[s-cars] While we are on the subject of WGFVs...

[Djdawson2 at aol.com]

In a message dated 1/11/2006 10:04:45 PM Mountain Standard Time,  
gailus at mindspring.com writes:

I've  never checked what happens when you pull off the hose
to the wastegate, so  I can't comment on that.

However, I just wanted to mention that it's  probably  more
accurate to consider the WGFV as bleeding off gas flow  from the
tap on the compressor scroll rather than bleeding off boost or  pressure.
The reason I say this is that the tap on the compressor  scroll
appears to be a nozzle with a very small orifice.
It's very  likely that this was designed to operate as a "sonic nozzle"
where the mass  flow is essentially independent of the downstream
pressure as long as the  difference in absolute pressures is more
than a critical value (somewhere  between 20 to 50% depending
on how well or how badly the nozzle was  designed).
Once this critical pressure ratio is exceeded, the flow  through
the throat of the nozzle remains at the speed of sound, and  the
flow can only be increased by increasing the upstream pressure
(and  therefore density) of the gas in the nozzle.

I strongly suspect that  the system was intentionally designed
to use this well-controlled flow to  make the system more
robust and repeatable than with just a pressure tap  off the scroll.
Plus, a "sonic nozzle" is more cool than just a hole  tapped
into cast aluminum.




Wow!
I've got to say that's the wildest theory I've heard to date.  That  must be 
the sonic boom I heard under my hood.
 
I strongly suspect you're over analyzing this.  We're talking about a  simple 
boost or vacuum signal, plumbed to the WG to either enhance or detract  from 
the spring pressure applied to the valve.  Nothing more, IMO.
 
Dave