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,…