Twin-turbo refers to a turbocharged engine on which two turbochargers compress the intake charge. There are two commonly used twin turbo configurations; parallel twin-turbo, and sequential twin-turbo. A third kind of twin-turbocharging called staged turbocharging exists, but is used more in
dieselracing applications than road cars.
Parallel twin-turbo refers to a turbocharger configuration in which two identical turbochargers equally split the turbocharging duties. Each turbocharger is driven by one half of the engine's spent exhaust energy. In most applications, the compressed air from both turbos is combined in a common intake manifold, and sent to the individual cylinders. Both turbos function simultaneously, unlike sequential twin-turbos. Commonly each turbocharger is mounted to its own individual exhaust/turbo manifold, however on inline-type engines both turbochargers could be mounted to a single turbo manifold. Parallel twin turbos are usually applied to V-shaped engines where one turbo is assigned to each cylinder bank, providing packaging symmetry, and simplifying plumbing; however, it is not uncommon for a parallel set-up to be used on an inline engine.
Nissan's RB26DETT is an inline-6that uses a twin-turbo set-up, as is BMW's N54. Toyota's 1992 Supra with the 1JZ-GTE (Japan only) 6 cylinder inline engine also used this same configuration, as does Volvo's 1998-01 2.8L B6284T and 2002-06 2.9L B6294T straight-six engines in the 1998-2006 S80 T6 and 2003-05 XC90 T6, Nissan's 1990-1996 Z32 300ZX with its V6 VG30DETT. Audi's 1997-2002 S4 (B5), 1997-2004 A6, and 2003-2004 RS6 are examples of a parallel twin turbo implementation where each turbo is driven by one bank of cylinders, as the motor in each is in a V formation.
While a parallel twin-turbo set-up theoretically has less turbo lag than a single turbocharger set up, because of marginally-reduced combined inertial resistance, and often simplified exhaust plumbing, the fact that both turbos spool at more or less the same time means that there is still a noticeable bit of lag, especially in high-flow turbo/high boost applications. One way to counter this is to use a light pressure set up where the turbos are designed to output less boost but spool earlier, however, this set up sacrifices top end power. Another system would be the use of
variable geometry turbochargers, this system changes the angle of the guide vanes depending on the exhaust pressure giving the system excellent power throughout the rev range. Once used mainly in turbocharged dieselengines, Porschewas the second to use it in a mass-production gasoline-powered vehicle in 2006 with the 911 Turbo (997).
Parallel operation of the turbochargers can still be used to great effect as demonstrated by the
Bugatti Veyron; which runs four relatively small turbochargers in parallel.
Sequential twin-turbo refers to a set up in which the motor can utilize only one turbocharger for lower engine speeds, and both turbochargers at higher engine speeds. During low to mid engine speeds, when available spent exhaust energy is minimal, only one turbocharger (the primary turbocharger) is active. During this period, all of the engine's exhaust energy is directed to the primary turbocharger only, lowering the boost threshold, and increasing power output at low engine speeds. Towards the end of this cycle, the secondary turbocharger is partially activated (both compressor and turbine flow) in order to pre-spool the secondary turbocharger prior to its full utilization. Once a preset engine speed or boost pressure is attained, valves controlling compressor and turbine flow through the secondary turbocharger are opened completely. At this point the engine is functioning in a full twin-turbocharger form, providing maximum power output.
Sequential twin-turbocharger systems provide a way to decrease turbo lag without compromising ultimate boost output and engine power. Examples of cars with a sequential twin-turbo setup include the 1993-1998 Toyota Supra Turbo (JZA8x), the 1992-2002 Mazda RX-7 Turbo (FD3S), the 1994-2005 JDM Subaru Legacy RS, and the 1986-1988
Porsche 959. With recent advancements in turbocharger design, sequential twin turbo systems have fallen out of favor because they are seen as unnecessarily costly and complex.
* [http://www.autozine.org/technical_school/engine/tech_engine_3.htm#Twin-turbo Autozine Technical School: Twin-Turbo]
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