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RE: K24 and boost - more on WGFV

> Manifold boost.  Saying that the WGFV switches between vacuum 
> and boost is 
> really not correct.  When it's off, whatever vacuum is in the 
> TI goes to the 
> WG.  It really isn't that significant, if it was, you would 
> see a longer 
> spool up time for the turbo between 0 vacuum and boost.  A 
> better way of 
> thinking of the WGFV is similar to a FPRegulator.  An FPR is 
> used to regulate 
> the amount of postitive fuel pressure, but the excess needs 
> to go somewhere.  
> Same principal to the WGFV.  

While I can see your point Scott, if Audi had simply intended to vent the WG
cap somewhere when the WGFV was not applying boost you'd think they could
find a more convenient place to connect the WGFV vent than the elbow right
at the turbo inlet.  I do think that there was some intent to actually
provide a slight vacuum on top of the WG.  Since the coiled steel spring
inside the WG works in concert with the pneumatic spring controlled by the
WGFV there will still be a spring holding the WG closed even when there is a
vacuum in the upper chamber of the WG.  Assuming that the WGFV switches to
the manifold pressure as soon as WOT is detected that would mean that there
should be no worry about vacuum holding the WG open ... it is simply waiting
for the manifold pressure to come up to help hold the WG closed.  I've
considered adding a check valve and small reservoir to the line that feeds
manifold pressure to the WGFV.  Once the reservoir was charged up by
operating with boost it would provide the stiffer pneumatic spring even
before the turbo had fully spooled up during subsequent WOT events.  That is
the least of my worries right now with my MC engine :) so I have yet to test
it.  Has anyone else done something similar?

While there are similarities between the WGFV and a FPR, I don't see it as a
good analogy to what is going on with the WGFV.  In the case of the FPR
there is flow to the controlled pressure destination, never back from it.
As you rightly point out, if the WGFV simply closed off the connection to
the upper chamber of the WG then some pressure would remain ... there must
be some small amount of flow back from the WG.  In the case of the FPR it is
more like a controlled leak to the return line, where the WGFV is actually
switching what the WG is plumbed to ...

Steve Buchholz
San Jose, CA (USA)