What is it?
The Mercedes SL 400 – the new six-cylinder version of a car as grand as a modern Mercedes SL. Which may seem a conflicted concept to get your head around, even with engine downsizing so key to success for car manufacturers these days.
Mercedes introduced the current SL two years ago having fitted one of the best V8 engines ever to grace the car to the SL 500, but carried over the lesser 3.5-litre V6 from the last version largely untouched.
Now, the SL 350 becomes the SL 400 – and that aged atmospheric V6 is replaced by a brand new 3.0-litre mill with twin turbochargers and direct injection.
Predictably enough, mid-range torque is what the new engine’s got, and what the old one never really had. The entry-level SL 400 develops more than 80lb ft of extra pulling power compared to its predecessor, and makes all 354lb ft available from just 1600rpm – half the crank speed you needed to get the old V6 to hit full stride.
Mercedes’ claimed 0-62mph sprint is also improved by seven-tenths of a second, to 5.2 seconds.
What’s it like?
Fairly fast. You’d bet that in-gear performance is even more greatly transformed than that 0-62mph improvement would suggest.
Response from the pair of turbochargers is good, and the accessible swell of acceleration they create suits the swift, relaxed ground covering that SLs are built to do very well.
The new engine also works better with Mercedes’ 7G-tronic automatic gearbox than the old V6 ever did, the former still slower and more unwilling to kickdown than competitors’ transmissions.
In outright terms the SL 400 is brisk enough to pick off everyday traffic effortlessly and certainly feels athletic. It isn’t quite as quick as the hotter six-cylinder versions of the Porsche 911 or Jaguar F-Type, but you’d imagine SL owners won’t care.
But, while performance is now where it should be for the bottom-rung version of Stuttgart’s blue-blooded roadster, other facets of the driving experience still need improvement.
The first is mechanical refinement: a shortcoming you don’t expect to find in such a smooth operator as the SL. The forced V6 is ever-so-slightly thrashy and grumbly at idle, booms a little bit under load and sends just-noticeable vibrations into the cabin through parts of the rev range.
The engine offers respectable economy – 30mpg in mixed use – but where the turbo V8 in the SL 500 can top twenty-six to the gallon in similar circumstances, that’d seem a poor reason to opt for the smaller engine.
Ride and handling in the SL remains fairly unashamedly biased towards the former, largely at the expense of brilliance at the latter. The SL has a light electromechanical steering setup configured for untroubling ease-of-use rather than involvement or feel.
Fitted with the optional hydraulic Active Body Control suspension of our test car, high-speed motorway ride is excellent. The fast back-road ride is good, but feels slightly wooden on occasion.
The continually adjusting chassis rates can also filter changeable weight and directional response into the car’s steering, which can make the car tricky to guide through a fast bend. Handling is quite direct and well-balanced, but could feel more natural. Body roll is almost entirely dialled out, but off-centre lateral grip and pace in the steering sometimes comes as a surprise.
Should I buy one?
Not if you can stretch to a V8. If you are going to have an entry-level V6 SL, we’d leave well enough alone and stick with the steel coils and adaptive dampers of the car’s standard suspension.
But we’d also still urge buyers to lavish the extra £9.5k required to buy an SL 500, even after the addition of this new powerplant to the SL range. In every dimension that matters with this car – performance, refinement, richness and flexibility – the V8 is worth that premium.
Mercedes SL 400 AMG Sport
Price £72,500; 0-62mph 5.2sec; Top speed 155mph (limited); Economy 36.7mpg; CO2 178g/km; Kerb weight 1730kg; Engine V6, twin-turbo, petrol; Power 328bhp at 5250-6500rpm; Torque 354lb ft at 1600-4000rpm; Gearbox 7-speed automatic

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