This is a classic David-versus-Goliath battle. Except, in this case it seems David — represented by a compact, low-riding Mercedes-AMG GT R — has the on-paper advantage over the gargantuan AMG GT 63 S 4-Door Coupe, the Goliath in this tussle.

We’re at the Circuit of the Americas, in Austin, Texas — where the Formula One circus will descend next weekend — and Bernd Schneider is flinging his silver GT R around the track as though it’s a qualifying session for the Deutsche Tourenwagen Masters (DTM), Germany’s premier saloon car racing category… in which he just happens to be a five-time champion.

Herr Schneider is the pacemaker for our track session in the brand-new AMG GT 63 S 4-Door Coupe, and his ultra-rapid progress is incentivising me to throw the two-tonne, five-metre-long behemoth into corners with entry speeds that shouldn’t be possible in a vehicle of its size and weight. Yet, the big Merc is soaking it all up and doing an uncannily adept job of staying within proximity of the hard-pedalling touring-car ace in the waist-high GT R.

You may be looking at the accompanying images and wondering what exactly the AMG GT 4-Door Coupe is and how it fits into a model line-up that already includes the E-Class sedan and coupe-wannabe CLS – and you’d be well justified in doing so. At face value, the Merc’s latest offering seems to be the answer to a question nobody asked.

It’s not an exercise in futility though, as the newcomer was conceived to fulfil a specific job description — going head-to-head with the Porsche Panamera Turbo — while leveraging the cachet of the two-door AMG GT that spearheads the three-pointed star’s go-faster range. How ironic, then, that the GT 4-Door doesn’t source its platform from the car whose name it borrows, but instead from the mainstream E-Class. The reason why this is the case is because the two-door GT’s aluminium spaceframe platform could not be stretched to the five-metre-plus scale of the four-door, and there’s also the fact that the former has a transaxle layout (transmission housed at the rear) that isn’t possible to package in a car with back seats and a full-size boot.

 

 

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But it wasn’t just a case of slapping a new body on the existing E-Class architecture and rolling the thing out. The GT 4-Door is the only sedan to be prepared in-house by AMG, and although the starting point was an E63, a wholesale revamp of the chassis and underfloor has been carried out to justify affixing a ‘GT’ badge on it.

There’s substantial aluminium bracing underneath to boost torsional stiffness, and the GT also gets a whole new rear subframe and bespoke front axle. Describing the magnitude of changes, AMG boss Tobias Moers says: “Even when compared with the AMG E63, we are speaking of another dimension.”

The engineering process is different, too. While the CLS went through the usual steps that any Mercedes-Benz does, the AMG GT 4-Door was designed and engineered in close cooperation between Moers and chief designer Gorden Wagener. What’s more, it’s built at AMG’s Affalterbach production facility, rather than in the Sindelfingen plant where the E-Class and CLS are put together.

Another key differentiator between the GT 63 S 4-Door and its siblings is the fact that stuffed under its angry snout is the most potent version to date of Merc’s trusty 4.0-litre twin-turbo V8. Thumping out 630bhp and a stupefying 900Nm of twist, it propels the two-tonne-plus sedan at the horizon with shock-and-awe violence.

A 0-100kph split of 3.2sec and top whack of 315kph is hardly sluggish but, more than that, it’s the huge wall of torque on tap from 2,500rpm onwards that really grabs your attention. It’s a large part of the reason why the GT 4-Door feels about 300kg lighter than it actually is.

Naturally, deploying this massive grunt without frying the tyres or making the GT 63 S an unruly monster requires a chassis that’s exceptionally well tied down. To this end, the AMG boffins have thrown every possible bit of tech at the car, including four-wheel steering, adaptive air suspension, rear-biased 4Matic+ four-wheel drive and an electromechanical differential that’s somehow able to make sense of massive torque and huge cornering loads applied simultaneously.

 

 

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The 5.5km long Circuit of the Americas (COTA for short) is not your run-of-the-mill racetrack. With 20 turns — some tight, others fast and sweeping — and 41m of elevation changes along the way, it’s a highly technical layout that punishes tyres and cruelly exposes any shortcomings in chassis design.

Up ahead, that man Schneider continues to push on at warp speed, but the GT 63 S is still hanging in there gamely. The tail gets a bit wiggly while barrelling through the tightening-radius esses (Turns 4, 5 and 6) with the stability control in ‘Sport Handling’ mode, but this setting strikes a fine balance between allowing leeway for some sideways action while still providing a safety net in case you run out of talent. The ESP works seamlessly with all the other chassis electronics so, perched behind the wheel, you’re barely aware that an arsenal of computer-controlled wizardry is working behind the scenes to help you make the fastest, safest progress.

We already know the 4.0-litre twin-turbo V8 well from other Merc-AMG models, but in the GT 63 S it’s positively monstrous. The blown V8’s massive low- and mid-range grunt rockets the big sedan out of the sequence of slower corners in the last sector of COTA, with satisfying drifts to be had if you so choose — although the car’s natural tendency is towards gentle, progressive understeer as you push towards its limits of traction and lateral adhesion.

The GT 63 S’s quad exhausts spit out the trademark AMG bellow, with an assortment of pops, crackles and fizzing when you lift off the gas. The beefed-up, wet-clutch nine-speed auto (‘Speedshift’ in AMG-speak) is a decently smooth and quick-shifting unit, but it occasionally fails to comply when you ask it to downshift manually. You can, of course, leave it to its own devices, and in Sport+ and Race modes upshifts occur later, while downshifts happen more readily. Leaving it in self-shifting mode is probably the quickest way to get around a racetrack, as the transmission’s software is generally very intuitive.

 

 

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The big carbon-ceramic stoppers (390mm diameter at the front, 360mm at the rear) also earn kudos as the brakes take an absolute hammering at COTA, especially in a heavy saloon that easily hits 250kph-plus on the front and back straights, both of which are followed by slow corners. You need to wipe off more than 150kph twice every lap, and the skill and heroics of pacemaker Schneider means I’m leaving braking until the last possible instant to stay in touch with the German ace. There’s a cool-down lap at the end of each four-lap stint, but then the cars go straight out again in the hands of other journos for more sustained punishment.

Thrashing around a racetrack is all well and good, but what you may be more interested in knowing is how the GT 4-Door fares in real-world conditions. Good news here, too, as the Merc-AMG hotshoe is adept at pulling off the Jekyll-Hyde transformation. We gleaned this during a road loop earlier in the day that saw us meander out of morning traffic in downtown Austin before tackling highways and winding country roads. Slotting the drive mode in its ‘Comfort’ setting results in the GT 4-Door morphing into an agreeable cruiser. The exhaust that’s so raucous in ‘Sport+’ and ‘Race’ modes recedes into the background, with nothing more than a muted bassy rumble permeating the cabin.

The fact the GT 63 S rolls on 20-inch rims wrapped in low-profile rubber means ride quality isn’t pillowy over sharp imperfections in the tarmac, but it’s not bone-jarring either. In the UAE, where roads are generally excellent, there should be little to complain about in the ride department.

The dash layout isn’t dissimilar to the CLS, with a full 31.2cm wide-screen digital display, doubling as the instrument cluster and the infotainment screen, with a virtual speedo that’s calibrated all the way to 360kph. Surprise-and-delight features include a Burmester surround sound system and LED cabin lighting that can serve up no less than 64 ambient colours for the interior. The turbine-mimicking air vents are also a nice touch, as are the aluminium-look highlights in the steering wheel spokes, door trims and seatbacks. The broad, elevated centre console is completely different to that of the CLS, though, and the transmission lever sits at the rear extremity of an inset section that’s modelled after a large NACA air duct.

Naturally, the GT 4-Door comes loaded with connectivity options, plus an armoury of driver-assist functions, as you’d nowadays expect in any high-end offering from Merc. You could effortlessly cross continents in the GT 63 S, as all seats — be they front or rear — are comfortable and accommodating. Incidentally, you can specify two individual buckets seats in the back or a three-seat bench — all depends on what you need. The boot is also reasonably capacious at 461 litres.

 

 

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Whether or not you warm to the GT 4-Door’s styling will be down to your individual taste. Personally, I think there’s a lot to like about its shark-like face and fastback derriere that’s characterised by slit-like taillights and a neatly integrated diffuser. What I’m not a big fan of is the slab-sided profile, which looks even more so when you see it parked next to the swoopy two-door GT R. But there’s obviously no way of hiding so much metal. This is, after all, a 5m long saloon — no matter how much it pretends to be a coupe.

Simply gluing on AMG GT badges on a rebodied E-Class and shoehorning in the uprated twin-turbo V8 would probably have been sufficient to guarantee the GT 63 S at least a modicum of sales success. It could have been a half-baked offering that merely clung to the coattails of the two-door GT.

But it’s pleasing to see Mercedes and AMG have done the job properly, engineering the car to the nth degree to ensure they’ve squeezed every last bit of performance potential out of it. Driven in anger, the GT 63 S is laugh-out-loud fun. Yet it’s also adept at doing the schlepping around town thing. It’s the real deal.