Twin-scroll turbochargers

A twin-scroll turbo is a turbo whose turbine housing has two separate passages, or scrolls, instead of one. A wall between them keeps the exhaust from two groups of cylinders apart until it reaches the turbine wheel.

It’s still one turbo. The name gets mixed up with twin-turbo, which means two whole turbos. How a turbocharger works listed twin-scroll as one of the ways engineers fight lag, and Exhaust manifolds showed the problem it solves: the pulses of exhaust from each cylinder getting in each other’s way. This lesson puts the two together.

The demo shows the exhaust side of the same four-cylinder engine as in the exhaust manifolds lesson, now with a turbo, and the throttle floored. Its ports feed the turbine below. Beside it is the torque the engine makes at each engine speed: the Mustard line is the turbo you’ve picked, and the grey line the other one.

It starts with a single-scroll turbo at 2,000 rpm. The four pipes meet in one passage before the turbine, so, as in the exhaust manifolds lesson, each cylinder’s pulse runs straight to the port of the cylinder before it in the firing order. That cylinder is in overlap, with both its valves open, so the pulse pushes burnt gas back into it. Watch the dots run back up its pipe. The pulses also blur into each other on the way to the turbine. About 10% of each cylinder is left full of burnt gas, and the turbo makes only 0.44 bar (6.4 psi) of boost, for 234 Nm (173 lb-ft).

Now press Twin scroll. Cylinders 1 and 4 share one passage, and 2 and 3 the other. Each pair fires a full turn apart, so each passage gets one sharp pulse every turn, and a pulse can never reach the cylinder in overlap, because that one is always in the other passage. Burnt gas left drops to 7%, boost rises to 0.73 bar (11 psi), and torque to 285 Nm (210 lb-ft).

Slide the engine speed up and down:

• Full boost of 1 bar (15 psi) arrives at about 2,250 rpm with twin scroll, against 2,700 rpm with one scroll.
• The torque limit. Like most modern turbo engines, this one has its torque capped by the ECU, here at 350 Nm (258 lb-ft), to protect the engine and gearbox. Twin scroll gets there at about 2,900 rpm instead of 3,300.
• Higher up, both turbos make full boost and the torque limit, so they’re the same. Twin scroll is about response low down, not more peak power.

The model is simplified, but the difference it shows is typical.

When an exhaust valve opens, the gas in the cylinder is still at several times the pressure of the air outside, and it bursts out as a sharp pulse. A turbine gets two kinds of energy from the exhaust: the steady flow of gas, and the punch of those pulses. At low revs there isn’t much flow, so the pulses are a big part of what spins the turbine.

A single scroll wastes some of that punch in two ways. The pulses spread out into a large shared passage, and they run into each other, so what reaches the turbine is smoother and weaker. Each scroll of a twin-scroll housing is narrower too, so the gas in it moves faster at low revs, much like the narrow gap the vanes make in a variable geometry turbo.

The cylinders benefit too. Less burnt gas pushed back in means more room for fresh air and a cooler charge, so the engine is less likely to knock. That lets engineers use more valve overlap, which helps the incoming air sweep out more of the burnt gas, and fire the spark a little earlier.

The trick is to group cylinders that fire evenly far apart, so each scroll gets a steady beat of pulses.

• Inline four, firing 1-3-4-2: cylinders 1 and 4 share a scroll, and 2 and 3 the other. Each scroll gets a pulse every full turn. The outer and inner pipes have to cross over, which makes the manifold a little awkward to build.
• Straight six, firing 1-5-3-6-2-4: the front three cylinders fire evenly, a third of a turn apart, and so do the back three. So the manifold just splits down the middle, front half to one scroll, back half to the other. That’s why BMW’s turbo straight sixes, such as the N55 and B58, use twin-scroll turbos.
• V8. Each bank of a road-car V8 fires unevenly, so a manifold on the outside of each bank can’t be paired neatly. BMW’s S63 V8, from the 2009 X5 M, put its twin-scroll turbos in the V between the banks, with pipes crossing over from one bank to the other so each scroll gets evenly spaced pulses.

Some designers go further. Mazda’s 2.5-litre turbo engine has valves in its manifold that narrow the passages at low revs, for extra speed and punch, and open them up higher up. And some larger turbos use a divided housing, where the two scrolls feed opposite halves of the wheel’s rim instead of sitting side by side, which does a similar job.

• A more complicated manifold. The passages have to stay separate all the way from the head to the turbine, with a divided gasket at each joint. Many modern engines cast a single-scroll manifold into the cylinder head to save weight, cost and warm-up time, which is harder to do with two separate passages.
• The wastegate. It has to let gas round the turbine from both scrolls evenly. Most use one gate that opens onto both; tuned cars with external wastegates often fit two, one per scroll.
• The top end. Two smaller passages can be a little more restrictive than one big one at very high revs. On a road car that rarely matters, but some drag-racing builds with huge turbos use an open, single-scroll housing for the most peak power.

Twin-scroll turbos suffer the same faults as any turbo, covered in How a turbocharger works and Wastegates. A few are their own:

• A cracked divider. The wall between the scrolls sits in the hottest gas, heated on both sides, and over years it can crack or burn away at its tip. The pulses can then mix again, so the turbo builds boost later, as if it had a single scroll. You often can’t see it without taking the turbo off.
• Leaking gaskets. A divided gasket that fails between the passages does the same as a cracked divider. One that fails to the outside makes the usual ticking exhaust leak, loudest from cold.
• A cracked manifold. The extra passages and the crossover pipes on a four-cylinder add more joints and thin walls to crack, especially on cars that see hard use and then sit idling.

• A bigger twin-scroll turbo is a common upgrade on cars that came with one, such as BMW straight sixes and many modern four-cylinders. Keeping two scrolls keeps most of the original’s quick response while the bigger wheels flow more at the top.
• The manifold and turbo have to match. A twin-scroll turbo on a single-passage manifold behaves like a single-scroll turbo, because the pulses have already mixed. Converting a car to twin scroll means a divided manifold and turbo together.
• A twin-scroll conversion on an engine that came with a single scroll, such as an older four-cylinder, gives quicker boost but costs more and takes more fitting than a straight swap.

Like any turbo change, these need a remap to go with them.