In 2006, BMW introduced a new mass-produced turbocharged engine for the first time in twenty years—the N54. Three years later, they released the N55 as a successor to the N54, which offered fuel efficiency and reliability improvements over its predecessor. While the two engines have their differences, at their core they’re both three-liter, turbocharged straight-six engines, both put out over 300 horsepower, and both were used for as the powerplant for numerous BMW models. But they have something else in common—a weak point.