Twin-Turbo

A twin-turbo engine uses two turbochargers to compress its intake air. The most common layout is parallel: two identical turbos, each fed by half of the exhaust. On a V6 or V8 each turbo can serve one cylinder bank, which cuts down on exhaust plumbing, and on a four-cylinder or straight-six both can bolt to a single manifold. The main point is to reduce turbo lag, since two small turbos spool up faster than one big one. The first production car with twin turbos was the 1981 to 1994 Maserati Biturbo.

A sequential setup uses one turbo at low engine speeds and brings in the second at higher speeds. That gets around the weakness of a big turbo, which gives little boost low in the rev range, without giving up power at the top. The first production car to use it was the 1986 to 1988 Porsche 959. At low rpm, all the exhaust from both cylinder banks went to one turbo so it would build boost quickly, and valves gradually brought the second up to speed until both worked together at high rpm.

In a series, or compound, setup the first turbo's output is compressed again by the second. If each one triples the pressure, the result is nine times atmospheric pressure. It usually brings plenty of lag, so it's mostly found on piston aircraft engines and some high-performance diesels. A twin-scroll turbo is a different thing, a single turbocharger with a divided housing.