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A Detailed View of the Most Powerful 4-Cylinder Engine, Mercedes-AMG's M139

The elevated level of craftsmanship and ingenuity of the Affalterbach engineers has brought us some of the most sought-after high-performance cars. With a few exceptions, they take standard Mercedes vehicles, which are already great, and make them even better by tweaking and upgrading every single part, from the engine to the bodywork.
Mercedes-AMG CLA 45 S 9 photos
Photo: Daimler AG
Mercedes-AMG A45Mercedes-AMG CLA45Mercedes-AMG M139 on CLA45Mercedes-AMG A45Mercedes-AMG A45Mercedes-AMG M139 on A45Mercedes-AMG CLA45 Shooting brakeMercedes-AMG GLA45
One great example is Mercedes-AMG’s M139 engine, which is the smallest member of the high-performance powertrain family. It is a redeveloped and improved version of the standard M260 Mercedes engine at the heart of compact models such as the A-Class, B-Cass, GLA, and CLA.

It had big shoes to fill, replacing the previous AMG four-cylinder called the M133, an engine that won the International Engine of the Year award three times.

The new engine is entirely built by hand employing AMG’s "One Man, One Engine" philosophy of having one single engineer build it from start to finish. This is done on a newly constructed, ultra-modern production line found on the first floor of the famous Affalterbach factory.

The turbocharged four-cylinder is available in two power output versions, able to produce 387 hp (285kW) and 421 hp (310 kW), respectively. The first powers “standard” AMG models while the second, which is the most powerful inline-four engine in production, is used for the top-shelf S versions.

Mercedes\-AMG M139 on CLA45
Photo: Daimler AG
It has been developed to be extremely agile and responsive and the designers carefully balanced the torque curve to achieve a power delivery similar to that of a naturally aspired engine.

The countless hours of hard work resulted in a peak torque of 500 Nm (369 ft-lb) for the S version and 480 Nm (354 ft-lb) for the basic model, which is available around 5000 rpm, while the engine’s maximum speed has been improved to 7200 rpm. This gives the small two-liter unit the agility and feel of a high-revving performance engine.

It has many improvements and innovative design features, like the new twin-scroll turbocharger that is extremely responsive at low engine speeds and provides a healthy dose of boost in the higher rpm range.

To achieve this, the turbine housing is divided into two flow passages that run parallel to each other while also receiving help from divided ducts in the exhaust manifold.

Additionally, roller bearings are fitted to the shafts of the compressor and turbine, an innovation that is borrowed from its famous AMG 4.0-liter V8 sibling. These bearings help minimize mechanical friction within the turbocharger.

To reduce friction between the pistons and cylinders, engineers opted to coat the linings using the patented NANOSLIDE technology that was pioneered on the Mercedes AMG Petronas Formula 1 engine.

Compared to its predecessor, the AMG M133, the new engine has larger exhaust valves that were made possible by repositioning and slightly angling the injection nozzles and spark plug system in the cylinder head.

Mercedes\-AMG CLA45 Shooting brake
Photo: Daimler AG
Another innovation is the two-stage fuel injection system, which is used by AMG for the first time on a four-cylinder. In the first stage, the fast and precise injectors supply fuel to the chamber at a pressure of up to 200 bar, while in the second stage, additional intake manifold injection is used aided by electronically-controlled solenoid valves.

The cooling system was also redesigned in order to handle the high-power output, so an additional radiator has been placed in the wheel arch to aid the main front-positioned unit.

The coolant flow is assisted by an electric high-performance water pump that operates independently of the engine speed. It ensures optimum heat dissipation over the entire engine speed range and also protects the engine when idling in very high ambient temperatures.

The 161 kg (355 lbs.) unit is another work of art by the Mercedes-AMG team that does a terrific job of transforming the A-Class, CLA, and GLA models into extremely fun to drive, high-performance compacts that are efficient enough for daily driving and powerful enough for a lap around a difficult racetrack.
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About the author: Vlad Radu
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Vlad's first car was custom coach built: an exotic he made out of wood, cardboard and a borrowed steering wheel at the age of five. Combining his previous experience in writing and car dealership years, his articles focus in depth on special cars of past and present times.
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