Although it seems somewhat unlikely that Mercedes-AMG’s new four-cylinder M139 engine will go on to enjoy the same hallowed status as its old M156 6.2-litre V8, it remains a remarkable piece of engineering.
In the base A45 alone (which isn’t coming to the UK), AMG has managed to extract 382bhp and 354lb ft from its 2.0 litres and four-cylinders – figures that rise to a frankly ludicrous 416bhp and 369lb ft in the range-topping A45 S model tested here. All told, that makes for a specific output of up to 209bhp per litre. A Ferrari 488 Pista’s 3.9-litre V8 manages 182bhp per litre, by contrast. Be in no doubt that the new A45’s motor is the most powerful turbo four-pot in series production.
The process of extracting such puissance from what is a fairly small engine is incredibly complex. While still mounted transversely at the car’s nose, it has been rotated 180deg so that its newly designed turbocharger and exhaust manifold are now sited rearwards and the intake system sits up front for improved airflow.
That turbocharger now has roller bearings (à la Mercedes-AMG GT 4-Door Coupé) for improved responses and an electronically controlled wastegate sharpens things even further. Cooling has been dramatically improved and the engine’s cylinder linings are coated in the same friction-reducing Nanoslide material that appears in Mercedes-AMG’s Formula 1 engines.
There’s a trick two-stage fuel injection system to help improve engine flexibility and reduce consumption and emissions, too. Meanwhile, clever calibration work enables its 369lb ft to arrive between 5000rpm and 5250rpm, with the theory being that this ‘torque shaping’ makes for a more naturally aspirated style of power delivery.
Power is directed to the road via an eight-speed dual-clutch gearbox and AMG’s 4Matic+ all-wheel drive system. This can send as much as 50% of the engine’s torque to the rear axle, where a new rear differential with two multi-disc clutches – one per wheel – can distribute the entirety of that punch as it sees fit.
This has also enabled AMG to install a Drift mode to sit alongside the myriad of other drive modes that govern the A45’s powertrain and steering response, four-wheel drive, stability control programmes and, if your car has them, adaptive dampers. Suspension is by way of MacPherson struts up front and a multi-link arrangement at the rear.