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Old 11-15-2006, 03:34 PM
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BMW X5 #3

X5 4.8i: The original Valvetronic V-8 delivers more power and torque with the same refinement

Under the X5 4.8i hood is a further evolution of this technologically advanced and unique V-8 engine. As such, it continues BMW’s revolutionary Valvetronic system, which controls engine power by varying valve lift instead of relying on a conventional throttle.

With Valvetronic, engine breathing – air induction – is controlled by varying valve lift rather than opening and closing a throttle. At minimum lift, the engine is idling or decelerating; at maximum lift, it delivers full power. Valvetronic varies valve lift according to the driver’s demands for power from the engine.

At smaller openings (lighter loads), a throttle imposes a restriction that incoming air must snake around. This causes so-called “pumping losses,” which take a greater proportion of engine power at lower speeds. By eliminating the throttle [1], Valvetronic essentially does away with these losses.

At the high end of the powerband, high valve lift contributes to high “specific power” (hp/liter). With conventional valve systems, where valve lift is always the same, there’s a limit to how high valve lift can be without degrading low-speed operation. With Valvetronic, lift is tailored precisely to operating conditions, and is extra-high at the top end. At the low end, in light-load operation and during idling, operation is especially smooth because valve lift is low.

Complementing the Valvetronic system is a dual-stage induction system, which also helps to improve the engine’s torque production over a wider rpm range. The new, higher torque peak of 350 lb-ft. occurs over a range from 3400 to 3800 rpm, compared to the predecessor’s 3600 rpm. Acceleration from 0 to 60 mph is 0.4 sec. quicker, though the perceived performance increase across the speed range may seem more impressive to many drivers.

[1] – Actually, there is an auxiliary throttle, for certain specific functions only: as a fail-safe measure for the unlikely event of Valvetronic malfunction; for diagnostic purposes; to control fuel-tank ventilation; and to improve cold starting. Under virtually all normal operating conditions, this throttle is open and imposes no restriction on incoming air.

The result of the increased displacement and additional enhancements improve the X5 4.8i’s performance significantly compared to the 4.4i. The new V-8 adds 35 hp and 26 lb-ft. of torque over previous 4.4-liter engine, yet retains the ultra-refined operation (smoothness) of its predecessor. Even though performance is increased significantly, EPA mileage ratings are little changed.

Standard on both models: an evolved 6-speed STEPTRONIC

Unlike the previous X5, the 2007 includes a 6-speed STEPTRONIC automatic as standard equipment. The previous 6-speed manual on the 6-cylinder X5 has been eliminated. This updated automatic also benefits from a number of refinements in the X5, designed to improve both performance and fuel efficiency—particularly in the case of the previous 5-speed in the 3.0i. Compared to the 5-speed automatic of the previous 3.0i model, the new transmission reduces fuel consumption at cruising speeds.

The new transmission also features an entirely new shifter. Instead of the previous mechanical lever, it is entirely electric/electronic and similar in concept to that of the
7 Series. With the X5’s console-mounted lever, Reverse, Neutral and Drive appear “inline” on the lever’s markings, and are selected by tipping the lever forward or rearward. Park is engaged via a separate button at the lever’s top. The transmission’s three modes—Drive, Sport and Manual—are selected by toggling the lever left or right.

In addition, this new transmission incorporates several new functional improvements that benefit efficiency or drivability. More effective internal vibration damping makes it possible to keep the converter’s lockup engaged more of the time. This means less slippage and a fuel-economy improvement of about 3%. The driver experiences sharper response to the accelerator pedal. Quicker shifting response—up to 40% quicker—means less lag between the time the accelerator pedal is depressed or the shift lever (or paddle) moved and the actual occurrence of a shift. Once a shift is initiated, shifting time is reduced by as much as 50%. Finally, the Adaptive shift control, which adjusts shift characteristics to driving style and conditions, now features three stages instead of two. This provides finer adaptation to driving style and conditions.

An additional advantage of the new shifter is that it provides significant freedom to optimize not only its location on the center console, but its size as well. The result: more space for storage compartments and large cupholders than would otherwise be possible.
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