WITH diesels like this, why would you bother with a petrol engine?

BMW’s latest oil-powered version of its barnstorming X5 soft-roader is $500 more expensive than its petrol-engined sibling, but the buyer gets more than that in return. A lot more.

Foremost is the turbo-boosted performance. Anything approaching 500Nm of torque is impressive at the best of times, but when you combine stump-pulling torque with almost excessive frugality you begin wondering where the inevitable compromises are.

As is seemingly the case with most turbo-diesels, there don’t appear to be any. If there are, they relate to things like noise levels, and a maybe slightly messy experience at the fuel pump.

But with a new breed of turbo-diesels sweeping through the SUV market, the former seems to have been relegated to history.

The BMW X5 3.0d, most of the time, could be passed off as a regular six-cylinder petrol engine. Only when it’s idling, and only if you pay special attention, does the typical diesel clatter enter your consciousness – vaguely.

The X5 3.0d benefits from Australia’s adopting of lower sulphur content diesel fuel, explaining why BMW brought the turbo-diesel here in the first place.

The previous version produced a mere 135kW and 370Nm of torque, but the new 3.0d winds out 150kW and a quite phenomenal 480Nm.

And then there’s the economy. With a combined quoted fuel consumption figure of 9.4 litres per 100km (8.6L/100km in manual transmission form), the automatic X5 3.0d’s specifications read more like an economy car than a fuel-slurping, heavyweight 4WD.

In fact, BMW claims the consumption is lower than the previous 3.0d engine which was already impressive enough.

All this, in a big, spacious SUV that weighs comfortably more than two tonnes.

The inline six-cylinder engine is a development of the previous turbo-diesel and has a new engine management system that, among other things, provides instant starting in temperatures as low as five degrees.

Even when the temperature drops to minus 20 degrees, the BMW diesel will take only two seconds to fire up.

The common-rail injector system also operates on a higher pressure than before, improving efficiency and performance. The short-stroke engine’s capacity has been upped slightly too, from 2.926 litres to 2.993 litres via an increase in the bore measurement from 88mm to 90mm. The stroke remains the same at 84mm.

The new engine is connected to new transmissions as well – a six-speed manual or a six-speed sequential auto as seen in 5 and 7 Series BMW sedans.

And there’s more.

Where previous X5s used three differentials to deliver full-time 4WD - with a normal-conditions 62-38 per cent power split favouring the rear wheels - the latest version uses a more complex system in which electronics play an even greater role in dictating where the power should be directed.

BMW calls it xDrive, and it is in constant conversation with the vehicle’s electronics determining wheel slip, yaw rate, road speed and steering angle before deciding the best place to send motive power.

It’s able to distribute the drive so selectively that up to 100 per cent can be sent to just the rear (or front) wheels if the occasion demands.

An electronically controlled multi-plate clutch is at the heart of the system. BMW says electronics are a faster and more efficient control method and suffer less delay than the more commonly used hydraulics.

This is further backed up by the ABS sensors that detect wheel slip and enable the X5 to regulate individual wheels, or sets of wheels, by applying brakes, or limiting engine power.

Because the xDrive system taps into other dynamic aspects than the usual wheel slip sensitive systems (steering wheel angle, yaw movement), BMW says it is more able to anticipate what is about to happen and tends to be proactive rather than reactive.

The extra information also helps on the road, enabling power to be apportioned between rear or front wheels in a way that contributes to dealing with potential under or oversteer.

How does it work in reality – especially off-road?

Well, having experienced other electronically managed 4WD systems in the past, a certain amount of scepticism initially prevailed. Our expectations were coloured by experiences with the previous top of the range Toyota Prado, which tended at times to go into a sort of electronic brainstorm, scrabbling for grip where a simple, manual locking-up of the centre differential quickly restored it. One was prompted to question how clever the electronics really were.

The X5, even on steep, minimal-traction climbs where you’d have thought the lack of a dual-range transmission would itself have presented a problem, managed amazingly well, much better than any other soft-roader that comes to mind.

The apportioning of tractive power to the appropriate wheels was so smooth and seamless you’d not have guessed how busy the electronics were.

This is one SUV that commands respect, especially when its off-road credentials are further helped by a full grab-bag of electronic aids including hill descent control (the driver can point the vehicle down a steep off-road slope and proceed safely and securely without touching the brakes), traction control and an automatic differential brake.

Similarly, the X5 is fast and effective on the tarmac, revealing its 7 Series roots and once again demonstrating why it is still considered the benchmark vehicle by other SUV hopefuls. It handles and drives with the confidence of a good sedan.

The X5 is assisted by systems such as cornering brake control to keep it tracking securely if the brakes are applied in mid-corner, and stability control to deter the onset of under or oversteer through selective braking of individual wheels. The xDrive system helps here too.

The ride is characterised by a certain firmness that is more confidence-inspiring than anything else. It steers with precision and maintains a level stance when cornering, yet manages to absorb larger bumps without excessive fuss.

And the revised 3.0-litre turbo-diesel engine?

Well, it’s hard to see why it shouldn’t be the obvious engine of choice for any X5 buyer not wanting to step up to a V8.

With its astronomical torque, outstanding economy and generally non-intrusive nature it makes more sense than the virtually identically priced 3.0-litre petrol engine.

The mid-range response is almost astounding – as you’d expect with 480Nm – and the noise levels are virtually the same as any good six-cylinder petrol engine. As we said earlier, it’s only when idling that a distant diesel clatter can be heard.

Accelerative performance is roughly the same as the petrol 3.0-litre engine, but the much stronger torque makes it a better bet for towing boats or horse floats - common applications in top-end SUVs.

And there’s no turbo lag apart from a very brief hesitancy off the line. The real bonus comes at the fuel pump, where the 92-litre tank promises a spectacular cruising range.

The sequential six-speed auto is a smooth, nicely intuitive transmission that will shift of its own accord into a lower gear on downgrades if the brakes are applied, and has a reverse-action manual shift pattern in which upshifts are effected by moving the lever backwards rather than pushing it forward as in the majority of cases. It’s merely a matter of acclimatisation.

That’s the dynamic side of the X5. What about the practicality, quality and comfort?

For 2004, the X5 has been given a subtle facelift that aligns the front-end with current BMW sedan themes as expressed in the 7 Series and the new 5 Series - but it’s not so radical that it’s about to alienate existing X5 owners.

The BMW grille is bolder and the bonnet reshaped to suggest some of the heavy sculpturing seen on the 5 and 7 Series. The front airdam – always the easiest and most popular target in a mid-life re-style – is color-coded and completely new, with a "central vertical section creating individual air ducts" and a new presentation for the foglights.

The tail-lights are slightly different, with new-look lenses.

Inside, the changes are less obvious and limited basically to a new steering wheel.

The X5 package is otherwise the same that is, generous accommodation for at least four adults, leather trimmed seats - manually adjusted in base form - double-fold split rear seat and a two-piece tailgate.

The manual front seats can be a little annoying at times, particularly the stepped backrest inclination that sometimes makes it difficult to find just the right angle, but they are unquestionably comfortable and supportive.

The X5 comes with no less than 10 airbags, including side and head airbags for the front and rear seats, climate-control air-conditioning, single CD player, trip computer, a restraining net for the rear luggage compartment and a full-size spare wheel.

Alloy wheels and roof rack are standard fare too, with options including things like satellite navigation and front and rear TV monitors linked in to the central display screen to allow viewing of normally out of sight objects.

The BMW X5 3.0d walks taller than most. The crisp handling is exactly the opposite of what we once believed of large 4WDs, the space and comfort are outstandingly good and the performance and economy of the latest turbo-diesel really make a nonsense of just about any petrol engine.

Is anyone imminently about to knock the X5 off its pedestal? It doesn’t appear so.