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Twin-turbo is a type of turbo layout in which two are used to compress the intake fuel/air mixture (or intake air, in the case of a direct-injection engine). The most common layout features two identical or mirrored turbochargers in parallel, each processing half of a V engine's produced exhaust through independent piping. The two turbochargers can either be matching or different sizes.
These can be applied to any of the five types of compressor setups (which theoretically could have 15 different setups):
Parallel configurations are well suited to V6 and V8 engines since each turbocharger can be assigned to one cylinder bank, reducing the amount of exhaust piping needed. In this case, each turbocharger is fed exhaust gases by a separate exhaust manifold. For four-cylinder engines and straight-six engines, both turbochargers can be mounted to a single exhaust manifold.
The aim of using parallel twin-turbos is to reduce turbo lag by being able to use smaller turbochargers than if a single turbocharger was used for the engine. On engines with multiple cylinder banks (e.g. and ) use of parallel twin-turbos can also simplify the exhaust system.
The 1981–1994 Maserati Biturbo was the first production car to use twin-turbochargers.
The system is arranged so that a small ("primary") turbocharger is active while the engine is operating at low RPM, which reduces the boost threshold (RPM at which effective boost is provided) and turbo lag. As RPM increases, a small amount of exhaust gas is fed to the larger ("secondary") turbocharger, to bring it up to operating speed. Then at high RPM, all of the exhaust gases are directed to the secondary turbocharger, so that it can provide the boost required by the engine at high RPM.
The first production car to use sequential turbocharging was the 1986–1988 Porsche 959, which used sequential twin-turbos on its flat-six engine.
A serial turbo can also be of use to a system where the output pressure must be greater than can be provided by a single turbo, commonly called a compound twin-turbo system. In this case, multiple similarly sized turbochargers are used in sequence, but constantly operating. The first turbo boosts provides the initial compression (for example to three times the intake pressure). Subsequent turbos take the charge from the previous stage and compress it further (for example to an additional three times intake pressure, for a total boost of nine times atmospheric pressure).
A downside of staged turbocharging is that it often leads to large amounts of turbo lag, therefore it is mostly used on piston engine aircraft which usually do not need to rapidly raise and lower engine speed. (and thus where turbo lag is not a primary design consideration), and where the intake pressure is quite low due to low atmospheric pressure at altitude, requiring a very high pressure ratio. High-performance diesel engines also sometimes use this configuration, since diesel engines do not suffer from pre-ignition issues and can therefore use high boost pressures.
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