2004 BMW Z4 Roadster - In Depth

Press Release

2004 BMW Z4 Roadster ??????????????????????????????????????? In Depth

Smooth, powerful 6-cylinder engines

In a sea of V-6 engines, BMW swims almost solo with its unusual ??????????????????????????????????????? but in BMW??????????????????????????????????????s opinion superior ??????????????????????????????????????? inline-6 configuration. The Z4 roadsters are powered by two versions of the M54 6-cylinder engine family; of the 3.0-liter version powering the Z4 3.0i, Automobile Magazine (August ??????????????????????????????????????03) wrote that ???????????????????????????????????Its torquey, 3.0-liter six is always ready to respond.???????????????????????????????? In addition to its inline six cylinders, this brilliant engine architecture features:

• Aluminum construction, contributing to vehicle performance by keeping down engine weight.
• Dual overhead camshafts and four valves per cylinder, for optimum torque and power.
• Chain camshaft drive, requiring no periodic replacement.
• Double VANOS steplessly variable valve timing, enhancing torque, power and emission control.
• Hydraulic valve adjustment for consistent sound and reduced maintenance.
• Electronic throttle system (???????????????????????????????????drive by wire????????????????????????????????) for smooth, precisely tuned throttle action and seamless integration of cruise control, Dynamic Stability Control and other features. This also facilitates an appealing option: Dynamic Driving Control, about which more later.
• Electronically controlled engine cooling, enabling precise and purposeful control of engine temperatures; contributes to fuel efficiency and heater effectiveness.

With all these elements of engine architecture shared, two versions of this powerplant power the two Z4 models: Z4 2.5i. 2.5-liter, with 184 horsepower and 175 lb.-ft. of torque. This unit delivers 0 to 60 mph in 7.1 sec. (manual trans., BMW AG test results) and achieves the amazing top speed of 146 mph. Its EPA mileage ratings are impressive too: 20 mpg city/28 mpg highway with 5-speed manual transmission, 20/29 with the optional 6-speed Sequential Manual Gearbox (SMG), and fully 21/28 with the available 5-speed automatic. Top speed is 137 mph with the 5-speed, 146 with SMG and 141 with the automatic. Z4 3.0i. 3.0-liter, with 225 hp and 214 lb-ft. of torque. This version is further refined for an even more exciting sound. Motor Trend noted in its May ??????????????????????????????????????03 issue that ???????????????????????????????????It delivers an impressive amount of power, with vibration evident only by its absence????????????????????????????????Underway, dual exhaust tips broadcast a turbine-like mechanical growl that??????????????????????????????????????s wired directly to the driver??????????????????????????????????????s right foot.???????????????????????????????? This engine powers the Z4 3.0i to 60 mph in just 5.9 sec. with the standard 6-speed manual transmission or optional SMG, and reaches 60 mph in only a tick more at 6.0 sec. with automatic (BMW AG test results). Top speed is electronically limited to 155 mph with the manual transmission or SMG; with the automatic it is a ???????????????????????????????????natural???????????????????????????????? 152 mph. With its manual transmission, the Z4 3.0i earns excellent EPA mileage ratings of 21 mpg city / 29 mpg highway; with SMG the ratings are 20/29 and with automatic 19/27.

5- and 6-speed manual transmissions

While the 2.5i model??????????????????????????????????????s standard transmission is a 5-speed manual, standard in the 3.0i is a 6-speed unit. Weighing no more than the 5-speed, this ???????????????????????????????????gearbox???????????????????????????????? incorporates refinements for even greater driving pleasure (and BMWs are already known for excellent manual transmissions):

• Even more effective synchronization of shifting
• Sportier shifting, via a 20-mm (0.8-in.) shorter shift lever and 10-mm (0.4-in) shorter shift ???????????????????????????????????throws???????????????????????????????? from neutral to each gear.

Optional STEPTRONIC

5-speed automatic transmission

Available for both models is the 5-speed STEPTRONIC unit that has garnered repeated praise from auto critics. Like all other current BMW automatics, it incorporates ???????????????????????????????????????

• A Sport mode that programs automatic shifts at higher engine speeds
• A Manual mode, which lets the driver choose shifts manually by ???????????????????????????????????flicking???????????????????????????????? the lever rearward or forward. Though basically similar to the transmission offered in other BMW 6-cylinder models, it is specially tailored to the Z4 with a ???????????????????????????????????tighter???????????????????????????????? torque converter (thus a more direct response feel) and specific shift characteristics.

???????????????????????????????????Third way????????????????????????????????

The Sequential Manual Gearbox

BMW currently offers two types of Sequential Manual Gearbox (SMG), both of which apply electrohydraulically actuated, electronically or driver-controlled shifting to a 6-speed manual transmission. Available on the super-performance M3 models is the very elaborate DRIVELOGIC version, which offers drivers a total of 11 shift programs. The version offered as optional equipment for Z4 roadsters (and, for ??????????????????????????????????????04, the 3 and 5 Series as well) is engineered for performance, convenience and (above all) driving pleasure; it offers a total of four shift programs:

• In its Automated mode, labeled D, a Normal program, and a Sport mode that executes shifts at higher engine speeds and makes the shifts themselves quicker.
• In its Manual mode, also a Normal and a Sport program; here the Sport program makes the shifts quicker, but the driver determines when the shifts occur.

In either mode, the Sport program is selected via a button on the console. There is no clutch pedal. The driver selects the desired operating range (N, R, D, S = Sequential) with a console-mounted selector lever, and can execute manual shifts with that lever or with two ???????????????????????????????????paddles???????????????????????????????? on the steering wheel. After starting the engine (which requires putting the lever in N and applying the brake pedal), the driver moves the lever to the right; this selects the Sequential mode, in which each tap of the lever or paddle(s) shifts the transmission up or down one gear. Moving the lever to the right toggles the unit to its Drive mode, in which shifts occur without the driver??????????????????????????????????????s intervention. To revert to Sequential shifting, the driver can toggle the lever to the right again (and simply tap it toward ???????????????????????????????????+???????????????????????????????? for an upshift or ?????????????????????????????????????????????????????????????????????????????????????????????????????????? for a downshift) or toggle one of the steering-wheel-mounted shift paddles. By pulling either steering wheel-mounted paddle, an upshift is executed. Pushing either paddle with your thumb produces a smooth downshift. In either the Drive or the Sequential mode, an instrument-cluster display indicates the gear currently engaged. In Drive, a ???????????????????????????????????D???????????????????????????????? is displayed next to the gear. When the Sport program is activated, an orange indicator light next to the Sport button illuminates. Though SMG does offer automated shifting, it is not meant to serve as a conventional automatic transmission; that role is played by the also available STEPTRONIC automatic. Just as with a manual transmission, power is interrupted for shifts ??????????????????????????????????????? though in hard, performance-oriented driving the shifts can be stunningly quick. SMG??????????????????????????????????????s appeal lies in these considerations:

• It is a racing technology, pioneered in Formula 1 competition and predominant in that sport today.
• It offers a new kind of actively enjoyable driving, and fascinating new things for an enthusiastic driver to learn.
• Relative to a manual transmission, it entails no performance loss and minimal increase in fuel consumption. In fact, it can match or even exceed the performance achieved by an expert driver.

Z4 running gear

This roadster is glued to the road

The Z4 raises enthusiasts?????????????????????????????????????? expectations of how a sports car should handle. Starting with a brand-new body/chassis structure, BMW chassis engineers developed a sports-car platform that is almost literally glued to the road. The basis for this remarkable platform is a body/chassis structure with exceptional stiffness for a roadster; it achieves 21 Hertz ??????????????????????????????????????? truly outstanding for a roadster, and close to the 25-26 Hz of today??????????????????????????????????????s BMW sedans. A number of specific features contribute to this rigidity:

• Y-form front longitudinal members. Each chassis rail carrying the engine (one per side) branches into a ???????????????????????????????????Y???????????????????????????????? to form a side sill and half of the central tunnel. In this regard, the Z4 is similar to the Z8 roadster (though the Z8 structure is of aluminum, the Z4??????????????????????????????????????s of steel).
• The side sills are configured to achieve maximum rigidity within acceptable bulk. (It??????????????????????????????????????s not acceptable simply to make them huge; this would cut into passenger space or make the car too bulky.)
• The underbody (floor pan) is designed to spread its strength evenly over its entire length and width.
• The underbody is further reinforced by two thrust plates, similar to those employed in the M3 models. The front one is of aluminum, the rear of steel. Also as in M3s, there are reinforcing braces from the front suspension??????????????????????????????????????s strut towers to the cowl area.
• High-strength steels are employed extensively for best strength without excess weight.
• To save additional weight, the hood is of aluminum.

In general terms, the Z4 suspension system applies concepts familiar from the 3 Series. Now, imagine a further developed system in vehicles almost 300 pounds lighter and with a significantly lower center of gravity, and you get an idea of the Z4??????????????????????????????????????s potential. Here are the particulars: Strut-type front suspension. with notable features:

• Forged aluminum lower arms, to reduce unsprung weight and thus improve ride and handling on rough road surfaces.
• Hollow strut rods, vs. solid; these weight 10% less than conventional solid rods.
• Relatively large positive caster to improve straight-line stability.
• Wide track ??????????????????????????????????????? 58.0 in.

Central Link rear suspension, a multi-link concept. The Central Link from which the system derives its name is a large, curved longitudinal arm, pivoted directly ahead of the rear wheel??????????????????????????????????????s vertical and horizontal centerpoint on a large rubber bushing of highly sophisticated design. Each wheel also has an upper and a lower lateral arm, for a total of three links per wheel. The system contributes to remarkable handling and riding comfort, yet is simpler than many multi-link concepts. Salient features include:

• Wide track, 60.0 in.
• Relatively large negative camber angles. Z4s have 2.25˚ negative camber at rest, visible in a slight inward tilt of the tops of the rear tires.
• Extra-firm forward bushings for the subframe that carries the suspension system.
• Aluminum upper transverse links to help reduce unsprung weight.

Overall suspension calibration. To underscore the Z4??????????????????????????????????????s sporting nature, relatively firm springs, shock absorbers and anti-roll (stabilizer) bars have been adapted. This means a firm ride and very ???????????????????????????????????flat???????????????????????????????? cornering. Because the standard suspension calibration is inherently sporty and the standard run-flat performance tires are relatively stiff, the available sport suspension (included in each model??????????????????????????????????????s Sport Package) does not employ firmer springs, shock absorbers and anti-roll bars as is customary with BMW sport suspension. Instead, its only difference is a 15-mm (0.6-in.) lower ride height. Every Sport Package-equipped Z4 comes with Dynamic Driving Control, which provides a Sport button on the console that selects ???????????????????????????????????????

• Firmer steering effort (less power assist) via the electric power steering
• Quicker accelerator response via the ???????????????????????????????????drive by wire???????????????????????????????? throttle system
• In vehicles with automatic transmission, an additional Sport mode beyond that selected with the shift lever.

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1 ??????????????????????????????????????? VANOS = VAriable NOckenwellen Steuerung = variable camshaft control, or variable valve timing.
2 ??????????????????????????????????????? Hertz, or cycles per second, quantifies a structure??????????????????????????????????????s rigidity in terms of its natural frequency of vibration when set into motion by some disturbance or input. Generally, the higher this frequency, the more rigid the structure.