Anybody expecting the outright performance of an 840d to underscore the car’s aggressive exterior design is inviting disappointment.

Our test car’s 0-60mph time was just a tenth short of the claimed 4.9sec, but even that is the best part of a second down on the Porsche Panamera Diesel we tested in 2017. Certainly, with ‘only’ 315bhp, there’s no risk of being thudded back into your seat by the propulsive force of this 2993cc straight-six turbodiesel. That’s the concern of the M850i, whose 3.7sec 0-62mph perches it at the sharp end of the sports car market, if not quite into the realm of bona fide supercars.

Simon Davis

Simon Davis

Road tester
Torque-rich turbodiesel engine hauls the 840d’s 1901kg chassis steeply uphill with little outward effort – indicative of strong real-world performance.

But in every other respect, this engine is worthy of praise, especially if you can accept the 8 Series as a brisk long-distance cruiser rather than an overtly expressive and exotic luxury GT in the mould of a traditional Aston Martin, Maserati or Bentley. And, although perhaps not searingly fast against the clock, it feels brisk in real-world use. That 501lb ft of peak torque briefly emerges at 1750-2250rpm, but the effect is not quite as ephemeral as those numbers suggest because, even at 4500rpm, the floor-hinged throttle pedal will still summon almost 370lb ft.

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The clandestine fast ground-covering abilities of this entry-level 8 Series can feel genuinely bemusing as a consequence, the engine’s flexibility especially evident if you’ve locked out the silky smooth automatic upshifts and are using the stubby paddles affixed to the wheel. In sixth gear, the time taken to heave from 40mph to 60mph is within touching distance of what the latest, 600bhp M5 can do –although you’ll not get the crank spinning past 5500rpm, even in manual mode.

Nevertheless, if you’ve not experienced one of BMW’s latest straight-six diesels, you’ll likely be very pleasantly surprised by the response and precision on offer. Few marques could so seamlessly integrate a diesel engine into an elegant coupé body and make it feel like it belongs there. Fewer still, if any, balance performance and fuel economy quite so ingeniously. Mega-hatch performance and a test average of almost 40mpg would be good for any car, let alone one whose dimensions are those of a four-wheeled super-yacht.

If, therefore, the question is whether diesel power is effective enough – and in possession of the requisite refinement – for a grand touring flagship, then the answer is yes. However, whether this engine captures the certain richness and character you might expect of such a car is a different matter.

The naturally aspirated V8 in a Lexus LC500 begs to differ, as does the cleverly electrified straight-six petrol in the Mercedes-AMG E53, or the lavish twin-turbo V8 in the S560 Coupé. After all, these cars are irrational purchases; for many, shoehorning in an element of objectivity, even one this well conceived, doesn’t feel quite as sweet.

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