A Diesel, A Pogo Stick
by David Finlay (20 Jan 00)

How, as a car manufacturer, do you double the sales of one of your models at a stroke? The usual answer is that you don't, but Mercedes-Benz reckons it can achieve the feat with its M-Class off-roader, by the simple trick of adding a diesel engine option to a line-up which has been petrol-only since its introduction in September 1998.

Adding a diesel to the range is one sign that Mercedes is beginning to take the business of luxury 4x4s a bit more seriously. A revised interior is another, and a third is the introduction of various safety aids including - as an option - side airbags for rear seat passengers.

The diesel unit contained under the bonnet of the ML270 CDI has five cylinders, 2.7 litres, common-rail direct injection and a turbo, all of which adds up to around 160bhp and, more importantly, huge amounts of torque. Although the M-Class is large, heavy and about as aerodynamic as a slightly smoothed-off Iceland, the diesel engine propels it along agreeably smartly.

Mercedes is making a very big deal of how quiet and refined this engine is, and even goes to some length to explain how this 'noise encapsulation' has been achieved. There's a cover running underneath the engine, for example, plus sound deadening in the carpets, the transmission tunnel and the bulkhead.

All of which is very fine, except that the ML270 is actually quite noisy. Even when not being pushed it makes its presence known, and if you are accelerating hard you can not only hear all the power strokes, you can actually feel them too.

For the ML270's basic price of £28,840 you get a six-speed manual gearbox, but for an extra £1450 you can have yours fitted with an excellent five-speed automatic.

According to the official figures the automatic is actually slightly quicker (in terms of top speed and 0-60mph time) than the manual, and comparably economical. The last time anyone tried to claim this it turned out to be a simple mistake (though this was never admitted), but there's a good reason for it in the case of the M-Class, and if you are keen to find out what it is, have a look at the bottom of this page.

The interior owes a lot to Mercedes saloons, and is every bit as comfortable, thanks partly to the tremendous headroom. The rear seat can be moved back from its standard position - in fact, because of the high floor it really has to be if you want to transport four adults comfortably over long distances.

Driving an ML270 is a relaxing process both on motorways and in town, and considering its bulk it does not a bad job on country roads either, as long as they have been recently ironed. If you happen to come across anything resembling a bump, the car flounders helplessly, as if nobody had warned the shock absorbers that anything like this might happen.

The lack of damping also shows up when you go off-road. After the current fashion, the car owes a lot of its all-terrain ability to electronics, and this shows up best when you are storming up steep slopes. You simply keep the foot down and let the microchips look for grip.

Coming down, which is the difficult bit in off-roading, isn't quite so easy. Sensors let the system know when to put the brakes on, but the process takes a little time. The feeling of being alternately caught and let go, caught and let go, caught and let go all the way to the bottom of a steep and gravelly slope was none too comforting, especially when the dampers started to lose the plot as well.

I don't know if you've ever tried pogo-sticking down a 30-degree slope, but I have to say it's not something I would recommend to the frail or congenitally nervous.

In other circumstances the M-Class performs reasonably well, but it's only an adequate off-roader, and if I were ever in a situation where something more than adequacy was required I would prefer to drive something else. Better to leave this car to people who value luxury, space and good visibility over anything more vigorous.

The mystery solved

Hello there, trivia fans. The new Mercedes ML270 CDI is unusual in that it has a 0-62mph time of 11.7 seconds in manual form and 11.6s as an automatic. Whatever anyone says about automatics, they are heavier and more power-sapping than any manual, so why is the ML270 auto quicker?

The answer is that there is a limited amount of space under the M-Class bonnet, and the biggest manual transmission that will fit is less robust than the equivalent automatic. Compounding this is the fact that a manual box allows for the possibility of driver error (muffing gearchanges, basically) which an auto does not.

The 2.7-litre CDI engine produces most of its torque between 1800 and 2600rpm. In the case of the automatic it musters up a whacking 294lb ft, but across this range the manual version is electronically detuned so that it produces a lesser, though still substantial, 272lb ft. By the time the maximum power speed of 4200rpm is reached both engines are equally efficient, but the extra torque between 1800 and 2600 gives the automatic an advantage which its less efficient transmission can't quite spoil.

A torquey engine is a relaxed engine, which may be why the auto is only 0.6mpg inferior to the manual on the extra urban cycle, and actually 0.3mpg better on the urban.

None of this explains Curious Statistic no. 3, namely the fact that the auto has a slightly higher top speed (116mph versus 115). A glance at the spec sheet shows that it is marginally lower-geared than the manual - yet another example of how the ML270 is a really rather singular motor car.

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