A Variable Geometry Turbo uses spring loaded vane tips to either expand or
contract the vane surface area that's pumping the air. As the turbo spins up
faster, the vanes creep out, pulling in even more air. At full RPM's the
turbo is sucking in as much air as a large turbo. At idle RPM's the turbo
vanes "retract" and the turbo acts like a small turbo, spinning up to speed
very quickly to eliminate "turbo lag". I had a VNT powered Shadow. I used to
enjoy watching the boost gauge. You could get 8 psi into the engine in a
heartbeat. It made the car very "skittish" as the lightest touch on the gas
pedal and the wheels were smoking rubber. A lot of practice made it so I
could use the car without melting the rubber or the clutch plate. Now from 8
psi, it moved more slowly into the 12+ range. At 15psi, the overpressure
valve would activate and would release excess pressure from the intake so you
didn't punch your pistons. The turbo was capable of 22psi without that valve
(or so I have heard). I understand Shelby's personal GLH has that unit on it
and it can handle the full boost, giving that little 2.2L engine almost 400 hp.
Variable Geometry turbos do not require wastegates and all their
pain-in-the-tush trappings. They are self governing for speed and pressure.
The pressure relief valve is only there so that they can "build down" the
engine, as the car was dangerous enough with 174hp stock. With a DOHC and a
full pressure VNT Turbo, you're talking HP into the 300+ range and that kind
of power in such a small car would kill people. So they limited boost at 15
psi with a pressure valve. But no wastgates needed. If you are really
interested in the VNT positives go to http://idt.net/~vnt4/vntrpt.html and you
will have all the information you need to know on Variable Geo Turbos...
Shaun H.
---original message---
Date: Thu, 9 Dec 1999 21:45:31 -0500
From: "nosdakota" <nosdakota@email.msn.com>
Subject: DML: Re: Twin Turbo 6er? Attn Shaun
I'm used to controlling a regular wastegate turbo but don't know anything
about the variable geometry turbos ( I believe chryco called it a vnt )
Would you be so kind to explain it to me or direct me somewhere that can?
Joe W.
87 Shelby Charger 13.9
98 Dakota 13.1
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