Sequential twin-turbo refers to a set up in which the motor utilizes one turbocharger for lower engine speeds, and a second or both turbochargers at higher engine speeds. During low to mid engine speeds, when available spent exhaust energy is minimal, only one relatively small 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, minimizing turbo lag, increasing power output at low engine speeds and providing the benefits of a small turbo. 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 (the primary turbocharger is deactivated at this point in some applications, such as the third generation Mazda RX-7). At this point the engine is functioning in a full twin-turbocharger form (or as in the RX-7 with a single large turbo), providing the benefits associated with a large turbo, including maximum power output, without the disadvantages such as increased turbo lag.
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 1986-1988 Porsche 959, the 1992-2002 Mazda RX-7 Turbo (FD3S), the 1993-1998 Toyota Supra Turbo (JZA8x), and the 1994-2005 JDM Subaru Legacy GT, GT-B, RS, RS-B & B4. With recent advancements in turbocharger design, and reductions in lag this has made possible, sequential twin turbo systems have fallen out of favor because they are seen as unnecessarily costly and complex.
Sequential twin turbo can also refer to a system where the output pressure must be much greater than atmospheric. In this case, two similarly sized turbochargers are used in sequence but with both operating all of the time. In this case the first turbo boosts pressure as much as it can (for example to three times the intake pressure) then the second turbo takes this charge and increases it further (for example to an additional three times intake pressure, for a total boost of nine times atmospheric pressure) to a pressure not possible by a single turbo. This is commonly found on piston engine aircraft which usually do not need to rapidly raise and lower engine speed (therefore turbo lag, while still present is not a problem) 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 use significantly higher boost pressure than Otto cycle engines.