Parallel Twin-turbo

The simplest twin-turbo arrangement is Parallel Twin-turbo. Both turbos work independently at the same time. Most twin-turbos on the market are this type.

Maserati was renowned for being the first to mass market twin-turbo, or in its own word, Biturbo. This picture shows its early 2.5-liter Biturbo V6. Each turbo was supplied by the exhaust gas from the nearby cylinder bank. Compressed fresh air from the two turbos joined in a common intake plenum and supplied all six cylinders. This simple arrangement is still being used by the majority of twin-turbo engines today.

Nissan VG30DETT engine on the last generation 300ZX had each turbo feed the opposite cylinder bank. This formed a feedback loop and automatically balance the power between the two banks. Most early twin-turbo engines, like Ferrari F40's and Lotus Esprit V8, had the same design. Modern engine management system can do the balance job by altering ignition, so the cross-feed arrangement is no longer necessary.

BMW's N54 twin-turbo straight-six has each of its turbo supplied by 3 adjacent cylinders. The compressed gas from both turbos joins and feeds all 6 cylinders. It's essentially the same as the Maserati design, just applied to straight engine.