Normally, when one climbs behind the wheel of a car with an AMG badge, you expect certain things. A V8 soundtrack that gurgles like someone’s pouring a mixture of lava and gunpowder down a long lift shaft, for a start. Tyres that, maybe, last 100 kilometres before disappearing in a haze of smoke. Acceleration that feels like a Saturn V rocket at maximum dynamic pressure.

You don’t expect this. You don’t expect (or at least haven’t done since the late nineties) a crisp, sharp, straight-six exhaust note. You don’t expect a lightness of touch and a deftness that’s absent on the big V8 bruisers. You don’t expect a hybrid. Welcome, then, to the new AMG ‘53’.

Specifically, welcome to the new Mercedes-AMG CLS 53. This is the third-generation CLS, since the four-door coupe was first introduced in 2004, creating (so Mercedes claims, although Rover might have a thing or two to say about it were it still around) the market for swoopy, slightly-less practical four-door vehicles.

Once again, the CLS is basically an E-Class wearing a slim-fit, tailored suit rather than the saloon’s sensible slacks-and-jacket. The platform, mechanical components and most of the interior is lifted straight from the E. The engines, too? Actually, the engines are mostly new, and for the moment all are straight-sixes, as Mercedes begins to replace its ageing V6 motors. The diesel engines have already been seen in the updated S-Class, but this is our first opportunity to try the new turbocharged straight-six petrol units, all of which get 48-volt mild hybrid assist.

That makes this CLS 53 the first AMG hybrid, but don’t worry — it hasn’t become some sort of Prius alternative just yet. The idea here, as with other mild hybrids, is to provide a bit of electrical boost from an electric motor integrated into the housing of the nine-speed automatic gearbox to cover up any holes in the power delivery. That’s when you’re pressing on. When you’re slinking about, the motor can take some of the strain of running ancillary systems off the engine, improving fuel consumption and emissions. It can’t drive the CLS all by itself, though.

Even so, this is a fearsomely clever engine. It produces 435hp and 520Nm of torque even before you account for the hybrid. That adds 22hp and a seriously useful 250Nm of torque. The extra electric torque should be enough all by itself to eliminate any sense of turbo lag, but the AMG engineers have gone further than that in the pursuit of instant response. There are two turbochargers, one strapped to either side of the engine. The larger one, which works at high rpm, is a conventional compressor, driven by the gases speeding through the manifold and out, down the exhaust. The other turbo is electrically-driven, and spools up to speed almost instantaneously. As well as kicking in hard from tiny revs, it can also act as a booster pump for the bigger turbo, sucking air through an extra pipe to spin up the impeller, improving the response times of the whole engine.

The results, on paper, don’t look all that exciting — a 4.5-second 0-100kph time is perhaps not especially noteworthy, and neither is the 250kph maximum speed. Ah, but this is an engine that for all its technological advancement, needs to be driven to be enjoyed. Around town it’s refined and smooth, shoving you meaningfully in the back as you slot into spaces in the traffic or try and find an extra car length when pulling away from traffic lights, but without the bombast and fireworks of a big AMG V8. Likewise on the highway, it’s easy-going and user-friendly, always ready with a surge of power, but equally happy just loafing along at minimum effort.

Get the CLS onto a challenging road, though, and it really comes alive. The noise is not like the deep, barrel-chested rumble of an AMG V8. Instead it sounds more like a racing engine from the 1960s, almost akin to a classic Aston Martin, or an older BMW. There’s a sharp-edged rasp to the noise, underlaid by a steady six-cylinder beat. It’s a gorgeous sound and there’s little enough sense that this is somehow a ‘lesser’ engine than its V8 brothers. In fact, thanks to that amazing electric-assisted throttle response, you’re able to keep up a really good rhythm on a tight, twisting, mountain road.

That might be, slightly, conduct unbecoming of a car like the CLS, which clearly is more about cutting a dash in town, or cutting rapidly across the country in total comfort. Thanks to some AMG tweaks to the four-link front, five-link rear suspension (with standard air springs here, which are optional on the rest of the CLS range), and 4Matic four-wheel drive (which is mostly rear-wheel drive until the system senses that the front tyres need to be brought into the action) the AMG 53 feels more than up to it. It’s not perhaps quite as reactive on these mountain roads as you’d like (a compact hot hatch would be more fun, arguably), but the beautifully-weighted steering means you can tuck that nose tightly into corners and the whole car responds like the thoroughbred that it is when you bring the pace up to serious levels. And then you get to the top of the mountain, find a bigger, wider, road and complete your journey in comfort and silence. It’s some combination.

Of course, if it’s silence you want, you should possibly go for the 3.0-litre straight-six diesel in the CLS 350 d. You could also try the 3.0-litre turbo-and-hybrid CLS 450 petrol, but the refinement of the 450 is slightly disappointing, with a distinct drone at low-to-medium revs that becomes annoying after a while. The diesel six, which we’ve already experienced in the S-Class, is definitely quieter, doubtless because its 600Nm of torque means you don’t have to stretch the engine’s resources half so much. With this engine (and there seems little point, to me, in trading up to the more expensive 400 d which only has an extra 100Nm of torque) the CLS becomes a proper Grand Tourer, wafting you along in total comfort, and a silence that you would normally only associate with an electric car. Diesel has come in for a world of pain in the media for the past two years (and is not at all popular around these parts in any case), but this engine proves it can still impress, even beguile.

Do you find the styling of the new CLS beguiling, though? I’m not so sure. It’s pretty, that’s for certain, and in the matte grey finish applied to the limited-run Edition 1 version, it looks striking, but I’m not sure it looks as striking, nor as pretty, as that 2004 original. It’s become a touch more generic, picking up too many cues from other Mercedes models, not least the new A-Class. That’s true too of the interior, which is fabulous in and of itself (the big digital screens, the ‘jet-turbine’ air vents marching across the dash, the seats, the comfort, the quality), but it’s basically the same as what you get in an E-Class, so if you’re paying to trade up from the saloon, you might wonder where your money’s gone.

It has gone, of course, on high-technology, hybrid assistance, clever turbos, impressive safety systems and more besides. Certainly in AMG 53 form, this new CLS is deeply satisfying, and it might just change your expectations of what you get from an AMG.