Mercedes-AMG prides itself on its “one man, one engine” principle, and this continues with the newly introduced M139 engine, which will go into the upcoming W177-based A 45. Touted as the world’s most powerful turbocharged four-cylinder engine in series production, the mill replaces the M133 used in the previous A 45, and there’s some big numbers to back that claim.

In its most powerful output version, the 160.5-kg, 2.0 litre-displacement unit makes 421 PS (416 hp) at 6,750 rpm and 500 Nm of torque from 5,000 rpm. These figures are reserved for the S-model of the new A 45, which gets a higher charge pressure of 2.1 bar for the single twin-scroll turbocharger attached to it.

Meanwhile the “basic” model with its 1.9 bar, makes just 387 PS (382 hp) at 6,500 rpm and 480 Nm from 4,750 rpm. By comparison, the M133 powerplant churns out a maximum of 381 PS (375 hp) at 6,000 rpm and 475 Nm from 2,250 rpm. The new engine also runs at a slightly higher compression ratio (9.0:1) compared to its predecessor (8.6:1).

The decision to ensure maximum torque is delivered at a higher rpm was deliberate, with the company saying the “torque curve was carefully balanced” to achieve a “power delivery similar to that of a naturally aspirated engine for an even more emotive driving experience.” It adds that this allows the engine to be more free-revving, with the maximum engine speed being set at 7,200 rpm compared to the M133’s 6,700 rpm.

Mercedes-AMG is also adamant to ensure a clear line of distinction between the A 35 and its new A 45 models, with the former packing a highly-strung version of the M260 (306 PS or 302 hp, 400 Nm) that is shared with the A 250.

As such, the mounting of the M139 sees the engine being rotated 180 degrees so the turbocharger and the exhaust manifold are now positioned at the rear. This allows the intake system to be placed at the front, which provides the benefit of better air ducting with shorter distances and fewer diversions – both on the intake and exhaust side.

The turbocharger itself has a turbine housing that features two flow passages running parallel to each other, which work with divided ducts in the exhaust manifold, allowing the exhaust flow to be fed to the turbine separately. This prevents the individual cylinders from negatively influencing each other during load cycles, while roller bearings reduce friction so the turbo spools up quicker to its maximum speed of 169,000 rpm.

Other aspects of the engine include an all-aluminium crankcase, forged steel crankshaft, forged aluminium pistons (residing within Nanoslide-coated cylinders) and baffle plates for the oil sump to ensure proper lubrication during spirited driving. The injector nozzles and spark plugs have also been repositioned for large exhaust valves compared to the M133 engine.

The technical details don’t end there, as the M139 also features port and direct injection, whereby the latter is called upon during lower loads in the first stage. To hit the maximum output, intake manifold injection is called into action using solenoid valves.

With this much power, keeping the engine cool is another aspect to consider, so the M139 is paired with a second radiator in the wheel arch to supplement the main one at the front, with an electric pump to get the coolant flowing. Cooling of the transmission oil is integrated into the engine’s coolant circuit, and is assisted by a heat exchanger mounted directly on the transmission.

Mercedes-AMG also notes the four-cylinder motor’s technology package will consist of an alternator management, ECO start/stop function with fast restarting, a petrol particulate filter and gliding function – the last bit involves the car’s dual-clutch transmission, which was previously confirmed by Mercedes-AMG CEO Tobias Moers to be an eight-speed unit.

While the engine sounds tantalising, it’s the task of human beings to bring it to life. As such, the company has developed a new digitally supported process and logistical organisation when it comes to building the M139.

A dedicated assembly cart contains all the necessary tools and items to assemble an engine, with an independent power supply, cordless tools, on-board operating fluids and an integrated tablet so employees can go paperless with instructions. Each assembly technician is also followed by a driverless transport system that has the necessary components for the engine.

Time is also saved by reducing the number of test stations by grouping several individual stations into three central stations – drag torque test, leakage test for oil/coolant and fuel systems, as well as for complete engine water jacket. Of course, there’s the obligatory plaque bearing the engine maker’s name to round things off.