Porsche 911 Turbo User Handbook Manual page 15

Variable Turbine Geometry.
Advancing the turbo concept.
Turbocharger with Variable Turbine Geometry (VTG)
The turbocharger has undergone The turbo dilemma.
a continuous evolution in the
hands of Porsche engineers. It's well understood that a smaller
Variations in turbine size, the turbine will generally reach
addition of the intercooler, the optimal speed more quickly than
move to twin turbochargers and a larger, heavier turbine. But as
advances in engine-management engine revolutions continue to
systems have all extracted climb, exhaust flow tends to
greater performance from the overwhelm the smaller turbine;
basic concept of an exhaust- the resulting back pressure robs
driven compressor. In the the engine of power at high rpm.
911 Turbo, Variable Turbine
Geometry (VTG) provides a Larger turbo units have the
revolutionary step forward. opposite tendencies. They work
well at mid- to high-range engine
speeds, but it takes them longer
to spin up to speed, resulting in
"turbo lag."
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A virtuoso performance. guide vanes. By changing the angle
of these vanes, the system can
The 911 Turbo offers a replicate the advantages of a small
revolutionary solution to this turbo at low rpm, and a larger
dilemma. Its twin intercooled turbo as the tachometer climbs.
turbochargers feature the
application of Variable Turbine While this technology has been
Geometry. With Variable Turbine used in diesel engines since
Geometry, the exhaust flow is 1996, Porsche uses Variable
channeled into the turbines by Turbine Geometry for the first
way of electronically adjustable time in the newest generation of
The 911 Turbo I Performance
911 Turbo. The system is capable
of handling the significantly hotter
exhaust from a gasoline engine
by using heat-resistant materials
first developed for aerospace.