Parallel twin-turbo

Parallel twin-turbo refers to a turbocharger configuration in which two identical turbochargers equally split the turbocharging duties with both turbos functioning simultaneously. 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. 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 applied to V-shaped engines are usually mounted with one turbo assigned to each cylinder bank, providing packaging symmetry, and simplifying plumbing over a single turbo setup. When used on inline engines the parallel twin turbos are commonly applied with two smaller turbos, which can provide similar performance with less turbo lag than a single larger turbo. Some examples of parallel twin-turbo inline engines are Nissan's RB26DETT, BMW's N54 and Volvo's B6284T and B6294T. Some examples of V formation engines with parallel twin-turbos include Mitsubishi's 6G72TT, Nissan's VG30DETT. Audi's 1997-2002 S4 (B5), 1997-2005 A6, and 2003-2004 RS6.

While a parallel twin-turbo set-up theoretically has less turbo lag than a single turbocharger set up, this is not always the case due to many factors, including, marginally-reduced combined inertial resistance, and often simplified exhaust plumbing and the fact that both turbos spool at more or less the same time means that there can still be a noticeable bit of lag, especially in high-flow turbo/high boost applications. Some ways to counter this are to use a light pressure set up with smaller turbos, where the turbos are designed to output less boost but spool earlier. This set up does sacrifice some top end power however it has less lag than a similar engine with a single turbo set up making the same 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 diesel engines, Porsche and Honda were first to use it in mass-production gasoline-powered vehicles, with the 911 Turbo (997) and Acura RDX respectively, both debuted in 2005 as model year 2006 vehicles.

It is possible to use parallel operation of more than two turbochargers, such as the Bugatti EB110 and Bugatti Veyron; both of which run four turbochargers in parallel. The EB110 runs 4 turbo's on a 3.5 litre V12 engine, producing 542 hp (404 kW) at 8000 rpm. In the Veyrons case, it is a 16 cylinder engine that generates 1,001 PS