To reduce turbo lag, some manufacturers opt for sequential twin-turbo. At low engine speed, all the limited amount of exhaust gas is directed to drive one of the turbos, leaving another idle. Therefore the first turbo can spool up more quickly. When the exhaust flow reaches sufficient amount to drive both turbos, the second turbo intervenes and helps reaching the maximum boost pressure. The switchover is implemented by a bypass valve, which is controlled by engine management system. Cars employing sequential twin-turbo include Porsche 959, Mazda RX-7 Mk3, Toyota Supra (last gen) and the 1990s Subaru Legacy.
Unfortunately, sequential twin-turbo requires very complicated connection of pipes, as both turbos have to be connected to all cylinders. This not only engages more space, but the longer pipes may offset some of the reduction in turbo lag.
As modern technology has largely reduced turbo lag, sequential twin-turbo is no longer deemed to be necessary. Today, it has disappeared from production.
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