A twin scroll turbo is a turbocharger with a turbine housing split into two separate exhaust channels, each fed by a different pair of engine cylinders. Instead of dumping all exhaust gas into one shared chamber, the divided design keeps exhaust pulses organized by firing order, spinning the turbine wheel more efficiently and reducing turbo lag. It’s one of the most common turbo designs in modern four-cylinder and six-cylinder engines.
How the Divided Housing Works
In a conventional single-scroll turbo, every cylinder’s exhaust flows into one shared passage before hitting the turbine wheel. That creates a problem: exhaust pulses from different cylinders can collide and interfere with each other inside the housing. When one cylinder is still pushing out exhaust gas while another’s pulse arrives, the energy partially cancels out. The turbine wheel receives a messy, overlapping stream instead of clean, distinct hits.
A twin scroll turbo solves this by physically dividing the exhaust manifold and turbine housing into two flow paths. Each path carries exhaust from a specific pair of cylinders, chosen so their firing events never overlap. The result is a “pulsed flow” where exhaust energy arrives in rapid, evenly spaced bursts, like alternating left-right punches on the turbine wheel. BorgWarner, one of the largest turbo manufacturers, describes it as a “one-two” alternating punch that keeps pulses in rapid succession. The divided design amplifies and arranges those pulses rather than letting them compete.
Why Cylinder Pairing Matters
The key to a twin scroll design is which cylinders share a scroll. The pairing follows the engine’s firing order so that two cylinders whose exhaust events are furthest apart in the combustion cycle feed the same channel. On a common inline four-cylinder engine with a 1-3-4-2 firing order, cylinders 1 and 4 feed one scroll while cylinders 2 and 3 feed the other. This spacing ensures that by the time one cylinder’s exhaust pulse enters the scroll, the previous pulse from the paired cylinder has already passed through and hit the turbine wheel.
If the pairing were wrong, say cylinders 1 and 2 sharing a scroll, their exhaust pulses would arrive too close together and interfere, defeating the entire purpose. On engines with more than four cylinders, the same principle applies but with different groupings. Inline-six engines split their six cylinders into two groups of three, again matched to firing order. Research on turbocharged engines with more than four cylinders shows that exhaust pulse interference in a single-scroll design “deteriorates notably,” making the twin scroll approach especially valuable as cylinder count increases.
Less Turbo Lag, Better Throttle Response
Turbo lag, the delay between pressing the accelerator and feeling boost pressure build, is largely a function of how quickly exhaust energy can spin the turbine up to speed. Because a twin scroll turbo delivers exhaust pulses as concentrated, sequential hits rather than a blended stream, it extracts more energy from each pulse at low engine speeds. The turbine wheel accelerates faster, and boost builds sooner.
This is most noticeable in everyday driving: pulling away from a stop, merging onto a highway, or accelerating out of a corner. The engine feels more responsive and less “laggy” compared to the same engine running a single-scroll turbo of equivalent size. At higher RPMs, where exhaust flow is abundant anyway, the difference narrows. But in the low-to-mid RPM range where most street driving happens, the improvement is significant.
Exhaust Scavenging and Efficiency
Beyond just spinning the turbo faster, keeping exhaust pulses separated has a second benefit: better exhaust scavenging. When a cylinder finishes its exhaust stroke, you want the spent gas to exit as completely as possible so the next intake charge of fresh air fills the cylinder fully. In a single-scroll system, pressure buildup from overlapping exhaust pulses can push back against a cylinder that’s trying to expel its gas, leaving residual exhaust behind. This back-pressure forces the engine to work harder to push exhaust out, wasting energy in what engineers call pumping losses.
In a twin scroll system, each scroll maintains lower, more consistent pressure because it only handles half the cylinders. The natural low-pressure wave that follows each exhaust pulse actually helps pull residual gas out of the next cylinder in the pair, a scavenging effect that improves cylinder filling. The result is a small but real gain in fuel efficiency and power output. Research on asymmetric twin scroll turbocharged engines has measured fuel economy improvements of roughly 2%, which adds up over thousands of miles of driving.
Twin Scroll vs. Twin Turbo
One of the most common points of confusion is the difference between a twin scroll turbo and a twin turbo setup. A twin scroll turbo is a single turbocharger with a divided housing. A twin turbo setup uses two separate turbochargers entirely. They’re fundamentally different approaches to the same goal of reducing lag and improving power delivery.
BMW’s branding has made this confusion worse. Since the N55 engine generation, BMW has labeled many of its engines “TwinPower Turbo,” which sounds like it means two turbos but actually refers to a single twin scroll turbocharger. The predecessor N54 engine was genuinely twin-turbocharged with two separate units. To add another layer, some BMW V8 engines (like the updated N63) use two turbochargers that each have twin scroll housings, combining both approaches. If you’re shopping for a BMW or reading its spec sheet, “TwinPower Turbo” almost always means a single twin scroll turbo, not two turbos.
A twin scroll setup offers similar power delivery to a twin turbo system in a smaller, lighter package. Twin turbo configurations require two of everything: two compressor housings, two wastegates, two sets of oil and coolant lines. A single twin scroll turbo achieves much of the same responsiveness with less complexity and lower weight.
Where Twin Scroll Turbos Are Used
Twin scroll turbochargers are most commonly found on inline four-cylinder and inline six-cylinder engines, where the even cylinder count divides neatly into two groups. They’re standard on many turbocharged sedans, hot hatches, and sports cars from BMW, Subaru, Hyundai, and others. The Subaru WRX, for example, switched from a single-scroll to a twin scroll turbo in its later generations, and the difference in low-end response was one of the most-discussed changes among owners.
The design does add manufacturing cost and complexity compared to a basic single-scroll turbo. The divided housing and matched exhaust manifold require tighter tolerances and more material. For very small engines or applications where cost is the top priority, a single-scroll turbo remains common. But for any performance-oriented turbocharged engine with four or more cylinders, twin scroll has become the default approach, offering a measurable improvement in response, efficiency, and how the engine feels in the real world.