Alright guys, after a heated argument with a friend not too long ago, I want to put this issue to rest. Some people don't let this kind of stuff bother them. I am not that type of person. It seems like everyone has their own ideas when it comes to what these terms mean:
"They are two different terminologies for what type of turbo configuration you have in your car. Example: Mercedes bi-turbo, or the Audi S4 2.7 bi-turbo. What this means is that there is a turbo for each bank of cylinders, and they are located on opposite ends of the motor. However, in a car like the Toyota Supra, that car is a twin turbo because it is an inline 6 motor, and the turbos are located next to each other. One feeds off the other (there is a smaller turbo that spools up to create less turbo lag, at which point a larger turbo kicks in for the higher boost). From what I understand, that is the "technical terminology"
The term twin turbo has been often given to vehicles that use sequential turbos, a small one for low rpm torque, and a larger one for higher rpms and power. The biturbo system uses two equally sized turbos, one fed by cylinders 1-3, and the other by cylinders 4-6. They are not sequential or in series, but in parallel.
"Biturbo is the same as "Sequential Twin Turbo"
Basically, you have 1 smaller turbo for lower RPM range, and a larger turbo for higher range. It helps greatly reduce turbo lag due to the smaller turbo can produce boost much earlier than a super single or twin turbo setup can do. At lower RPM, the smaller turbo spools and produces boost - then when you reach a preset RPM or boost level, a staging valve opens and directs air to turbo 2(large one) and it now produces the boost for the engine. Usually, stage one turbo is turned off, or its boost is sent into the compressor intake of the stage 2 turbo to achieve a higher output pressure.
Do not confuse Biturbo / Sequential Twin Turbo with "Twin Turbo".
Twin turbo is having two EQUAL size turbo's. Each runs off its own cylinder bank in V or H type (boxer) engines, or off of a turbo manifoldon inline type engines. Both turbos share the load equally in producing boost air. (Like a dual core processor in your PC - both run at same speed, sharing the load equally) So, turbo lag still effects this type of setup, as both turbos will spool at the same time."
From Wikipedia (The Eternal Source Of Trueness and Correctness):
Twin-turbo or biturbo refers to a turbocharged engine, in which two turbochargers compress the intake charge.
So, let me just make this clear (I hope I'm not wrong, that would make me sound dumb). Twin turbo and Biturbo mean the exact same thing. They mean two turbos. There is no difference. It's marketing speak. Mercedes uses the term Biturbo because it sounds
uppity classier? BMW uses the term TwinPowerTurbo because it sounds stupid sportier. It has nothing to do with the turbos being sequential or parallel, it has nothing to do with which specific cylinders feed the turbos, it has nothing to do with the size of the turbos, it has nothing to do with where they are located. Bi=Two=Twin. That is all. End rant.