For 2006, the Z4 Series undergoes its first major evolution. Heading its extensive list of updates and improvements are all-new, more powerful engines; 6-speed manual and automatic transmissions across the board; more powerful brakes, standard 17-in. wheels and tires now on both models, and a more advanced Dynamic Stability Control system. Along with all this driving-oriented progress, design and materials refinements inside and out enhance both esthetics and function.
Once again the Z4 Series consists of two Roadster models, but even their names have changed: Z4 Roadster 3.0i, replacing the former 2.5i model and base-priced at $36,295 including destination charge; and the Z4 Roadster 3.0si, replacing the former 3.0i model and priced at $42,795.
All-new engines: 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 model vs. previous M54 3.0-liter):
- Greater power, 255 hp vs. 225 hp. This amounts to 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 engine’s “redline” is 7000 rpm, vs. 6500 for the predecessor.
- Superior fuel efficiency. Though official U.S. EPA mileage ratings are not yet available, they are expected to reflect improvements similar to those of the 3 Series.
- Reduced weight - 22 lb. less. Had BMW engineers developed 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.
Z4 3.0i and 3.0si: the two engine versions
Instead of the previous strategy of a 2.5- and 3.0-liter engines in the two BMW Z4 models, both new models use a 3.0-liter N52, equipped and calibrated to provide two levels of performance:
- Z4 3.0i - 215 hp @ 6250 rpm, 185 lb-ft. torque @ 2750 rpm
- Z4 3.0si - 255 hp @ 6600 rpm, 220 lb-ft. torque @ 2750 rpm.
Significant differences between the two engines are as follows:
- Whereas the 3.0si engine has the 3-stage induction system, the 3.0i unit has a single-stage intake manifold.
- Engine software differs between the two models.
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.
Valvetronic variable valve lift. This exclusive, patented innovation appears in the N52 in evolved 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, Valvetronic 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. (This is new; the previous engine has bucket-type hydraulic valve lifters.) On the intake side, there is an additional element between the cam lobe and rocker arm, called an intermediate follower.
Upon contact by the camshaft 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 6600 rpm, vs. 5900 rpm before. Yet low- to medium-speed operation is not compromised.
- A “flatter” 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. In light-load operation, 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, high-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.
Transmissions: all choices are now 6-speeds
The previous Z4 2.5i had a 5-speed manual transmission, the 3.0i model a 6-speed. For ’06, both models (3.0i and 3.0si) come standard with a 6-speed.
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 unit, it’s fully 10% lighter, has a more efficient torque converter, actually operates with fewer internal clutches, and can reduce fuel consumption, particularly at cruising speeds when 6th gear is in frequent use.
Z4 suspension: firmly planted on the road
The Z4’s front and rear suspension systems are evolutions of the technology that has given BMW’s M3s their award-winning road capabilities. At the front, this means that the Z4 strut-type front suspension features:
- Forged aluminum lower arms to reduce unsprung weight and thus improve ride and handling on rough road surfaces.
- Hollow strut rods for a 10% weight savings in these components.
- Evolved geometry - large positive caster to improve straight-line stability.
- Wide track.
At the rear is a multi-link system, also similar to what’s found in the M3s and capable of keeping the rear wheels at virtually ideal angles relative to the road. With the proven capabilities of these suspension systems, the BMW Z4 has the “right stuff” for awesome handling. To underscore its sports-car character, relatively firm springs, shock absorbers and anti-roll bars are used, meaning:
- Amazingly stable and predictable handling, for example changing direction very little when the driver lets off the accelerator while cornering.
- A firm ride, communicating clearly that the Z4 is a serious sports car.
- Remarkably flat cornering, achieved via a low center of gravity and the firm suspension calibration.
- Impressive cornering ability: Road & Track (March ‘05) recorded a 0.92g figure for the Z4 3.0i with Sport Package.
Electric power steering: innovation with clear benefits
Uniquely among BMWs, Z4 steering is assisted by an electric servo motor instead of a conventional hydraulic pump. Benefits of this feature, all important to sport-minded drivers, include these:
- Facilitates specific tuning of steering characteristics, achieved via software.
- Vehicle-speed-sensitive power assist (Servotronic). Other current BMW models with this type of power assist are the 5, 6 and 7 Series as well as 3 Series Sedans and Sports Wagon with Active Steering.
- Reduced vehicle fuel consumption, because the electric motor operates only when the steering wheel is turned. A conventional hydraulic pump runs all the time.
All Z4 brake systems are engineered for powerful performance. For ’06, both models have upgraded brakes to match their greater performance:
- Z4 3.0i - front 300-mm/11.8-in. diameter, up from 286/11.3 on the previous Z4 Roadster 2.5i model; rear 294 mm/11.6 in., up from 280/11.0 and now ventilated so that this model has 4-wheel ventilated discs
- Z4 3.0si - front 325-mm/12.8-in. diameter, up from 300/11.8 on the previous Z4 Roadster 3.0i; the rear brakes remain at 294-mm/11.6-in. diameter, and all continue with ventilated discs
Inside the new Z4
Design and materials refinements add quality and visual appeal. As before, the Z4 Roadster’s cockpit features contemporary design and materials. It hasn’t changed fundamentally for ’06, but gets several refinements that enhance its quality and visual appeal.
There are four upholstery schemes:
- Leatherette – standard in Z4 3.0i, available in Black. A leather-wrapped steering wheel and handbrake grip are included; the trim material on dash and console is in a Graphite finish.
- Active Sport cloth/leather, optional in 3.0i only. This and both leather interiors include a new-design brushed-aluminum trim on dash and console; the available Montego Blue and Black colors are new for ’06.
- Oregon leather – standard in Z4 3.0is, optional in 3.0i. Leather seats, upper door-panel inserts and dash kneepads; available in Black, Beige, Dream Red and new Pearl Gray.
- New England extended leather – optional in 3.0is only. This finest Z4 treatment has special leather stitching, and adds leather to the windshield frame, sun visors and doors. Trim on the shift paddles, shift knob, gearshift-boot surround, door pulls and rollbar cladding is also upgraded. This interior is available in Black, Beige, Dream Red and new Saddle Brown.
Two audio systems available
A handsomely integrated 10-speaker audio system with single-disc CD player is standard in the 3.0i model. Optional in the 3.0i and standard in the 3.0si is a Premium Sound System with –
- Upgraded speakers throughout, including two 160-mm subwoofers that incorporate Carver long-stroke technology.
- A powerful Carver amplifier and 7-band Digital Sound Processing.
The Z4 Series just became the first European cars to win the coveted THX Certification for their premium audio system. THX Ltd. was established in 1983 by filmmaker George Lucas, and has taken its mix of technology and certification programs to emerging entertainment media, including home theater systems, DVDs, video games and high-performance automotive audio systems. THX engineers worked closely with BMW to analyze Z4 models‘ cabin acoustics, speakers and other audio components to determine whether they perform to THX standards. They did; the result is a THX Certified Premium Sound System that is designed to deliver a music experience as the artist intended.
In a comparison test of three high-end automotive audio systems, Germany’s auto motor und sport magazine (April 16, ’03) reported just how excellent this premium audio system is: “In the DSP’s neutral setting, the Z4 system delivers amazingly weighty, tight reproduction of drums that goes perfectly with the Roadster’s sporty character. Then, when one uses the DSP equalizer to compensate for the system’s truly minor weaknesses, the fun really begins.”