What is the difference between "twin turbo" and "bi-turbo"?
These terms essentially mean the same thing. When a car is labelled "twin turbo" or "bi-turbo", it means the engine has two turbochargers.
Different car manufacturers have different preferences for the use of terminology. Usually, two turbochargers are used in V6, V8 or V12 engines. Using one turbocharger for each bank of cylinders has a number of packaging and efficiency advantages.
A single turbocharger is sufficient for most four-cylinder engines. At times, it is used in V6 and inline-6 engines too.
Another term that you may have come across is "twin-scroll turbocharger". Now, this says nothing about the number of turbos used. Instead, it describes the type of turbocharger.
The "scroll" is a spiral path that the exhaust gases take when they enter the turbine section of a turbocharger.
In order to separate the exhaust pulses to prevent gas-flow interference, the exhaust tracts of consecutively firing cylinders are separated all the way to the mouth of the turbine.
Continuing into the turbo, the exhaust gases continue to maintain separate channels by entering two separate scrolls - hence the term "twin-scroll" - within the body of the turbocharger unit.
Twin-scroll turbochargers offer higher levels of gas-flow efficiency, reduced turbo lag and allow engines to be tuned for slightly more power than the single-scroll variety. Twin-scroll turbochargers are used in many of the latest engines.
So remember, twin-turbo and bi-turbo mean the same thing, but twin-scroll turbo is another thing.