We like
Quicker than the 4.4i, a more rewarding drive, heaps of equipment

Room for improvement
Thirstier than the 4.4i, some losses in ride quality, pricey

BMW turned the 4WD market on its head, both here and abroad, when it released the X5 in 2000. Never before had a vehicle of this type been offered with such dynamic, car-like abilities, particularly in terms of performance, ride and handling, and especially given a kerb weight of about 2.2 tonnes.

Instead of trying to challenge the accepted off-road class leaders like LandCruiser and Range Rover, BMW opted for the soft-roader approach when it designed the X5, with a focus skewed heavily towards the on-road driving experience rather than traditional off-road, mud-plugging abilities.

The result is a vehicle that combines all that is good about a 5-Series Touring, with the ground clearance, high seating position and cabin space that make 4WDs so inherently practical.

With the German marque's latest X5 model, the 4.6is, the performance wick has been turned up even further to give the off-roader some serious on-road abilities.

The X5 4.4i, on which the 4.6is is based, is no slouch itself when it comes to acceleration performance - its 4.4-litre all-alloy V8 pumping out 210kW of power and 440Nm peak torque.

With the 4.6is, BMW has increased engine capacity by just over 0.2 of a litre - through a 1mm bigger bore and a 2.3mm longer stroke - as well as lifting the compression ratio slightly and modifying the breathing and engine management systems, to lift the outputs to 255kW and 480Nm.

That makes it the second most powerful engine in the BMW family, outdoing even the latest M3 model in straight power and torque terms, while bowing only to the 294kW 4.9-litre powerplant in the M5 sedan.

It is enough power, in fact, to enable the 4.6is to light up all four wheels from take-off on dirt, simply by stomping on the loud pedal.

It's also good enough to push the weighty X5 to 100km/h from standstill in just 6.5 seconds, a domain that has recently belonged to the local V8 sports sedans from Ford and Holden, as well as cars like Nissan's 200SX turbo terror.

But 6.5 seconds is the manufacturer's claimed acceleration figure and clearly a tad ambitious in the real world. Independent tests in this country have been consistently in the region of 7.5 seconds - still extremely quick for a 4WD mind you.

The engine specs for the 4.6is put it on a par with its main rival, the ML55 AMG from fellow German manufacturer Mercedes-Benz. The ML puts out the same 255kW but from a larger capacity 5.4-litre powerplant, however there's also more torque on tap - 30Nm more at 510Nm.

Regardless, from the figures it is easy to see the 4.6-litre unit is no ordinary engine, certainly not one you would normally find beneath the bonnet of a humble off-roader..

There is a slight lumpiness to the engine on cold start-up, just like many true performance engines of years gone by, but it settles into a smooth and refined idle once warm.

The 4.6-litre powerplant is extremely free-revving and just as happy bouncing off the rev limiter as it is pulling cleanly from the lower reaches of the rev range. The real power starts to come on at about 3000rpm, while by 4000rpm the engine note has taken on a VANOS (variable valve timing) induced hard edged growl, as it opens up, breathes better and starts to really pull hard.

And when the throttle is nudging the firewall, the 4.6 engine sounds more like a highly-tuned sports sedan, or even a race car, than it does a 4x4 workhorse.

The basic X5 suspension design remains unchanged, although sports settings have been applied to cope with the extra engine performance. Other changes include an upgraded braking system, similar to that on the M5 and the new 7 Series.

The handling is much the same as the 4.4i - which is exceptional by 4WD standards anyway - but it has also been tweaked for even better on-road responses. Body roll is kept to a minimum while the vehicle's limits, in terms of tyre squeal and understeer, are much higher than you would expect.

Despite its more dynamic qualities - engine, suspension, brakes and wheels/tyres - the 4.6is is still no sports car and when it comes to throwing it around the suburban streets, can feel quite cumbersome. There is no hiding the fact it is a big, heavy car, regardless of how many performance modifications it has over its lesser-equipped X5 stablemates.

On the open road the opposite is true, as it can be hustled along like no other four-wheel drive currently on the market. It points well, tracks truly and is remarkably nimble for something this size. It also comfortably tackles corners at well above the suggested speed limit, which is quite amazing given its weight, high centre of gravity and overriding 4WD design.

The suspension is not overly compliant when it comes to ride quality, but still fares better than any vehicle wearing monstrous 20-inch wheels and ultra low profile tyres (40 series front and 35 series rear) has any right to.

Naturally, it is not as smooth as its lesser tyred 3.0i and 4.4i siblings, but still manages to soak up all but the sharpest of road hazards with aplomb, leaving the occupants to enjoy the superb sounding stereo system and climate controlled, leather-clad interior.

Put 20-inch wheels on anything else and you could almost guarantee it would shake your fillings loose.

The cost of replacing those enormous tyres, while still expensive, is not as outrageous as might be expected - around $650 for the front tyres and $675 for those at the rear. These prices are unlikely to cause an owner to blink when it comes time to replace them, especially one who is prepared to hand over $150,000 for the privilege of having the latest model to impress the neighbours.

Despite the asking price, the ultimate X5 continues BMW's tradition of having an extensive options list. It is still possible to load up the 4.6is with over $11,000 worth of extras, covering such items as double-glazed side windows, electric rear seat backrest adjustment and rear seat heating, an extendible load floor and a mobile phone package.

The standard equipment list covers almost everything else you could want in a motor vehicle, including satellite navigation, television, sunroof and 10 airbags, as well as the increasingly familiar catalogue of electronic driver aids. For this X5 it includes ABS, ADB-X, ASC-X, CBC, DBC, DSC, HDC and PDC.

In English that's anti-lock brakes, automatic differential brake, automatic stability control and traction, cornering brake control, dynamic brake control, dynamic stability control, hill descent control and park distance control - enough techno-wizardry to help the handbook blow out to encyclopaedia-like proportions.

If you're wondering what the X5 4.6is is like off-road, you probably need to ask yourself this question: would you honestly take a $150,000 vehicle with 20-inch wheels and low profile road tyres for a session in the bush?

Not likely, which is just the conclusion we came to. This performance-oriented X5 will spend most of its time on the bitumen - a dirt road to the beach house or hobby farm is about as adventurous as it is likely to get.

Change the tyres to an off-road style tread pattern and it will no doubt go just as far as any other X5, which is further than some but not as far as most true off-roaders.

Having said all that, everything is relative. Lined up against an M5 or even the 540i, the accolades levelled at the 4.6is here are hardly likely to be as free-flowing, but for its market segment and against its competitors, the 4.6is exists on another plane altogether.

The real reason for this car's existence has got to be that it is simply a fun machine to drive. It ups the dynamic abilities of the already competent 4.4i, which makes it even more rewarding for the driver.

Without having driven the 4.6is, it would be easy to question why BMW felt the need to build a car like this: what call there was for the company to combine M-level performance with a four-wheel drive, even one as good as the X5.

But once you have driven it, it becomes much clearer.