Forced Air Induction
The Garret Aviation VNT-25 The idea of forced air induction by turbine, or
turbo, is not new and has it\'s mass production roots in WWII fighter planes.

What is new, however, is its application to passenger automobiles. Unlike a near
constant high RPM fighter engine, an automobile requires wide-open throttle (WOT)
power availability throughout its entire operating range. Previous automotive
turbo applications acted like an on-off power switch with a five second delay,
decreasing drivability, rather than providing the smooth linear powerband of a
normally aspirated engine. Because the turbine is in a fixed position in the
exhaust stream, it was plagued with sometimes uncontrolled production from the
compressor at high engine speeds, commonly referred to as boost creep, and a
significant decrease fuel economy versus a similar, but naturally aspirated
engine. The Garret Aviation produced VNT-25 solved all of these problems with
its innovative Variable Nozzle Turbine. Hands down it is the most advanced turbo
ever mass-produced and it was the first of its kind on production cars. One of
the most talked about problems with turbo charged engines is the lengthy time it
takes for the turbo itself to accelerate to operational speeds. This is commonly
referred to as turbo lag or turbo spool up time. Under WOT, turbo lag results in
a seemingly underpowered engine that suddenly comes to life as a delayed tire
melting rush of acceleration. Previously, turbo lag was limited by decreasing
the size of the turbo itself. This resulted in lower rotating mass and more
importantly, a smaller cross sectional area, which accelerated exhaust gasses at
lower engine speeds. Although the turbo is able to spool quicker due to its
size, for the same reason its ability to move and compress large amounts of air
efficiently is significantly reduced. Inherently a smaller turbo will produce
less maximum horsepower than if it were replaced by larger turbo on the same
engine. Previous turbochargers also used a fixed position turbine that powered
the centrifugal compressor directly. Because the turbine is located directly in
the exhaust stream, the turbine is a huge exhaust restriction. This restriction
creates a constant exhaust backpressure that decreases fuel economy even when
the turbo is not in use. At high engine speeds, the restriction creates enough
pressure in front of the turbine (back pressure) that the wastegate can no
longer limit turbine power by bypassing the exhaust around the turbine. The
result is that turbo compresses more air into the engine than is wanted. For
example, a turbo was set to produce a maximum 12psi boost pressure, but during a
period of sustained wide open throttle high engine speeds the turbo is now
producing 14.5psi of boost and still rising. This unwanted phenomenon is called
boost creep. The VNT-25 solves all of these problems with an innovative turbine
called a Variable Nozzle Turbine. Rather than a fixed turbine the VNT-25 uses a
ring of 12 moveable paddles aligned around a central, but very small turbine
wheel. The entire exhaust charge is then directed to the small turbine by the
paddles. Moving the paddles varied the crossectional area that the exhaust must
pass through. When the paddles are nearly closed the exhaust is accelerated
towards the turbine wheel to increase power. Decreasing the crossectional area
of flow accelerates normally slow, low engine speed, gasses and nearly
eliminates turbo lag while allowing a large and efficient compressor wheel for
excellent maximum engine power. Opening the paddles allowed the exhaust to flow
slower and bypass the turbine to limit power. This unique arrangement
significantly reduced backpressure, greatly improved fuel economy, and allows
excellent control turbine power at sustained high engine speed, without the use
of a bulky external wastegate. The Garret VNT isn\'t without its drawbacks. In
high performance applications it is a turbo that has little to be desired. The
engineers of this turbo, in their effort to reduce turbo lag as much as
possible, kept the compressor and turbine as small as possible. The smaller size
of the turbine and the compressor decreases the size and therefore the weight of
the turbo internals. Keeping the weight as light as possible reduces rotational
inertia to an absolute minimum, which results in a much more responsive turbo.

Because the exducer, that is the compressor, is of a compressor type,
operational speeds are very high. It is not unlikely for a VNT to reach maximum
operational speeds of 173 thousand revolutions per minute even though resting or
"cruise" speed of the turbine is only 2000-6000 RPMs. It is this
latency of the turbo to accelerate to operating speeds that is referred to as
turbo lag. Although