Porsche 911 Turbo User Handbook Manual page 18

Variable Turbine Geometry (VTG).
The ideal turbo for every scenario.
Turbocharger with Variable Turbine Geometry (VTG)
The 911 Turbo has always been increased. Since compression also
synonymous with performance. causes an increase in temperature,
The latest models are more the air must be cooled in a
capable than ever thanks to a device known as an 'intercooler'.
twin-turbo system with Variable With more oxygen present in
Turbine Geometry (VTG). each cylinder charge, more fuel
can be burnt yielding greater
On a conventional turbocharger, energy. Since higher exhaust
the exhaust flow drives a turbine pressures generate greater
that is connected to a compressor loads on the intake side, the intake
on the intake side. By compressing pressure must be carefully
the incoming air, the amount controlled in order to protect the
of oxygen in a given volume is engine. This 'boost pressure' is
· 42 ·
usually limited using a 'wastegate' spinning up easily to its optimum
valve that bypasses excess speed. The key disadvantage
pressure around the turbine. of using a smaller turbo is that
the back-pressure generated at
Another important factor in the higher engine speeds causes a
system is the size of the turbo- significant reduction in performance.
chargers. Since a smaller turbine Resistance is caused by the
has a lower mass, it responds smaller cross-sectional area through
more quickly to increasing pressure, which the exhaust is required
911 Turbo | Drive
to flow. Larger turbo units, which This phenomenon, known as
create lower back-pressure at 'turbo lag', means there is
higher rpm, take considerably virtually no turbocharging effect
longer to spin up under power due at lower engine speeds.
to the large cross-sectional area
and relative inertia of the heavier
turbine. Generally, this type of
turbo will only be effective in the
medium rpm range.
· 43 ·
To overcome this problem, the the optimum gas-flow characteris-
twin water-cooled turbochargers tics. The guide vanes are controlled
on the 911 Turbo models feature by the engine management system.
Variable Turbine Geometry (VTG).
With this technology, the gas-flow The result is a high turbine
from the engine is channelled speed – and therefore higher
onto the turbines via electron- boost pressure – even at low
ically adjustable guide vanes. engine rpm. With more air
By changing the vane angle, available, the combustion is
the system can replicate the increased, yielding greater power
geometry in all types of turbo, and torque. Maximum torque
large or small, and thus achieve is reached at lower rpm and is
· 44 ·
Turbocharger guide vane adjuster
retained across a wider
rev range. A full 620 Nm is
available from as low as
1,950 rpm up to 5,000 rpm.
Every throttle input is met
with exceptional response
and phenomenal acceleration.