Garret VNT's (was Re: Turbo vs. Supercharger.)

[drew at scirocco.cs.uoguelph.ca (Drew MacPherson)]

An interesting aside on this discussion concerning the VNT.  The following 
is from Garret's Formula SAE FAQ concerning the use of VNT in Form. SAE 
projects - note the hint that the VNT (built for primarily diesel 
applications) may not take to prolonged use on a gasoline engine.

4. What precautions should be taken when using the variable nozzle turbo?
Safety first: VNTs are designed to broaden the usable range of a
particular turbocharger. With vanes closed, the unit acts like a small
turbo, and can generate the appropriate amount of air flow very quickly.
With vanes open, the unit acts like a much larger turbo, and can generate
a higher amount of flow. Garrett VNTs are designed with a vacuum actuator
such that, if the vane actuation system were to fail, the vanes would fail
open. This prevents the turbo from overspeeding, and helps to protect the
engine from any damage. If you alter the actuation system, be sure to
allow for the vanes to go "full-open" in the event of a failure. The
second consideration is temperature: most VNTs are designed for Diesel
applications, and thus, are not capable of handling gasoline exhaust
temperatures for the long term.  If your team decides to use a VNT, and
you desire to move the vanes during engine operation, be advised that the
vanes may become sluggish above 825 C. For any other VNT specific issues,
please contact your team's Garrett support engineer.


...Drew

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