Re: Subject: Re: 4000 MC conversion, now 20vt info

[C1J1Miller@aol.com]

In a message dated 12/31/98 11:06:22 AM Eastern Standard Time, quk@isham-
research.demon.co.uk writes:

> > The 20-valve turbo used in the new body style cars ...
>  > ... revised motronic ECU allowing overboost, ...
>  
>  Yes.  As you probably know, the RR has no connection to the wastegate
>  upper chamber - it's open to atmosphere.  I've always wondered why
>  Audi took such an apparently backwards step.

As I understand it, the 3B (which also has the open to atmosphere wg upper
chamber) uses the wgfv to provide either manifold pressure to the lower
chamber (opening the waste gate against the spring tension, the default mode)
or to provide vacuum to the lower chamber, keeping the wg closed and
increasing boost.  The stock spring tension is enough for 2.25 bar per Scott
Mockry; my thoughts are that the higher value (than the 1.8 bar stock ECU
application) is to limit waste gate "cracking".  This system also keeps boost
limited to 1.4 bar when the wgfv breaks or is unplugged.  I think it reacts
faster than the older version, where pressure is applied to the top of the
chamber.

The 3B uses an atmospheric pressure sensing system to limit boost at higher
altitudes (the thinner air means the turbo has to spin faster to produce the
same boost, thus the potential to overspin and burn up the turbo bearings at
high altitude under high boost).  This limiting of boost is done through a
sensor in the driver's footwell in USA 200q 20v cars, which sends a signal to
the motronic to limit the boost.  With the open to atmosphere wg top chamber,
at altitude there is less pressure on the diaphram than there is at sea level;
perhaps this allows the default no wgfv control mode of 1.4 bar to be reduced
slightly at altitude?
chris miller, windham nh, c1j1miller@aol.com
'91 200q ==> http://members.aol.com/c1j1miller/index.html