What is it?
Mercedes’ new M-class off-roader, now equipped with a grunty V8 diesel. The new V8, launched in the E 420 CDI late last year, is actually a 4.0-litre unit, not the 4.2-litre engine the badge on the tailgate suggests.
Based around the same four-valve-per-cylinder architecture as Merc’s smaller V6 diesel, it runs a pair of turbochargers and the latest common-rail piezo injector technology for combustion efficiency.
It produces 306bhp at 3600rpm – or exactly the same output as the ML 500’s long- running three-valve-per-cylinder 5.0-litre V8 petrol engine and 82bhp more than the ML 320 CDI’s 3.0-litre V6 oil-burner.
The ML 420 CDI’s trump card, however, is its vast torque. With 516lb ft available between 2000 and 2600rpm, it is incessantly rapid.
What’s it like?
Quick. It’s the fastest accelerating M-class model save for the 6.2-litre V8 petrol ML 63 AMG. Despite tipping the scales at 2315kg, the ML 420 shoots from zero to 62mph in 6.5sec. And while it can’t quite match the ML 500’s electronically limited 149mph top speed, the ML 420 CDI’s in-gear performance is superior.
You can tell it’s a diesel, but only just. There’s no discernible clatter once the engine has reached its optimal operating temperature, just a distant thrum as you lay the throttle back. In sound and feel it behaves much like an old pushrod V8 petrol unit, albeit with far better economy. With a combined cycle return of 24.9mpg, the ML 420 CDI can’t match the ML 320 CDI’s 29.4mpg but it betters the ML 500’s 21.1mpg.
Performance and economy are helped by the standard 7G-Tronic seven-speed automatic transmission. The ’box shifts crisply and always keeps the engine’s revs plum in the optimal operating range. Despite all that torque, Mercedes says towing ability remains the same as the rest of the M-class line-up at a claimed 3500kg.
Should I buy one?
Until the new 5.5-litre V8 petrol arrives in next year’s new ML 500, the 420 will be the automatic choice for those seeking an ordinary M-class with a performance edge.
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