For 2006, the Z4 has undergone its first major evolution. Heading an extensive list of updates and improvements are all-new, more powerful engines; 6-speed manual and automatic transmissions available on all models; more powerful brakes, standard 17-in. wheels and tires across the board, and a more advanced Dynamic Stability Control system. Along with all this driving-oriented progress, design and materials refinements inside and out enhance both aesthetics and function.
And now, still within the 2006 model year, the Z4 acquires a new family member: the Z4 Coupe 3.0si. As the Series‘ first closed-body model, the new Coupe encloses its two seats and a larger cargo area in a graceful fastback body that retains the Z4’s unique aesthetic character while looking expressively new. With this addition, the Z4 lineup now consists of three models: Z4 Roadster 3.0i at $36,295 including destination as the “base“ variant; Z4 Roadster 3.0si at $42,795 with higher performance and other increments of technology and features; and the new Z4 Coupe 3.0si, which shares its performance and equipment level with the Roadster 3.0si. The Coupe has a base price of $40,795 including destination charge; like the Roadsters it is produced at BMW’s newly configured and updated production facility in Spartanburg, South Carolina. Production of the Coupe begins on May 1, 2006.
The Coupe shares a comprehensive freshening with the ‘06 Roadsters, whose production began on February 1:
Performance & efficiency
- All-new, more powerful N52 engines for all models –
- Magnesium/aluminum composite construction
- Valvetronic variable intake-valve lift
- Electric water pump
- Volume-controlled oil pump
- Roadster 3.0i – 215 hp/185 lb-ft. torque vs. former 2.5i’s 184 hp/175 lb-ft.
- Roadster and Coupe 3.0si – 255 hp/220 lb-ft. torque vs. former Roadster 3.0i’s 225 hp/214 lb-ft.
- New final drive ratios, numerically higher to further improve acceleration (Coupe ratios differ from those of Roadster 3.0si)
- 6-speed manual transmission now standard on all models (former 2.5i model had a 5-speed)
- 6-speed automatic transmission optional on all models (formerly 5-speed), with new features:
- Shift paddles on steering wheel
- Direct Selection of Manual shift mode via paddle downshift
- Sportier overall programming
Handling, ride & braking
- 17-in. wheels and tires now standard on all models; new 17-in. differentiated front/rear sizes as part of 3.0i Sport Package; new wheel designs throughout
- Dynamic Stability Control system incorporates new braking functions –
- Brake Fade Compensation
- Brake Standby
- Brake Drying
- Start-off Assistant
- Modulated ABS function, for smoother operation
The new Coupe body
Reporting the Coupe’s debut at the Geneva Automobile Salon, Switzerland’s authoritative Automobil Revue described the design in its February 1, ‘06 issue: “The Z Coupe embodies the new BMW design in its purest form. It is characterized by round, flowing lines. Curved, long hood, pronounced wheel arches and a “trough“ down the roof’s center are classic sports-car design cues, but here they’re newly mixed in the BMW way. The roof flows in an arc to the rear window and on to the vertical rear cutoff point.“ America’s Automobile (December ‘05) lent this note of opinion: “There’s no controversy over the way the forthcoming BMW Z4 Coupe looks. It’s stunning, and it’s heading to showrooms next summer.“
So it is. In its lower-body contours, the Coupe corresponds mostly to the Z4, with its newly refined front end and hood lines; long-hood, cabin-back proportions; and double beltline in profile view. In particular, the lower, stylistically primary beltline begins above the outer headlight, curves upward over the wheels and then downward into the door, and “hikes“ sharply upward again, finally heading downward to conclude at the rear edge. Because the Coupe’s upper beltline has a visually longer continuity as it extends on past the new rear-quarter window with its characteristic BMW “reverse kink,“ the lower one even more boldly evokes the fender line of classic Grand Touring coupes, almost proclaiming for the Z4, “Yes, I was meant to be a Coupe too!“
Even stronger in its aesthetic impact is the fastback roofline. For one thing, in cross-section its center depression or “trough“ is reminiscent of certain GT coupes of the Italian coachbuilder Zagato in the Fifties and later. In profile view, it flows in a line parallel to 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.
This hatch, including the entire roof from the rear window’s top edge to the bumper, opens wide and high to provide excellent access to a relatively roomy cargo area that can accommodate two golf bags.
All-new engine: BMW’s advanced N52 6-cylinder concept
Most vehicle manufacturers’ 6-cylinder engines are in the V-6 format, whose compactness is advantageous for small or midsize cars with front-wheel drive. By contrast, BMW’s inline 6-cylinder engines are brilliant for their smoothness and sound, and BMW customers as well as professional auto critics have come to treasure them for these attributes. BMW has retained this inline format while developing it toward reduced weight, more compact dimensions – and even more brilliant performance, smoothness and sound. An increase in fuel efficiency and even tighter control of emissions were also set as goals for the new engine.
The result of this quest is a new generation of 6-cylinder engines, called N52. Compared to its predecessor, the M54 engine family, the N52 achieves notable progress on all fronts (N52 3.0-liter of 3.0si models vs. previous M54 3.0-liter):
- Greater power, 255 hp vs. 225 hp – a very impressive 85 hp/liter.
- Greater torque, 220 lb-ft. vs. 214, and even stronger torque delivery across the broad range of engine speeds.
- Higher revving ability. The new “redline” is 7000 rpm, vs. 6500 previously.
- Superior fuel efficiency. Both 3.0si models (Roadster and Coupe) with automatic transmission are EPA-rated at 21 mpg city/29 mpg highway, up significantly from their 3.0i predecessor model’s 19/27. With manual transmission, the 3.0si Roadster and Coupe essentially match their predecessor with 20/30 mpg vs. the previous 21/29.
- Reduced weight – 22 lb. less. Had BMW engineers evolved the previous engine to meet their goals, it would have weighed 52 lb. more than the new engine does.
- More compact – Because there is just one external drive belt, vs. the previous two, overall engine length is about an inch shorter.
Here’s how this dramatic progress was achieved – over an engine that was already outstanding in every respect.
Valvetronic variable valve lift. This exclusive, patented innovation appears in the N52 in refined form.
Variable valve lift is a step beyond variable valve timing – which this and all other current BMW gasoline engines also have. Valvetronic varies lift to a far greater degree than other variable-lift systems; indeed, this concept varies lift so extensively that it replaces the traditional engine throttle; engine breathing is controlled by the valves rather than a throttle or throttles.
The Valvetronic mechanism sits atop the intake valves. Each of the engine’s 24 valves is actuated as the camshaft lobe deflects a finger-type rocker arm. On the intake side, there is an additional element between the camshaft lobe and rocker arm, called an intermediate follower.
Upon contact by the lobe, this follower actuates the rocker arm and, in turn, the valve. The follower is positioned by an eccentric shaft that a servo motor rotates in response to the driver’s accelerator-pedal movements; the eccentrics on this shaft determine each intermediate follower’s pivot point and thus varies the valve lift.
Here are the highlights of Valvetronic:
- Intake valves assume function of throttle. Engine breathing – air intake – is controlled by varying valve lift. The driver’s foot gives the commands; valve lift varies accordingly. At minimum lift, the engine is idling or decelerating; at maximum lift, it delivers full power.
- Greater efficiency. As a throttle closes, it imposes a restriction that incoming air must snake around. This causes “pumping losses,” which take a greater proportion of engine power at lower speeds. By eliminating the throttle , Valvetronic essentially eliminates pumping losses.
- More spontaneous engine response. Again, because there is no conventional throttle.
- More power. High valve lift contributes to high power output. With conventional valvegear, there’s a limit to how high valve lift can go without degrading low-speed operation. With Valvetronic, lift is tailored precisely to all operating conditions, and is extra-high at the top end. The 3.0si engine’s power peak (255 hp) comes at a relatively high 6600 rpm. Yet low- to medium-speed operation is not compromised.
- “Fatter” torque curve. Not only does the new engine produce more torque; torque also peaks at a lower engine speed, 2750 rpm vs. the previous 3500 rpm. This means stronger low- to midrange response.
- More refined engine operation. Light-load operation is especially smooth because valve lift is low.
- Low friction, precision components. Every “rubbing point” in the Valvetronic mechanism is not a rubbing (friction) point at all. Instead, low-friction rollers transmit the motion: from cam lobe to intermediate follower, follower to rocker arm, eccentric shaft to follower. The follower itself is a precision component – now even more so in this new, higher-rpm evolution of Valvetronic. As before, zero valve clearance is maintained hydraulically to ensure quiet operation, though by a different mechanism.
How Valvetronic has evolved. As dramatic as these fundamental advantages of Valvetronic are, with this new engine they become even more significant. Though highly technical and detailed, the evolution of Valvetronic can be understood in these broad terms:
- Maximum engine speed increased by 500 rpm. This was enabled by making Valvetronic’s reciprocating parts more rigid.
- Maximum intake-valve lift increased from 9.7 mm to 9.9 mm, which contributes to the increase in maximum power output.
- Greater maximum intake-valve acceleration. Less time is spent opening and closing the valves; thus they are effectively open longer, further reducing pumping losses.
- Phasing of the two intake valves. Starting from minimal intake-valve lift (i.e. idling), an increase in engine load causes the lift and timing of intake valve 1 to increase faster than that of valve 2. At its maximum, this phasing has valve 1 lifting 1.8 mm more than valve 2; at about 6 mm, the two valves are again “in synch.” This achieves an asymmetric distribution of the fuel/air mixture that enhances fuel economy under low-load driving conditions.
Improved combustion chambers. Subtle refinements to the combustion-chamber shape conspire with the intake-valve phasing to create more stable combustion, with benefits to fuel efficiency and emission control.
Further evolved VANOS. Double VANOS variable intake- and exhaust-valve timing is a familiar feature of all current BMW engines. The range over which intake-valve timing can be varied has been increased by 10˚, achieving yet another de-throttling effect.
3-stage induction system. Previous Z4 engines had a 2-stage (or dual-resonance) system: one intake-path length for lower rpm, the other for higher rpm. This system further optimizes the engine’s power delivery by providing an additional “middle” stage. Electrically switched, the three stages are:
- Low-speed: idle-3250 rpm
- Medium-range: 3250-4500 rpm
- High-speed: 4500-7000 rpm.
(This feature is present only on the Z4 3.0si engine; the 3.0i engine has a single-stage intake manifold.)
Higher fuel-injection pressure, up from 3.5 to 5 bar (51.4 to 73.5 lb/sq in.), results in an improved injection spray, helping reduce raw hydrocarbon emissions in a cold engine.
All-new engine electronics. The number of variables (inputs) feeding into the engine’s electronic management system has increased significantly; a completely new system was developed. Among many innovative details, the basic ignition and valve-timing functions are duplicated. The first part was optimized for fuel consumption and emissions; the second part was determined according to pure driving parameters. Depending upon how perfectly the engine is running at any time, control interpolates between the two strategies. Under ideal conditions, the engine always runs with its lowest fuel consumption. In case of poor fuel quality or unfavorable environmental conditions, the control parameters prioritize driveability.
Magnesium/aluminum composite construction
Although the direct customer benefits of this unique and pioneering construction are subtler than those of Valvetronic, this is an important innovation – a world’s first in modern times and exclusive to BMW.
Structurally, the new engine block consists of three major castings:
Bedplate (magnesium alloy ). This casting forms the lower portion of the block (crankcase), and is similar in concept to a construction element found in some racing engines – as well as the 500-hp V-10 engine that powers the BMW M5 and M6. The bedplate combines with the upper crankcase to form the outer shell of the cylinder block, resulting in an ultra-rigid engine structure.
Upper crankcase (magnesium alloy). Joining the bedplate at the level of the crankshaft (main) bearings, this too is a weight-saving casting. It is mounted onto the bedplate from above.
Insert (aluminum alloy). Forms the cylinders and their coolant passages. Whereas the previous engine has an aluminum block with cast-iron sleeves as the cylinders’ working surfaces, this insert is of silicon-impregnated aluminum (Alusil). Silicon particles are thus cast into the block; a “soft honing” machine removes just enough of the aluminum to leave the crystals as the ultra-hard cylinder surfaces. In this sense, the N52’s block construction resembles that of current BMW V-8 and V-12 engines, though these blocks are all-Alusil.
How it goes together. First, the aluminum insert is cast by conventional methods. Then, during a newly developed die-casting method, the magnesium upper shell shrinks onto the insert while cooling; structural rigidity and stability are ensured by interlocking ribs where the two castings meet.
In the next step, the upper crankcase, consisting of magnesium shell and aluminum insert, is mounted onto the magnesium bedplate from above. The sintered-steel main bearings’ lower halves are in place in the bedplate, the upper halves in the upper crankcase. After the bedplate and upper crankcase have been bolted together, a liquid sealing compound is injected into a groove on the contact surface between the two components. Special aluminum bolts are used to attach parts, such as the engine mounting brackets, to the magnesium/aluminum castings.
As in all BMW engines for decades, the cylinder head is of aluminum; however, the head of an inline 6-cylinder engine must be cast with great precision because its relatively great length implies relatively large contraction as it cools after casting. The casting process used here is called “lost-foam”; this process, which employs a polystyrene “dummy” of the head to form the mold into which the aluminum is poured, results in an extremely precise casting.
Other weight-saving materials. Though the magnesium/aluminum composite crankcase construction is the most sensational example, other materials and production innovations also help pare weight from the N52 engine. The second most productive material innovation was the adoption of hollow camshafts, which save a remarkable 2.6 lb. Beginning as steel tubes, the camshafts are shaped in a hydroforming procedure, subjected from the inside to a water pressure of 4000 bar (58,000 lb./sq in.) against outer forms to achieve the cam profiles. All this takes place in a cold state – nothing melts – and as a final step the cams are polished to a finish quality of 1/1000 mm.
The engine’s camshaft cover is of weight-saving magnesium. And the chain camshaft drive, a high-durability, low-maintenance feature of all current BMW engines, has an aluminum chain tensioner that also saves weight. Instead of being a separate casting, the camshaft drive’s housing is integrally cast into the magnesium structure, eliminating a production step and sealing components. As one final weight-reducing element, the exhaust headers’ flanges are formed from 2-mm-thick steel, significantly lighter than the 12-mm flanges used previously.
Electric water pump. A conventional engine water (coolant) pump is driven by a belt at a speed directly proportional to engine rpm. This innovation is electrically driven and electronically controlled according to the engine’s coolant and oil temperatures at any moment. Thus it runs only as much as needed, and in doing so consumes a maximum of 200 watts vs. up to 2 kilowatts (10 times as much) for a conventional pump. The electric pump has numerous tangible benefits:
- By requiring less power, contributes to the engine’s increased power output.
- Faster engine warmup, because it doesn’t circulate coolant when the engine is cold.
- By eliminating an external drive belt, makes the engine shorter.
Variable-volume oil pump. Conventional oil pumps, too, deliver oil in direct proportion to engine speed. To supply pressure to the VANOS system (which employs oil pressure to rotate the camshafts and thus vary valve timing) at all speeds and temperatures without excess capacity at high engine speeds, BMW engineers developed a new type of oil pump. By varying the output of its pump element according to engine oil pressure, this pump always delivers sufficient pressure to lubricate the engine and operate VANOS, yet never pumps more oil than is necessary. Thus it –
- Contributes to the engine’s increased power output, by requiring less power from the engine.
- Doesn’t require a bypass to divert excess flow, which can be up to 80% with a conventional pump. This also avoids possible excess oil temperatures and oil foaming.
Oil/coolant heat exchanger. Another feature that speeds engine warmup; during this phase of operation, it transfers heat from the coolant to the oil circuit. Under conditions of high engine power and high oil temperatures, it performs the reverse, transferring heat from the oil circuit to the coolant, from which the engine cooling system then removes excess heat.
Transmissions: all choices are 6-speeds
The previous Z4 2.5i had a 5-speed manual transmission, the 3.0i model a 6-speed. For ’06, all Z4 models come standard with a 6-speed like the one Motor Trend praised in its August ’04 issue: “Like the engine,” commented the magazine, “the gearbox is a sweetheart. Said one editor, ‘The shifter has that positive, spring-loaded feel.’ Another wrote, ‘It snicks into its gates with an oiled authority.’”
There’s also good news for those who like their shifting automated – at least part of the time. Now the Z4 joins other BMW Series in offering a 6-speed automatic; compared to the previous 5-speed it’s fully 10% lighter, has a more efficient torque converter and new shift programming for improved responsiveness, actually operates with fewer internal clutches, and can reduce fuel consumption, particularly at cruising speeds when 6th gear is in frequent use.
As with other current BMW automatics, the STEPTRONIC feature offers drivers a choice of Normal, Sport and Manual modes. The Sport mode, in which shifts occur at higher engine speeds, is engaged by moving the lever leftward from Drive; the Manual mode is engaged when the driver manually chooses a gear by tipping the lever forward (for downshifts) or rearward (for upshifts).
Performance with a conscience
BMW strives to produce its motor vehicles and other products with the utmost attention to environmental compatibility and protection. Integrated into the design and development of BMW automobiles are such criteria as resource efficiency and emission control in production; environmentally responsible selection of materials; recyclability during production and within the vehicle; elimination of CFCs and hazardous materials in production; and continuing research into environmentally friendly automotive power sources. Tangible results of these efforts include the recycling of bumper cladding into other vehicle components; water-based paint color coats and powder clear coats; near-future availability of hydrogen-powered models; and various design and engineering elements that help make BMWs easier to dismantle at the end of their service life