We've had almost a year to get used to the idea of a hatchback hitting 62mph in under four seconds and nudging 170mph, but it still sounds bloody mad, doesn't it? As does the near £51k price tag required to put a Mercedes-AMG A45 S on the driveway. Thanks to the prevalence of the lesser AMG-Line trim, it could easily be mistaken for a car worth half its asking price - and that's a problem when you're trying to convince potential buyers that an Affalterbach engine and chassis are worth the premium. And that's a shame. Because it would be nice if we all owned a new A45 S.
For a start, it feels like an AMG inside. The latest A-Class is impressive anyway thanks to its widescreen slab of an infotainment system and instrument cluster that migrates into the five-door hatchback from larger models, but the A45 adds all the right tinsel - not least the well-bolstered leather seats and Alcantara-wrapped AMG steering wheel. Sure, there are some scratchy plastics on the B-pillar trim and lower transmission tunnel but they don't significantly detract from the model's range-topping status. And that's before you fire up its party piece.
If the M139 needs further introduction it's only to reiterate that it bears no similarities with the 2.0-litre engine fitted to the Mercedes-AMG A35. That motor is white bread. Very decent, does-its-job white bread. But filler nonetheless. The M139 is more like a Perigord truffle: rarified, small and apparently made by God Himself. How else to explain the gathering muscle of the low end or the explosiveness beyond? All four-cylinder engines want to be mistaken for something bigger capacity but the M139 actually follows through with the sort of elastic response that makes it feel fast from virtually any start point.
It's so rapid that it makes the eight-speed dual clutch auto seem like it's got shortened ratios. They're rattled off in such short order that it makes the downshifts seem a little hesitant. Hesitant for a tarmac rally car, that is. Elsewhere the frenzy is matched with a pyrotechnic soundtrack, the high-pitched four-pot backed by exhaust crackles and snorts to confirm that you're really on it.
The M139's partner in crime is the AMG Torque Control driveline, which uses a rear differential with clutch packs for each wheel. The 4Matic+ system is fully variable, so it can send a majority of the motor's torque rearwards and then split it depending on your inputs, giving the car rear-drive-like traits backed by all-wheel drive security. Over the crest and bumps of South Wales, the A45 S does a brilliant job of keeping all four wheels pinned no matter the mode (although obviously Sport+ does it best), leaving you and the torque split to determine how to attack each corner. In its stiffer damper setting the A45 S turns in eagerly - so eagerly in fact that even in the damp it's possible to really load up the outside edges of the Michelin Pilot 4Ss, just before you default to the accelerator pedal to determine which line you're about to take.
You can achieve several degrees of mid-corner rotation without ever encountering the unnerving rear-drive sensation that overcorrection is about fire you into the opposite ditch. The corrections can be small and neat, so long as you stay on throttle and allow the front wheels to help haul you forwards and back into line. Of course, for a 'straighter' fast line all the way through you can turn in off-throttle and rely the A45's initial safety understeer, before squeezing back onto the power when the car's settled, a tactic that suited the A45's predecessor best. But the idea of rewarding anyone still committed after turn-in is all-new, multi-dimensional and extremely moreish.
If there's a complaint, it's that the steering system itself still lacks feel, and must be supplemented by the feedback arriving in the seatbacks. Fortunately, the architecture is stiff enough to do the channeling even if it does result in undoubted firmness at low speeds. Above them the chassis possesses no small amount of composure and is quick enough in its responses that it does not require an overly fast steering rack or all-wheel steering hardware.
It's possible of course that the conditions in Wales this past week somewhat flatter the A45; after all, on damp roads the car's limits are made to seem more approachable - while highlighting just how quick it is in all weathers. But that's finding fault where arguably none exists. Ultimately, AMG has plumbed an additional strata into the A45's handling character - and that is to be celebrated no matter where it crops up in the overall experience. How good is it in the final analysis? Well, no-one is going to suggest that its maker has replicated the kind of fingertip purity you get from an Alpine A110 or Porsche 718 - but the A45 is quicker in a straight line and no less amusing when you really set your mind to it. For a car with back doors and a hatchback it speaks to its exceptionalism. Which is something else we're going to have to get used to...
SPECIFICATION - MERCEDES-AMG A45 S
Engine: 1,991cc, four-cyl turbo
Transmission: 8-speed dual-clutch auto, all-wheel drive
Power (hp): 421@6,750rpm
Torque (lb ft): 369@5,000-5,250rpm
0-62mph: 3.9 seconds
Top speed: 168mph
CO2: from 189g/km
Photos | Stan Papior