Mercedes’ M139 Inline-Four Is the Most Powerful Production 4-Cylinder Ever—But It’s Not Winning Any Fans

Mercedes-AMG’s M139 inline-four has earned bragging rights as the world’s most powerful production four-cylinder engine, but that technical triumph hasn’t translated into customer enthusiasm. The 2.0-liter turbocharged unit debuted in 2019 with up to 416 horsepower in its non-hybrid form, then received a 2023 upgrade to 469 horsepower as the M139l, powering E-Performance models such as the GLC 63. Its advanced features—including an electric turbocharger, a mix of direct and port fuel injection, and Nanoslide cylinder coating—made it a marvel of engineering.

Yet despite these innovations, the M139 has failed to resonate with Mercedes’ traditional buyers. According to an Autocar UK report citing a company insider, the engine’s lack of character—specifically, the absence of the growl and soul associated with a V8—left enthusiasts cold. Car and Driver’s 2024 test of the C63 confirmed this, describing the M139’s exhaust note as buzzy and thin, despite headline figures of 671 horsepower and 752 pound-feet of torque.

The added weight of the hybrid system further undermined performance, as evidenced in Edmunds’ comparison with the 543-horsepower BMW M3 CS, where the Mercedes suffered an 859-pound weight disadvantage. Not all reviews were negative: the M139 was praised for its charismatic engine note in the non-hybrid AMG CLA45 (Car and Driver, 2020) and the A45’s exhaust sound (Car Magazine), while Top Gear hailed it as a ‘monster of a thing’ for its technical prowess. The issue, it seems, lies not with the engine itself but with its application in models like the C63 and GLC 63.

Mercedes-AMG boss Michael Schiebe told Edmunds that stricter Euro 7 emissions regulations pose a significant hurdle, requiring substantial investment to maintain compliance. With resources better deployed elsewhere—such as the new S-Class’ Euro 7-compliant flatplane-cranked V8 (M177 Evo)—Mercedes is phasing out the M139 across many compact models. The company is shifting its focus back to inline-six and V8 powertrains, though hybridization will remain part of the strategy.