The variable turbine geometry of the twin water-cooled exhaust gas turbochargers goes a long way to resolving the conflict of aims of normal turbochargers. With this technology, the gas flow from the engine is channelled onto the turbines via electronically adjustable guide vanes. By changing the vane angle, the system can replicate the geometry in all types of turbo, large or small. And thus achieve the optimum gas-flow characteristics. The guide vanes are controlled by the engine management system.
This results in a high turbine speed – and therefore greater boost pressure – even at low engine rpm. With more air available, the combustion is increased, yielding better power and torque. The torque curve reaches its maximum level much sooner – and stays there. Variable turbine geometry also improves the response of the turbo engine with dynamic boost pressure development.
When the boost pressure reaches its maximum value, the guide vanes are opened further. By varying the vane angle, it is possible to achieve the required boost pressure over the entire engine speed range. So excess pressure valves are no longer required.
The fuel economy that is achieved despite the high power output is impressive. Simply efficient, just as it should be with a turbo. A 911 Turbo to be precise.