Bi-turbo generally applies to having one turbocharger for each bank of cylinders in a V-type engine or a horizontally opposed engine (such as in the Porsche 911). Many commonly use the term interchangeably with “twin turbo,” which technically means having two turbochargers on an inline engine.
On a bi-turbo V-type engine, both turbochargers work simultaneously. On a twin-turbo setup, both turbochargers may operate at the same time or they may be staged so that one kicks in at low engine speeds and the other comes in at higher engine speeds. One advantage to having two smaller turbos instead of one larger one is that smaller units tend to activate at lower engine speeds, so there is less lag before they force more air into the engine and increase power.
Mercedes-Benz’s 4.6-liter V-8 used in the CLS550 is an example of a bi-turbo engine, as is BMW’s twin-turbocharged 4.4-liter V-8 used in the 550i.
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