BMW Z4 M Coupe (2006)

Even stronger in its esthetic impact is the new fastback roofline. For one thing, in cross-section its center depression or "channel" is reminiscent of certain GT coupes of the Italian coachbuilder Zagato in the Fifties and later. In profile view, the roof parallels the side windows' top edge, then continues on down to meet a crisp, stand-out cutoff point that forms a rear spoiler. Then it concludes at the bumper as the bottom edge of a rear hatch. Everywhere you look, the contours are masterfully drawn.

This hatch, encompassing the entire roof portion from the rear window's top edge to the bumper, opens wide and high to provide excellent access to a relatively roomy, velour-lined cargo area that can accommodate two golf bags.

The BMW M look: subtle and purposeful

The transformation from a regular-production BMW model to a BMW M creation is always subtle, always purposeful. The M Coupe is clearly in this tradition.

At the front, the M model has a complete distinct facia - bumper, spoiler, grille framing - that makes a statement both esthetic and functional. The traditional BMW "kidney" grilles are set deeper into their openings. Their vertical slats are black, harmonizing with the three black-trimmed air-intake openings underneath. To satisfy engine and brake cooling requirements, these are somewhat larger than the Z4's single opening. Standard Xenon headlights with auto-leveling include BMW's popular luminous rings on the inboard lamps, functioning as parking lights but also remaining illuminated when the headlights are on. A high-intensity headlight cleaning system is also standard.

As on Z4s, the hood is aluminum, positively influencing weight distribution and overall vehicle weight; the M version adds two longitudinal "precision lines" not present on Z4s. At the sides, the 18-in. M Double Spoke wheels and their ultra-low-profile performance tires reflect the BMW Z4 M Coupe's performance priorities, while an M logo just behind the characteristic diagonal slash and its BMW logo/turn indicator confirms the Coupe's heritage.

In its rear view, the BMW Z4 M Coupe also has its own bumper ensemble; this includes a diffusor and cutouts for the four chrome exhaust tips. As on '06 Z4s, the taillights are new, and feature upper sections with three horizontal LED strips that serve the second stage of illumination provided by BMW's Adaptive Brake Lights under hard braking. The M logo appears again here, on the left side of the hatch lid.

In this intimate cabin for two, M also means luxury

In its interior design, the Coupe adopts the primary design themes and materials of the Z4 M Roadster.

Doorsill trims display the M logo (with its three colors) in a large, handsome format. Once inside the BMW Z4 M Coupe cockpit, the driver and passenger enjoy a uniquely sporty, elegant atmosphere blending BMW M's style of abundant luxury with typical BMW functional design. Premium Nappa leather M sport seats (choice of manual or power) include embossed M logos on their head restraints. Leather also appears on the door panels, console armpad, kneepads, handbrake grip and shift knob. Trim materials are special: The largest areas (across the dash, center console) are in a sporty-technical Aluminum Hexagon surface, while pearl-gloss galvanic trim appears on the climate controls, interior door handles and pulls, and the "tubes" into which the main instruments are recessed. Automatic climate control is standard in the M; its rotary knobs include the new rubber touch surfaces that enhance their tactile feel.

Tactile feel is also a strong point of the M sport steering wheel, which displays its heritage with the BMW M logo on the bottom spoke and M-color stitching of its thick leather rim. Fingertip audio controls and tilt/telescopic adjustment are included on the wheel.

As in other M models, instrumentation is special, both optically and functionally. The instrument cluster reflect the M's performance capabilities with special features:

• 9000-rpm tachometer scale
• Variable tachometer warning segment that lowers the redline according to engine temperature: lowest with a cold engine, then increasing in increments to an 8000-rpm limit with a fully warmed-up engine. Although this feature appears in some regular-production BMW models, the BMW M graphics (with an arc of orange and red LEDs) are distinctive.
• Oil-temperature gauge at bottom of tachometer dial
• 180-mph / 300 km/h speedometer scale
• Distinctive graphics throughout, including red-on-gray dial pointers.

Two alternate trim materials are offered in place of the standard Aluminum Hexagon: Carbon Leather at no extra cost, and the rich Walnut Madeira wood (that's also offered in the M5 and M6) at extra cost.

Also available is even more luxurious M Extended Leather upholstery; see options for details on this and other available optional choices.

Two audio systems available

A handsomely integrated 10-speaker audio system with single-disc CD player and MP3 capability is standard in the BMW Z4 M Coupe. Optional, as part of the Premium Package, is a premium system with:

• Audiophile-quality speakers throughout, including two 160-mm subwoofers incorporating Carver long-stroke technology
• A powerful Carver amplifier and 7-band Digital Sound Processing
• An FM reception-enhancing diversity antenna system.

The Z4 premium audio system just became the first in a European car to win the coveted THX Certification for audio systems. For more details on the system and this recognition of its excellence, see options.

The BMW M 6-cylinder engine: thoroughbred performance machine

Already known from its application in the award-winning BMW M3 Series, this engine has been aptly characterized as "ferociously powerful" by Car and Driver (May '03) and "an engine that likes to rev" (Road & Track, December '03). The British magazine Engine Technology International has named it Best Engine of the Year for 3.0 to 4.0 liters for five years in a row. Now this remarkable unit gets to power a lighter, even more agile BMW.

Statistically speaking, the results are dramatic: delivering 330 horsepower at a high-revving 7900 rpm, the internally designated S54 engine propels the 3200-pound Roadster to 60 mph in just 4.9 sec. Maximum speed is electronically limited to 155 mph; speculation about what the top speed might be without the limiter will intrigue enthusiasts, but BMW makes no claim in this regard.

As multi-talented as any M Car is - and the M Roadster certainly is that - its heart is always the engine. Under the M's aluminum hood with its distinctive added contour lines is an engine like none other. In its broad concept, this engine shares its inline format with other BMW 6-cylinder engines. Virtually all its engineering details, however, are unique and targeted to the very highest level of performance.

Unlike other BMW 3-Series engines, which employ lighter materials for their block and cylinder head, the S54 combines a cast-iron block with its aluminum head; this was adopted to achieve the engine's 3.2-liter displacement without lengthening the block.

The block accommodates this engine's bore and stroke of 87.0 x 91.0 mm, both larger than the dimensions of BMW's 3-liter engines. However, playing larger roles than increased displacement in the 75-hp increase over the most powerful regular-production 6-cylinder engine is the M's induction, combustion and exhaust engineering, together with its execution as a high-rpm engine. The starring role here is played by a cylinder head that could be termed "exotic." Its key features include:

Double VANOS steplessly variable valve timing on the intake and exhaust camshafts. Though this system appears in all other current BMW engines, in the M unit it varies timing over a wider range and contributes in a major way to the engine's ultra-high power output.

VANOS pressure pump. In regular-production BMW engines the main oil pump supplies the pressure to operate VANOS; here the VANOS system has its own radial-piston hydraulic pump. Integral to the exhaust camshaft's VANOS mechanism, the pump produces up to 120 bar (1740 lb./sq in.) of pressure to vary valve timing more quickly at the very high rpm this engine reaches. BMW M refers to its VANOS system as High-Pressure Double VANOS; it is also used in the M5's 500-hp V-10 engine.

Unique valve mechanism. All BMW engines have an efficient valve mechanism. Yet for an engine with the S54's rpm potential, BMW M engineers needed less reciprocating mass than in the regular-production engines.

To achieve this, they created a unique actuating mechanism, using extra-light finger-type rocker arms. Pivoting on their own shafts (one on the intake side, one on the exhaust), these small arms provide the actuating surface between camshaft and valve. As the entire arm does not move the distance of valve lift, its effective reciprocating mass is less than its actual mass. All told, the valvetrain's effective mass is 30% less than in the BMW 6-cylinder engines that have bucket-type hydraulic lifters; in turn, this allows lighter valve springs, which further reduce inertia. The system also has less friction.

As there is no hydraulic maintenance of valve clearance, it does have to be inspected periodically. It is unlikely that clearance will ever require adjustment, but if so it is done with shims (tiny metal discs of various thickness) without removing the camshafts.

Whereas the "regular" 6-cylinder engines have simplex (single) camshaft drive chain or chains, the S54 has a full duplex (double) chain driving its camshafts. As usual with BMW engines, the chain is hydraulically tensioned and needs no periodic adjustment or replacement.

Extra-high compression ratio. At 11.5:1, the M engine has the next-highest ratio in current BMW production, topped only by the M5-M6's 12.0:1.

Machined surfaces. More "engineering finery" again: Combustion chambers and intake ports are completely machined, for smoothness that facilitates airflow. The exhaust ports are partially machined. For durability, the valve seats are of especially hard steel. A 3-layer stainless-steel head gasket ensures effective sealing of the head to the block.

Head casting and sealing. Extreme strength in the cylinder head is achieved by making it a single aluminum casting. Though elaborate, this construction also saves a significant 29 lb. As this weight reduction is at the top of the engine, it helps lower the car's center of gravity.

Induction system: BMW M tradition, state-of-the-art technology

The S54 engine manifests an important BMW M tradition: an individual throttle for each cylinder. Positioned much nearer to the cylinders than a single throttle can be, these bring atmospheric pressure practically right to the cylinder. The "lag time" inherent in airflow into the cylinders is thus greatly reduced and the engine can react more quickly to throttle movements.

Via the accelerator pedal and its two potentiometers, the driver gives the commands, in turn processed by the engine control module and received by a DC servo motor. The motor drives the throttle shaft through a tiny gearbox.

Upstream of the throttle bodies are the six intake trumpets, made of weight-efficient fiberglass-reinforced PA6 thermoplastic; these are laser-welded into the induction plenum of the same material to form a single assembly.

M Dynamic Driving Control provides Normal and Sport settings for throttle response. In Sport, selected via a console switch, the ratio of throttle opening to pedal movement is increased so that apparent engine response is even quicker. Even the transitional response of the electronic engine controls is altered to suit. Drivers tend to find one or the other setting more to their liking, or choose them according to driving conditions or mood. The system always defaults to Normal when the engine is started.

Together with the VANOS, this elaborate induction system adds to the engine's immense breathing and fuel/air processing capabilities.

Exhaust system: engineered for free flow

For the M Roadster and BMW Z4 M Coupe, it was necessary to develop a newly routed exhaust system because its underbody is quite different from that of the M3s that the S54 engine also powers.

After the partially machined exhaust ports, the system begins with two snaking stainless-steel headers serving three cylinders each. These elaborate headers are "hydroformed" under high pressure with liquid inside them, which ensures even distribution of the forming pressure and thus consistent wall thickness. In turn, this process allows stainless-steel walls only 1 mm thick (about 1/25th of an inch), not only helping save weight but also hastening engine warmup as there is less metal to heat up after a cold start. Each header is a single piece, thus not welded-up as are most headers.

For effective emission control during engine warmup, the forward catalytic converters are right at the headers. From these converters rearward, the dual exhaust system continues through mufflers, resonators, two additional catalysts and finally four chrome outlets that speak the authoritative tones of M Power. The more constricted space available under the Z4 body costs 3 hp compared to the BMW M3; but then the BMW Z4 M Coupe weighs 185 lb. less than the M3 Coupe, more than offsetting the slight reduction in power. In its sound quality, the M Coupe exhaust system is tuned for a somewhat milder tone than that of the M3.

High-performance lubrication and cooling

To help ensure adequate lubrication under the high cornering, acceleration and braking loads the BMW Z4 M Coupe attains, the S54 engine employs a "semi-dry-sump" oiling system. Particularly in hard cornering to the left, it is critical to ensure return of oil to the pan; therefore, integrated into the gear-type pressure pump is a scavenging pump that collects oil from the right side of the small forward oil sump and pumps it back into the main, larger rear sump. This rear sump is almost completely closed off from the rest of the system, and thus able to hold the oil necessary for lubrication throughout the engine. Specific return passages are also incorporated into the intake (left) side of the engine to help ensure ideal oil flow under all operating conditions.

The graphite-coated aluminum pistons are cooled by oil spray; each valve's rocker arm is sprayed with oil just before it is loaded by its camshaft lobe.

A thermal sender monitors oil level and temperature. If the level drops low, a warning appears in the instrument cluster; the tachometer face includes the oil-temperature gauge.

The cylinder head incorporates crossflow cooling; this promotes consistent temperatures from the front to the rear of the head, helping minimize distortion and wear under the extreme heat such a high-performance engine develops when its full power is being exploited.

The high-rpm concept

High engine speeds are essential to achieving high power from moderate displacement, but they pose challenges; engineers must ensure that durability standards are met at these levels. The S54 engine's maximum power occurs just below its 8000-rpm limit at 7900 rpm.

To achieve this revving capability, the engineers applied a number of detail measures. A forged, nitro-carbonized steel crankshaft provides great strength in this critical component. Forged-steel "crack" connecting rods eliminate the need for bolt sleeves and thus reduce reciprocating weight.

A unique water pump plays a role too. The crossflow cooling requires high coolant flow. To achieve this, the engineers developed a pump with 3-dimensionally contoured vanes. Such contours would have been inordinately costly to produce in metal, so BMW M developed a novel pump design. Each vane is a small plastic casting, pressed into an also-plastic rotor and then welded into place. Also adding cooling efficiency is a ring thermostat, which imposes less resistance to coolant flow than a conventional plate thermostat.

Electronics play their role too. The engine's control module - newly developed for the M Roadster application - calculates and adjusts the ignition and fuel injection at each cylinder individually every 6 degrees of crankshaft rotation, it. Ignition occurs through a very small-diameter "pencil" coil at each cylinder.

The BMW Z4 M Coupe chassis: extensive new engineering by BMW M

Starting with the Z4's already excellent chassis engineering - Motor Trend commented in its May '05 issue that the Z4's "sport suspension and solid chassis make this car a thrill to drive on canyon roads" - BMW M has endowed the BMW Z4 M Coupe (along with its Roadster stablemate) with typical M modifications that further enhance handling and tailor the car's road capabilities to M's higher performance level.

New front suspension. While retaining the Z4's basic concept - strut-type with forged-aluminum lower arms - the M engineers created an essentially new version with:

• A wider front track (by 0.5 in.)
• Newly configured lower arms, still of forged aluminum; more complex shape with open sections for lightness, similar to those of M3s
• New steering knuckles, bolted to the struts (save weight, sharpens steering response)

Such changes, subtle yet deep-reaching, are - as good as the basic BMW model is - key to achieving the "beyond exceptional" reflexes of an M model.

Rear suspension. Though modifications to the Z4's Central Link (multi-link) rear suspension are less extensive than at the front, they are significant. Among others, the following major M elements are shared with M3s:

• Rear subframe - larger and stronger; accommodates differential with M Variable Differential Lock
• Wheel bearings
• Central (longitudinal) Links
• Arrangement of anti-roll bar.

As at the front, these modifications sharpen handling capabilities.

Suspension calibration. The M gets an all-around new calibration of springs, shock absorbers, anti-roll bars and bushings to BMW M parameters; this includes specific auxiliary springs at the front and rear.

A new steering system. Alone among BMWs, Z4s have electrically assisted power steering. With this system's vehicle-speed-sensitive power assist, it is notable for its combination of reduced driver effort in parking and at low vehicle speeds, and good road feel at higher driving speeds.

BMW M has somewhat different priorities: a greater emphasis on absolutely accurate road feedback and ultra-spontaneous response to the driver's commands at the steering wheel. In this context, higher parking and low-speed efforts are acceptable. Thus M gave the Coupe what is essentially the M3 steering system, with:

• The hydraulically assisted rack and pinion of the M3s
• A power-steering pump similar to that of M3s, but with assist characteristics specifically optimized for the BMW Z4 M Coupe.

As with M3s and other BMW models with engine-speed-sensitive power assist, this system reduces parking efforts somewhat, but not to the extent that vehicle-speed-sensitive assist does. Interacting with the M front suspension, this M steering system helps deliver the sort of reflexes that a Road & Track editor commented upon in December '03: "The M3 is the sharpest tool for carving up a back road."

BMW M's most advanced brake system. The BMW Z4 M Coupe comes standard with BMW M's most elaborate brake concept, one that is part of the M3's Competition Package and is standard on the BMW M5 and BMW M6: compound, cross-drilled 4-wheel ventilated discs.

The brake rotors are 2-piece, with an aluminum "hat" and a cast-iron outer portion that is the actual friction surface. The hat and outer portion are connected by steel pins on which the aluminum and cast-iron portions, because of their differing expansion rates, can move relative to each other. This is an elaborate, costly construction that cuts unsprung weight and eliminates rotor deformation under hard-braking, high-heat conditions. In practical terms, this means virtually no tendency of the brakes to vibrate when hot, and also virtually no likelihood of the rotors cracking under the extreme heat conditions that typically might be encountered in driving on a race track.

Dimensions are as on the M3 with Competition Package and the Z4 M Roadster. At the front, the discs are of 345-mm diameter x 28-mm thickness (13.7 x 1.10 in.); at the rear, the discs measure 328 x 20 mm (12.9 x 0.79 in.). Cross-drilling, visible through the wheels, enhances heat dissipation beyond that provided by the rotors' internal ventilation, further increasing the brakes' resistance to fade, improving braking effectiveness in wet weather and reducing unsprung weight.

Wheels and tires: ultimate style and grip. Except for wheel design, M Roadster/Coupe wheel and tire equipment is the same as that of M3s. M Double Spoke wheels, with a very open configuration to help get plenty of cooling air to the brakes, complement the Coupe's new body design with a boldly sporty, yet elegant look. Sized 18 x 8.0 front and 18 x 9.0 rear, they are connected to the road by suitably wide, low-profile, high-speed-rated tires sized 225/45ZR-18 front / 255/40ZR-18 rear .

As with other BMW M models, the Coupe's tires are not run-fats; run-flat tires meeting M requirements are not yet available. Also, the M exhaust system's bulk precludes a spare tire. Thus like other BMW M models, the Coupe comes standard with the M Mobility System for use in case of a flat tire. A Flat Tire Monitor alerts the driver to a loss of tire pressure.

M Mobility consists of a container of rapid sealant, a small compressor, and a hose to connect the compressor to the damaged tire. All this is carried in a container in the trunk; the system can seal punctures up to approximately 1/4 inch across. Omitting the spare tire saves about 45 lb. of weight.

The BMW Z4 M Coupe's wheels incorporate the so-called Extended Hump rim shape, which helps keep the tire on the rim in case of deflation.

Optimum weight distribution. The BMW Z4 M Coupe is laid out for almost precisely even front/rear weight distribution: 50.2% front/49.8% rear.