Mercedes-Benz will shortly reveal a dramatically overhauled version of the S-Class for 2026 - complete with a fresh look, uprated interior tech and a new V8 with a flat-plane crankshaft.

The company said this is the "most extensive update in one generation" for its flagship saloon, which is in its seventh generation, having been launched in 1972.

More than 50% of the components have been redeveloped or re-engineered for this facelift, with the refreshed S-Class "taking a leading role in the most ambitious product launch programme in Mercedes-Benz history".

The camouflage is set to come off in the next few weeks, as prototypes of the revamped S-Class are said to be "very close to series production" - and Autocar was invited on board as road testing entered its final phase (see below). 

Full details of the changes under the skin are yet to be revealed, but Autocar has been told that the 'M176' V8 in the most potent S-Class variants will be replaced by a new M177 engine that swaps the conventional cross-plane crankshaft for a flat-plane design - like in the old AMG GT Black Series.

The 'M177' – still 4.0 litres in capacity and twin-turbocharged like its forebear – already features in various top-drawer AMG models (including the S63), as well as the Aston Martin DB12, DBX and Vantage – but the mild-hybrid S580 will be the first to use the new flat-plane-crank format.

The ultra-luxurious Maybach S580 will follow shortly after, with the same engine cranked to 603bhp to replace the V12 it currently uses, which Mercedes will retire in Europe.

There's no word on when AMG will reveal the updated S63, but it will no doubt follow suit. The engine is also set to be introduced to other top-link AMG models, including the upcoming CLE 63. 

A flat-plane crank has its pins arranged at 180deg intervals around the shaft, giving a flat end-on profile, while a traditional cross-plane V8 has its pins at 90deg intervals, giving an X-shaped profile - hence the names.

The two can be most obviously told apart by their soundtracks, with a cross-plane V8 characterised by the off-beat, low-rev 'burble' that comes with an uneven firing order, while a flat-plane crank engine has evenly spaced firing and has a distinctive higher-pitched engine note at high revs.

The move, engineers told Autocar, will help to reduce emissions, but not at the expense of power: the new engine increases output from 496bhp to 530bhp, which should help trim the S580's 0-62mph sprint time down closer to 4.0sec.

Mercedes doesn't currently offer the S580 in the UK, meaning V8 power is restricted to the top-drawer S63, but it remains to be seen whether the modifications make it more viable for import.

Autocar understands the straight-six S-Class variants – including the plug-in hybrid S580e – will continue to be sold here.

The S580e's petrol six will be boosted from 362bhp to 443bhp, with the accompanying electric motor kicking out 161bhp (up from 148bhp) to give a combined 577bhp.

Inside, meanwhile, the new S-Class will be recipient of a significantly updated version of Mercedes' MB.OS operating system, which comes with "a new service-oriented electrical and electronic architecture".

The new S-Class is set to go on sale in the UK later this year, commanding a slight premium over its predecessor, which currently starts at around £100,000.

So what's it like, then?

Passenger ride-alongs can sometimes feel slightly inconclusive from an impression-gathering perspective, but this is one of those rare cases where the back seat experience is just as important as the driving one - even more so, maybe.

Mercedes took me on a brief tour of its Sindelfingen factory in a long-wheelbase S580 with the new flat-plane-crank V8, and I'm pleased to report that playing rear gunner in one of these behemoths is still among the most enjoyably opulent experiences you can have on the road.

The standard S-Class is spacious enough, of course, but the longer version's extra 30cm of wheelbase makes the rear footwell approximately the same length as an Olympic swimming pool, and the twin reclining captain's chairs feel like they've been lifted straight out of an A380's first-class cabin – properly sumptuous.

You can really stretch out in here, and if you close your eyes, you can almost forget you're in a car at all, so impressive is the S-Class's bump isolation and noise cancelling. 

The ride is still supreme, natch, and the noise isolation highly impressive - save for when Jeeves winds up the V8 and lets it fly, whereupon you're treated to a subtle yet enticing woofle from the front end as the car surges towards motorway speeds effortlessly and with total composure.

Flat-plane crankshafts give a less muscular low-end rumble, which does deprive a V8 of some of its star character at idle and low speeds, but from where I was sitting (about three metres aft of the bell housing), it still sounded like a pretty enthusiastic unit. It emits a brutish but creamy growl that feels at once slightly out of step with the S-Class's inherent equanimity but also indicative of the sheer power it has at its disposal. Don't worry too much about it sounding like an anonymous four-cylinder. 

Local speed limits and weekday traffic precluded any attempt to reach the higher limits of the rev range, so it's hard to say whether this revamped limo will match the old GT Black Series' gravelly, brutish bark at full chat. But all indications suggest this most incongruous of combinations is as enticing as ever: sit back, relax and enjoy the glide.