Twin-turbo or twin-scroll turbo: which is better?

On other occasions we have discussed the types and the operation of forced induction, focusing on the advantages of forcing more air into the cylinders than an engine can draw in on its own when it comes to increasing performance. The good thing about the turbocharger is that it is a very effective way to increase an engine’s power, and its use has spread widely in both petrol and diesel engines.

So much so that there are practically no new cars left with naturally aspirated engines, except small models intended for urban use where not much power is needed. Or in hybrid cars, where the electric motor can perform the functions of a turbo – on a very different scale – providing an extra boost in certain situations by adding its thrust to the combustion engine.

The downside of turbocharged cars is that this system is quite complex, as it is a mechanism capable of spinning at very high revs and requiring special care, especially regarding its lubrication. A turbocharger failure can be one of the most expensive breakdowns you may encounter: replacing a turbo on a 4-cylinder turbodiesel engine can cost between 1,000 and 1,500 euros.

Engines with more than one turbocharger

Imagine if we talk about cars with more than one turbocharger. A problem with turbocharging is “turbo lag”, or delayed turbo response, since its operation depends on the pressure of the exhaust gas flow; in other words, on the engine load. To solve this, variable geometry turbochargers, electrically assisted turbochargers, the use of more than one turbocharger, and twin-scroll turbochargers emerged.

A small turbo allows the exhaust gases to spin the turbine up quickly and the response is immediate, but it limits the possibility of increasing performance. And vice versa: a large turbo can deliver a lot of power, but it takes considerable effort to overcome the turbine’s inertia so the turbo actually “blows” properly. Combining a small or low-pressure turbo with a larger or high-pressure one is a very common solution in supercharged inline-cylinder engines.

Technically, a V-engine with a single turbo for each bank of cylinders would not have this double boosting, although in both cases they would be “bi-turbo” engines. There are also V-engines with just one turbo for both banks… or with two turbos in series working together for all cylinders. And BMW, in its 6-cylinder inline turbodiesel engine, now uses no fewer than four turbochargers, two low-pressure and two high-pressure. Mind you, we are talking about four turbos for a single bank of cylinders. The Bugatti Chiron, for example, also has 4 turbos… but in a 16-cylinder engine, which is like four 4-cylinder engines, with one turbo per bank.

The twin-scroll turbo: better response from low revs

In fact, BMW was one of the first to embrace the advantages of twin-scroll turbos, which it used in place of the twin turbo. The word “twin” can cause confusion, especially when people speak of “twin-turbo” or “twin-scroll”, since some refer to a twin-scroll turbo using either term, when a “twin-turbo” engine may have two turbos, neither of which is twin-scroll.