Variable vane turbo chargers are a common part of the modern diesel engine and are now starting to appear on petrol engines. The main advantages with the variable vane turbo chargers are the improved throttle response also known as lag, as well as the ability to produce more torque at lower engine revs.
Turbo lag is caused when the turbo is not spinning fast enough to produce the induction pressure that is required for combustion. If the cylinders have not got enough air in them and you add too much fuel, then this can cause high emissions and often black/grey smoke is produced from the exhaust.
If a small turbo charger is fitted then the turbo lag problem can be overcome but at higher engine revs, the same problem arises with insufficient air in the cylinders.
The way the variable vane turbo charger works is to reduce the area that the exhaust gasses have to flow through. This reduced area increases the exhaust gas speed which enables the turbo to spin faster producing more air for the combustion chamber.
This end view of the turbo gives you some idea of how the principle works. The turbine that is driven by the exhaust gasses can be seen in the centre, and the paddle shaped pieces are seen surrounding the turbine, these are the veins that open and close, reducing and increasing the exhaust gas area.
As you can see the adjustable vanes are in the closed position which is restricting the exhaust exit. This restriction will increase the exhaust gasses velocity driving the turbos turbine faster.
These vanes can have their movement restricted from build ups of carbon, and because the movements are computer controlled often with the use of an electric stepper motor, engine faults can often occur.