First ride: 2019 Mercedes-AMG A45 S prototype

With a bore and stroke of 83.0mm and 92.0mm, it features twin overhead camshafts and four valves per cylinder as well as a newly developed twin-scroll turbocharger and otto particulate filter. The changes over the standard powerplant are so comprehensive it is considered by the German car maker to be an all-new engine.

In standard form, it delivers 383bhp, which is quite an achievement in itself. But with added turbocharger boost pressure and some other yet to be revealed tweaks, in the new range-topping A45S 4Matic+ its output increases to a rather exceptional 416bhp. This equates to 209bhp per litre or 46bhp more than the specific litre output of the twin-turbocharged 4.0-litre V8 engine used by AMG’s headlining GT R.

It also endows the new top-of-the-line A45 4Matic with 46bhp more than its predecessor, 12bhp more than the M2 Competition and 22bhp more than perhaps its keenest hyperhatch rival, the four-wheel drive Audi RS3.

The real secret to its performance, however, lays in the adoption of a new four-wheel drive system developed in a partnership between AMG and Austrian engineering specialist, Magna Powertrain. The active system uses electro-hydraulic actuation instead of the electro-mechanical operation of the old reactive system used by the previous A45 4Matic. It retains much of the hardware used to dole out drive to the front wheels, but it replaces the single rear differential with two separate clutches that are capable of apportioning up to 100 per cent of power not only to the rear axle but individually to each rear wheel depending on factors such as prevailing traction, throttle position, yaw angle and steering angle. Overall, it operates in a similar manner to the set-up used by the latest E63S 4Matic+.

Our very first outing in the A45 4Matic+ prototype consisted of a short journey along snow-strewn roads from a base near Arjeplog to a test track upon a frozen lake where AMG conducts much of its winter testing in northern Sweden. There, we rode shotgun around a giant skidpan and other handling courses in a brief but intensive introduction to the new AMG model.

There’s nothing too surprising about the look of the new AMG model when you get up close. Behind the unique camouflage of the prototype is a regular fourth-generation A-class bodyshell that has been modified in various areas, including the front fenders, which have been widened to accommodate a slightly wider front track than that used by the A35 4Matic. Like its lesser sibling, it also adopts a prominent wing to smooth airflow along the roof and increase downforce at the rear at speed. Other details, such as the design of the front and rear bumpers, are hidden for now.

Inside, the similarities with the A35 4Matic continue; you sit low on heavily contoured front seats peering ahead to somewhat cluttered multi-function steering wheel and a dashboard topped by a long high panel housing both digital instrument and infotainment displays that form part of Mercedes-Benz’s excellent new MBUX operating system. Renger says the benefits of the new four-wheel drive system are instantly noticeable out on the ice. “There’s clearly more traction. It is an active system; we can send more power to the rear wheels and then apportion it left and right with greater precision. You’re aware of it straight away,” he says.

Engaging the drift mode, though, requires more than a press of a button. First, the AMG test driver scrolls through the comfort, sport, sport+, slippery and individual settings on the drive mode indicator with the touch pad located between the front seats before selecting race. He then thumbs the ESP button to the off position and pulls both the left- and right-hand steering wheel shift paddles forward to select manual prior to one last pull of the right-hand shift paddle. “It’s the confirmation command. Without it, the system defaults back to the nominal race mode,” says Renger.

When the multi-stage set-up is complete the word “Drift” appears in the middle of the instrument display, and we’re off. Almost immediately, the engine is running at over 5000rpm and with a small tug of the steering wheel the rear end of the A45 4Matic swings around as if by command. Within 100 metres our sight is no longer focused through the windscreen but out the passenger side window as we speed anti-clockwise around the huge skidpan with the steering where almost on a full turn of lock.

As well as being able to send power to the rear wheels and also apportion power between the rear wheels, the new four wheel drive system is also claimed to boast faster reaction times. “There’s improved response to throttle inputs. And the shift of drive between each rear wheel is more precise. It’s not as brutal as before,” explains Renger.