Variable Geometry Turbochargers and Twin Scroll Turbochargers each have their own unique advantages, but what if you could combine the technology together?
Borgwarner (known for TwinTurbo in Saab 9-3 TTiD) has developed a turbocharger concept where a twin scroll turbocharger has a valve which can redirect exhaust airflow into just a single scroll, or vary the amount the valve opens and allow for the gases to completely split between the two scrolls.
The disadvantage of variable geometry turbochargers is that they are too expensive for gasoline applications because the materials have to be strong enough to withstand the high exhaust temperatures resulting from gasoline combustion.
Diesel applications tend to have lower exhaust temperatures so the technology is far more common with diesel engines. By implementing a valve with the twin scroll turbo, it can act very similarly without worrying about exotic materials to make it work in gasoline engines.