raising a partition that separates the trunk from the roof-storage cavity above it. The regular Z4's trunk is a bit larger at 7.8 or 9.2 cubic feet. The Z4 has run-flat tires, while the M version has what BMW calls the M Mobility System and I call sealant goo and a pump. All M models use this system to avoid a space-robbing spare tire as well as the higher unsprung weight and associated limitations that run-flat tires would bring.
In comparison, the Audi TT roadster has a 6.4-cubic-foot trunk, the Mercedes-Benz SLK55 AMG has 6.5 cubic feet with the top down and 9.8 cubic feet when it's up, and the Porsche Boxster's top has no effect on the trunk space, but its 9.5 cubic feet is divided evenly between front and rear trunks.
Z4 M in the Market
The small-roadster market is saturated, and possibly oversaturated, but the ultra-performance variants are less common. The Boxster S is in the same ballpark, but there's no souped-up version of the Audi TT. The Chrysler Crossfire SRT6 roadster is comparable, if you can bear driving an automatic sports car. The alternatives are outside the M roadster's price, size or performance range. I don't think the Z4 is one of BMW's best models, but the M roadster is the best Z4, and disproportionately so.
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