Turbo vs. Supercharger.

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

On Wed, 2 Jul 2003, Eric S wrote:

> As for wastegate control, yes, I am familiar with turbo design.  Now
> Larry, have you seen the latest VNT turbos?  No wastegate...quite
> interesting design actually...all the TDI engines in the Beetles and
> Golf/Jettas after 99.5 use a VNT-15 turbo, which is called a "variable
> vane" turbo....I'm not completely sure exactly how it works, but from
> what I have been told there is a nozzle of sorts that is constantly
> varying how much exhaust flows into the turbo.

VNT (variable nozzle turbo) has been around for a while - It was actually
available on Shelby ed.  Shadows etc. in limited numbers back in '89/90.  
The nozzle position is controlled by either manifold pressure (simplest
method) or an electronically (ECU) controlled vacuum modulator (in the
case of the TDI.)  The nozzle position is moved from the outside edge of
the impeller at low exhaust velocity or manifold pressure, to spool the
impeller quickly.  As boost builds, the nozzles are redirected towards the
inside of the impeller to slow the impeller (and regulate boost.)

The system works very well (as anyone who has driven a VNT TDI could tell 
you - well, except for Cathy - I'm pretty sure hers has taken a vacation)  
If I could find a reliable (!!!) method of regulating the nozzle, I would 
retrofit one of these on my 1.6 TD.

> Well, I'll tell you, turbo lag on these really dosen't exist...the
> VNT-15 on the TDI engine is at full boost by the time you hit 1700 rpm
> or so...basically almost right off idle.

Yes, the VNT build boost very quickly - which is why messing with it is 
rather dangerous... :)

Drew

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