Motoring: The Independent Road Test: You big smoothie

THIS IS one of those cars that always lurks in the background - discreet, low-key, slightly upper-crust. Glamorous it is not, high-muscle versions excepted. Expensive it is. Too expensive, actually, for a car seen in its thousands, painted cream and plying for hire at Germany's airports.

That's the Mercedes-Benz E-class. Look hard, and you'll see it has been very slightly facelifted. The number of changes totals about 1,800, but little is outwardly obvious beyond tidier bumpers and a lower profile to the bonnet and front wings. Mere twigs on the evolutionary tree.

Such changes won't suck Mercedes-Benz into the mainstream icon-executive territory patrolled by BMW and Audi, at least not in the UK. The E-class costs too much, and buyers feel is less dynamic to drive. But, long-term, Volkswagen is gearing up to be a full-on Mercedes rival. Then we'll see what happens to the prices. For the moment, we'll let the revised Es speak for themselves.

Are they different to drive now? Up to a point, yes. Among the changes are new engines and a new gearbox. Some people buy big Benzes with manual transmissions, though the foot-operated parking brake makes for tricky clutch control when starting on a hill. (Mercedes-Benz justifies this need for three-footed acrobatics, saying on having no handbrake frees storage space in the cabin.) Apart from the brake pantomime, manual Mercs have not been known for the sweetness of their gearshift action, either. The new gearbox has a more positive feel, with six forward ratios instead of five. Sixth is long-legged, for quieter and more frugal cruising. But no-one will discover this in the UK, because buyers here will continue to buy automatics.

The most intriguing of the new engines, and the only one to be offered here, is autobox-only. It's a 3.2-litre, straight-six-cylinder, common- rail, direct-injection turbodiesel. And it's truly terrific. Not only does this engine deliver 197bhp, a healthy figure for a three-litre petrol engine and near-miraculous for 3.2-litre diesel, but it also generates a monstrous 347lb ft of torque.

This explosion of energy is made possible by that buzz-phrase among the diesel faithful, common-rail fuelling. The fuel is bled off, under extremely precise electronic control, from a highly-pressurised reservoir which is shared (hence "common") by all the injectors along the feed-pipe, or "rail". Exactly the right amount of fuel is injected when needed, no more (to stop a sooty exhaust) and no less (so the power potential is always met).

In the E320 CDI, it's akin to driving a steam engine. Solid acceleration and ample overtaking ability are an ankle-flex away, and all that torque means it does not matters that the power fades at 4,500rpm. The transmission simply shifts up a gear and the surge is repeated, smoothly and quietly with a sophisticated six-cylinder hum and hardly any dieselly clatter. This is a quick car, as the figures above confirm. It's quicker than the E280 V6, a petrol-fuelled E-class which, unaccountably, costs more. The V6 is a rougher drive, because its automatic transmission has to shift more often at higher engine speeds. Add the diesel's big economy advantage, and it's an easy choice. The automatic gearbox has a manual override allowing you flick the selector to one side to change up, to the other to change down. And the other changes to Mercedes-Benz's most successful model? Airbags above the doors are standard. So is ESP, the Electronic Stability Program, which automatically corrects skids and slithers by applying the brakes to individual wheels as required and .COMAND (cockpit management and data system') which appears to do everything except settle domestic disputes.

The E-class is a safe, stable, tidily-handling car anyway, especially in lowered-suspension Avantgarde guise. Maybe next year ...

Specifications

Model: Mercedes-Benz E320 CDI.

Price: from pounds 31,780.

Engine: 3,226cc, six cylinders, 24 valves, 197bhp at 4,200rpm.

Transmission: five-speed automatic gearbox, rear-wheel drive.

Performance: 143mph, 0-60 in 8.1sec, 31-36mpg

Rivals

Audi A6 2.5 TDI SE: pounds 29,001. Most interesting-looking of the diesel rivals, smooth and punchy. Common-rail version comes soon.

BMW 530d SE: pounds 31,920. Comes closest to Mercedes for sheer muscle-power and refinement, thanks again to the efficiency of common-rail fuelling.

Mercedes-Benz E280: pounds 33,285. V6 petrol-engined alternative Benz costs more than E320 CDI, is less

powerful and does not offer such a smooth drive.

Volvo S80 2.5tdi: pounds 25,405: Uses the old Audi five-cylinder direct-injection engine; lowish-tech but sounds good. S80 is better to look at than to drive.