An engine of this boorish strength gives you licence to disregard the laws of physics, it seems. With a full tank of fuel, our test car weighed in at 2055kg – a touch above the claimed figure but not deceitfully so – and then dispatched 60mph from rest in just 3.7sec.
Moreover, it rattled off the 30-70mph dash in 3.3sec, some 0.2sec quicker than Audi’s considerably lighter but not much less powerful RS4 Avant could manage. The Mercedes-AMG’s engine isn’t quite as flexible, though, taking just under a second longer to dispatch the same increment while locked in fourth gear, but you’ll find keeping the tachometer needle within and indeed extending it beyond the broad 5500-6250rpm window where all 503bhp is metered out to be no chore at all.
Twin-turbocharged it may be, but this 4.0-litre V8 spins freely out to the 7000rpm redline, and how. Indeed, where this car outshines anything else you might reasonably consider a rival – except, perhaps, the Alfa Romeo Stelvio Quadrifoglio – concerns its soundtrack. With the £1000 AMG performance exhaust fitted, there is simply no respite because, even at idle, you get a swollen chug audible from some distance away.
The more theatrical inclinations of Mercedes-AMG’s engineers are particularly evident when the chrome mode switch toggle on the centre console is used to select Race. At this point, the fuelling strategy ensures upshifts are accompanied by such a deafening crack that you really do have to exercise this engine with some sympathy for your surroundings.
And so to fuelling. Naturally, that V8 is unashamed in its thirst, returning a touring economy of 26.3mpg despite the on-board computer reading 31.3mpg. With a 66-litre tank, the car’s consequent motorway range will nudge 400 miles, which is poor without being embarrassing, although owners nevertheless seem destined for crushing fuel bills. Driven in anger on track, the GLC63 S managed just 7.9mpg.
Meanwhile, the nine-speed transmission is a mixed bag. Although fast-acting and tolerant of usefully early downshifts for engine braking, it is not always smooth in normal driving and particularly at low speeds. This contributes to the car’s general lack of refinement, more on which in the following section.