Twin Turbo or Biturbo engines use two turbochargers to compress the incoming air to the engine as opposed to a
single (usually larger) turbo.
While the name twin turbo may sound self explanatory an explanation is still required as there are various ways
and advantages in implementing the two turbos.
The two main methods of implementing a twin or bi turbo setup on an engine are Parallel and Sequential which will be
explained as follows.
Parallel Turbos are when the turbos are identical in size and function.
Parallel turbos are commonly found on V engines as a turbo is applied to each
bank since a V engine has separate exhaust manifolds and plumbing for both sides of the V. This application on the V engine
is what's referred to as biturbo.
The advantages here are simplicity as it would be more complicated to connect both manifolds to one turbo and a
reduction in turbo lag as the two smaller turbos would spool up faster than a single larger one.
Parallel turbos are sometimes applied to inline engines (mostly inline 6) as well. This application trades a little
complication for reduction in turbo lag as mounting two turbos to a single manifold is slightly more complicated than one.
Sequential Turbos are not identical in their function but operate as a primary and secondary turbo.
Sequential turbos can be broken down into two sub categories as (1) both turbos may be identical in size or (2) they may
be of different sizes (smaller-primary, larger-secondary) with this type being sometimes referred to as Variable Turbos.
Sequentials is generally more complicated than parallel as both turbos usually have separate plumbing to the same manifold(s)
and the ability to restrict access of the exhaust gasses to secondary or both of them.
How it works is in low RPM and engine speeds the gasses is sent only to the primary turbo which wont require much to
spool up but as the RPMs or engine speed increase a valve is opened allowing the gasses access to the secondary turbo.
When both turbos are of the same size, they both provide boost at high RPMs/engine speeds but in variable turbos the
primary turbo can be blocked leaving only the secondary to provide boost.
The main benefit of sequential/variable turbocharging is providing usable boost along a wider spread of the RPM range
especially at lower RPMs. It should be noted than variable turbos are commonly found on twin turbo diesel engines.
The plumbing after the turbos can go to one intercooler but some vehicles especially
those with parallel turbos (eg. BMW F10 M5) use 2 intercoolers.
Twin turbo, biturbo and other terms related to the use of two turbos are sometimes interchanged but I've incorporated as
much terms and names as I can in the explanation.