A turbocharger can significantly increase an engine’s horsepower without adding a lot of weight, which is partly why so many automakers today are adding turbos to their lineups. Turbocharging allows vehicles to use smaller engines and achieve greater fuel economy without sacrificing performance.
A turbocharger works by using exhaust gas to spin a turbine which in turn powers a pump that sucks in additional air that is directed back into the engine for an added boost. One of the biggest downsides of a turbocharger is that it can take time to wind up and apply the boost, a problem called turbo lag.
Lag problems are sometimes addressed through a twin sequential turbocharger system, in which one turbo handles low-RPM conditions and another kicks in for high-RPM conditions. In 2012, BMW introduced a tri-turbo system for the European market.
The third turbocharger awakens in the upper rev limits to achieve peak performance, but apparently three were still not enough.