More on WGFV

[QSHIPQ@aol.com]

>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.

Hmm.  Not with the thinking Steve.  A couple of things...  A WGFV needs to 
vent the unused boost from the FV.  In a wet oil pcv system that has to be 
closed loop, or the EPA boys would be on the tail of audi.  Also, since the 
air that enters the WGFV is measured air (post fuel measure), it has to stay 
close loop.  That gives you a limited amount of options as to where to vent 
the unused boost.  Since the manifold is now at positive pressure (on boost - 
WGFV operating), it has to be somewhere where there is vacuum all the time.  
The ONLY place for that is between the turbo to the fuel flap.  Remember a FV 
needs a feed and a vent.  You can find this in several other places than the 
WG.  The v8 EGR comes to mind, as does the Fuel pressure regulation (feed vs 
return to tank) in 10vt cis cars.  


>  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.  

exactly.  

>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?

A stock ck valve in reverse with a tank?  I'm not sure you will find great 
results here, for those with k24 turbos, you will get surging.  If you use 
the .5-.7 * maximum boost spring theory, you will get the same results 
cheaper.  WG cracking is an issue, but can be addressed more simply with 
baseline spring pressure.

>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)

Audi uses the term "vent" when speaking of the return side of most FV 
operations.  In the 20vt WGFV for instance:
"Depending on the amound of charge pressure, the frequency valve will either 
increase the amound of charge pressure to the lower chamber of the WG to 
lower charge pressure (opening the wastegate), or ****>vent<*** charge 
pressure to the inlet side of the turbo to raise charge pressure (allowing 
the WG to remain closed)."  

Speaking of a WGFV as a "vacuum" regulator just doesn't jibe with all audis 
TSB and ST manuals regarding FV operation.  They speak of a feed source and a 
"vent", no FV exceptions (that I've found anyhow).  

HTH

Scott J