In 2002, BMW introduced the high-performance X5 4.6is. As the top dog in the X5 line of sport-utes, its base price was $67,495, about 17 grand more than the 4.4i model. Although that's some serious cheese, it turns out that about $9000 of that price increase was the result of making almost every option on the 4.4i model standard on the 4.6is. The remaining $7800 was for 20-inch wheels and tires, 0.9-inch-larger front brakes, and an upgraded V-8. More 2005 models...

In the 2002 4.4i model, the V-8 displaced 4398cc and made 290 horsepower and 324 pound-feet of torque. Upgrading that V-8 to 4.6 liters added another 221cc, 50 horses, and 26 pound-feet of torque. More important, the extra oomph boosted the X5's performance, matching the Mercedes-Benz ML55 AMG's 6.5-second run to 60 and 15.1-second quarter-mile—at the time, the quickest straight-line acceleration numbers for an SUV. Then in 2003, the $45,000 Infiniti FX45 had a brief reign until Porsche claimed the acceleration honors with the 90K Cayenne Turbo, which scampers to 60 mph in five seconds flat and turns a 13.5-second quarter-mile.

BMW has raised the bar yet again, replacing the 4.6is with the 2004 4.8is, but the company doesn't bring the stones to outdo the Turbo; it's the same formula as the 4.6is—slightly more displacement, the same massive 20-inch wheel-and-tire combo, and nearly every X5 4.4i option now standard.

In this iteration, the 32-valve DOHC 4.6-liter engine was further stroked by 3.3 millimeters to achieve the extra 200cc displayed on the badge. This engine's added displacement is teamed up with the other '04 updates that all V-8 X5s received—Valvetronic (BMW's throttle-less variable-valve-lift system), a six-speed automatic transmission, and xDrive all-wheel drive (an electronically controlled limited-slip center differential replaces an open differential that relied on traction control to distribute torque).

The high-end truck now turns out 355 horsepower and 360 pound-feet of torque, which is 40 more horsepower and 36 more pound-feet than the '04 4.4-liter X5, but only an additional 15 horsepower and 10 pound-feet over the 4.6is.

Firing up a 4.8is begins with a menacing braaap followed by a throaty burble at idle, loud enough for passing shopping-cart pushers to take notice. The exhaust note is satisfying not only in the parking lot but also while cruising, and it never has an annoying drone.

The newfound power is sent through a six-speed transmission that provides smooth shifting when the driver is light on the throttle and firms up the shifts and holds gears appropriately with large throttle inputs. However, this new automatic strangely executes early downshifts—even when not in sport mode—resulting in noticeable engine braking when coming to a stop.

These seemingly incremental increases—additional power, a six-speed (the 4.6is had a five-speed), and slightly less weight (at 5090 pounds, the 4.8is weighs 37 fewer pounds than a 4.6is)—provide a surprising leap in acceleration.

At six seconds flat, 0-to-60 passes 0.5 second quicker, the quarter-mile mark comes 0.6 second sooner and 4 mph faster (14.5 seconds at 96 mph), and the speedo swings to 100 mph in 16.0 seconds, 2.1 seconds quicker than the 4.6is.

Fitted with the X5 sport-suspension package and spinning low-profile dubs—the same absurdly large Michelin Diamaris tires from the 4.6is (275/40R-20s in front and 315/35R-20s in back)—it's no shocker that the 4.8is doesn't pitch and roll to excess and pulls 0.83 g on the skidpad. The surprise was ride quality that is less punishing than we had anticipated on Michigan's awful roads.

Residing behind the wheels are the same 14.0-inch-front and 12.8-inch-rear brake rotors from the 4.6is, good for a sports-car-like 168-foot stop from 70 mph. These huge brakes trade performance for comfort, which amounts to extremely grabby behavior around town. The first half-inch of brake-pedal travel gets you exactly nothing, after which the binders bite down, causing immediate and sudden deceleration. The touchy brakes combined with the transmission's premature downshifting make smooth stopping nearly impossible.

The only performance category where the 4.8is didn't impress involved its top speed. BMW claims an unrestricted 153 mph, but our test car was slipped a 128-mph governor for some reason.

Although the price of the quickest X5 is up, now $70,495 for a 4.8is ($3000 more than an '03 4.6is), the performance is considerably better, and settling for a second-place finish to the Cayenne Turbo saves you $19,170. That's not a bad trade-off.

by Dave Vanderwerp
Car and Driver
November 2004