The new in-line unit is smoother and more refined than the V6 it replaces. Barely audible at idle, revs build quickly, smoothly and linearly, with the S-Class’s new nine-speed torque converter 9G-Tronic gearbox, which operates in combination with a 2.47:1 final drive ratio, programmed to exploit the strong torque characteristics.
While lacking the electric motor assistance of the 2.9-litre, in-line six-cylinder, petrol-powered S500, speed nevertheless builds quickly with merely a distant hum from the new engine detectable from the well-isolated cabin. With a strapping 516lb ft on offer at just 1200rpm, the new 2000kg S400d 4Matic’s in-gear performance is wonderfully punchy from anything above idle through to where it peaks at 3200rpm. Mercedes-Benz quotes a 0-62mph time of 5.2sec and a (limited) 155mph top speed, with official fuel consumption and CO2 emissions of 50.5mpg and 147g/km.
Behind the wheel, it's the superb ride that sets the new S-Class apart from the upper luxury car competition. On smooth surfaces it glides along in the finest traditions of its celebrated predecessors. The electronically controlled air sprung suspension maintains a pre-set ride height and sponges away blacktop scars with authority. Even over nasty transverse ruts, our 400d 4Matic managed to maintain ironfisted composure with the sort of inherent control and sensitivity unmatched by the latest 740d and the outgoing A8 3.0 TDI.
Switch the standard Dynamic Select system into Sport mode and it distinguishes itself with outstanding road holding and, with the optional Air Body Control suspension fitted to our long wheelbase model, exceptional body control given its generous dimensions.
The steering remains extremely light and a little vague in the first few degrees off centre, but it weights up nicely and is sufficiently precise with a quarter turn of lock to engage the driver. The four-wheel-drive S400d 4Matic serves up exceptional grip and the sort of cornering tenacity to allow swift and spirited progress over more challenging roads without constant interruptions from the traction and stability control systems.
Inside, the front seats offer a tremendous comfort over longer journeys. The cabin styling has been subtly refined, with higher-grade materials within the dashboard, which remains dominated by two high definition displays for the instruments and infotainment system.
In the rear, the long wheelbase provides exceptional leg room – something that arguably matters most at this end of the luxury car spectrum.