Mercedes-AMG M 139: the world’s most powerful four-cylinder for the A 45

AMG - three letters forever linked with brawny V8s - also wants to be the “queen” of four-cylinder engines. The new M 139, destined for the upcoming A 45, is set to become the world’s most powerful four-cylinder unit, delivering a remarkable 421 hp in S form.

That’s especially striking when you remember this new engine still displaces just 2.0 l - in other words, it means (just) over 210 hp/l! You can call Germany’s power wars pointless, but the outcomes remain hard not to admire.

M 139: genuinely all-new

Mercedes-AMG says the M 139 is not simply an evolution of the previous M 133 used in the “45” range until now - according to AMG, only a handful of nuts and bolts carry over from the earlier unit.

To meet emissions regulations, the packaging demands of the models it will be fitted to, and the goal of combining more output with lower weight, the engine had to be comprehensively reworked.

One of the standout changes is that AMG has rotated the engine 180º around its vertical axis. As a result, the turbocharger and exhaust manifolds now sit at the rear, close to the bulkhead separating the engine bay from the cabin. Naturally, the intake system is now positioned at the front.

This layout brings multiple benefits: aerodynamically, it helps optimise the front-end design; and in terms of airflow, it not only enables more air to be captured, but also shortens the distance air must travel, creating a straighter path with fewer diversions - on both the intake and exhaust sides.

“AMG didn’t want the M 139 to replicate the typical response of a Diesel, but rather that of a naturally aspirated engine”

One turbo is enough

Another notable point is that, despite the very high specific output, there is still only a single turbocharger. It’s a twinscroll unit running 1,9 bar or 2,1 bar depending on version - 387 hp (A 45) and 421 hp (A 45 S), respectively.

As with the turbos used on Affalterbach’s V8, this new turbo uses bearings on the compressor and turbine shafts, cutting mechanical friction and ensuring it reaches its maximum speed of 169 000 rpm more quickly.

To sharpen low-end response, the turbo housing incorporates separate, parallel passages for exhaust-gas flow, and the exhaust manifolds also use divided runners. This keeps exhaust pulses separated, directing a dedicated flow towards the turbine.

The M 139 also features a new aluminium sump, a forged steel crankshaft and forged aluminium pistons - all designed to cope with a new 7200 rpm redline, with peak power arriving at 6750 rpm, 750 rpm higher than on the M 133.

A different kind of response

A major priority was throttle response, particularly the shape of the torque curve. Maximum torque now rises to 500 Nm (or 480 Nm in the base version), delivered between 5000 rpm and 5200 rpm (or 4750–5000 rpm in the base version). That’s unusually high for a turbocharged engine - the M 133 produced 475 Nm from just 2250 rpm, holding it through to 5000 rpm.

This was a deliberate decision. AMG didn’t want the M 139 to behave like a Diesel; instead, it aimed for the feel of a naturally aspirated engine. In practice, that means a character that, like a strong NA unit, encourages regular visits to higher revs, with a more free-revving nature rather than relying on the mid-range.

Even so, AMG still promises strong responsiveness across the rev range, including at the very bottom end.

Keeping the horses cool

With outputs this high - it’s the most powerful four-cylinder in the world - cooling becomes critical, not only for the engine itself but also to keep compressed intake-air temperatures in the optimal window.

The toolkit includes redesigned water and oil circuits, separate cooling systems for the cylinder head and engine block, electric water pumps, plus an additional radiator housed in the wheel arch to supplement the main front radiator.

To keep the transmission at the right operating temperature, its oil is cooled via the engine’s cooling circuit, and a heat exchanger is mounted directly on the gearbox. The engine control unit hasn’t been overlooked either: it sits within the air-filter housing and is cooled by the airflow.

Specifications

Mercedes-AMG	** M 139**
Architecture	Inline 4-cylinder
Capacity	1991 cm³
Bore x Stroke	83 mm x 92.0 mm
Power	310 kW (421 hp) at 6750 rpm (S)
285 kW (387 hp) at 6500 rpm (base)
Torque	500 Nm between 5000 rpm and 5250 rpm (S)
480 Nm between 4750 rpm and 5000 rpm (base)
Maximum engine speed	7200 rpm
Compression ratio	9.0:1
Turbocharger	Twinscroll with ball bearings for compressor and turbine
Maximum turbocharger boost	2.1 bar (S)
1.9 bar (base)
Cylinder head	Two adjustable camshafts, 16 valves, CAMTRONIC (variable adjustment for exhaust valves)
Weight	160,5 kg with fluids

We’ll see the M 139 - the most powerful production four-cylinder engine in the world - arrive first in the Mercedes-AMG A 45 and A 45 S, with everything pointing to as soon as next month, followed by the CLA and later the GLA.

As with other AMG engines, each unit is assembled by a single person. Mercedes-AMG also announced that the production line for these engines has been improved with new methods and tools, cutting build time per unit by around 20 to 25% - enabling output of 140 M 139 engines per day, across two shifts.

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