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.
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.)
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