Twin-Scroll Turbochargers

In a standard single-scroll turbocharger, all of the exhaust pulses from all cylinders are collected into one passage (scroll) that feeds the turbine wheel. The problem is that exhaust pulses can interfere with each other. On a four-cylinder engine firing 1-3-4-2, when cylinder 1 fires, its exhaust pulse can push back into cylinder 2 whose exhaust valve is about to open. This back-pressure reduces the scavenging effect and slows spool-up of the turbo.

A twin-scroll turbocharger divides the exhaust into two separate passages (scrolls) inside the turbine housing. Cylinders that would interfere with each other are separated into different scrolls. On a 4-cylinder with 1-3-4-2 firing order, cylinders 1 and 4 share one scroll, and cylinders 2 and 3 share the other. Each scroll feeds the turbine wheel from a different direction. The pulses arrive at the turbine wheel sequentially without interfering, which spins the turbine faster at lower RPM — meaning less turbo lag and better low-end torque.

A twin-scroll turbo requires a divided exhaust manifold — the two groups of cylinders must remain separate all the way to the turbine housing. A standard 4-into-1 exhaust manifold will not work. On engines with integrated exhaust manifolds (where the manifold is cast into the head), the twin-scroll division is designed into the head casting. This is one reason integrated exhaust manifolds and twin-scroll turbos go together so often.

BMW uses twin-scroll turbochargers on the B48 four-cylinder (used in 2 Series, 3 Series, X1, X3, and others) and the B58 six-cylinder (3 Series, Supra, X3 M40i, X5 40i). These are some of the most common engines in the BMW lineup, so you will see a lot of twin-scroll turbos. Hyundai and Kia use twin-scroll on some 2.0T Smartstream engines. Subaru uses a twin-scroll turbo on the FA24 engine in the 2022+ WRX. MINI (BMW) uses them across the range.

From a service perspective, twin-scroll turbos are maintained the same as single-scroll — oil changes, coolant system maintenance, and listening for shaft play or unusual noises. The divided exhaust manifold and turbine housing are the only physical differences you will notice. If a twin-scroll turbine housing cracks between the two scrolls (the dividing wall), exhaust pulses will mix and you lose the twin-scroll benefit. This shows up as increased turbo lag and sometimes a change in exhaust sound.