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

[Paul Gailus]

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.

Paul

----- Original Message -----
From: Tom Mullane <tmullane at gmail.com>
To: <Djdawson2 at aol.com>
Cc: <S-CAR-List at audifans.com>
Sent: Wednesday, January 11, 2006 8:10 PM
Subject: Re: [s-cars] While we are on the subject of WGFVs...


Dave,

I can't explain the mysteries of the universe, but I do know that the
basic WGFV functions as Teddy and I described it.  When power is
applied, all ports are joined.  All you need is a WGFV, a battery, and
breath of air to test and confirm this.

Remember that the spring only needs to hold the WG shut against
exhaust gas pressure inside the exhaust manifold.  The only boost to
affect the spring comes from the WGFV when it is closed.  Exhaust
manifold pressure is related to the exhaust system flow, not boost.
Think about what happens when you disconnect the WGFV from the WG -
boost goes sky high.  In this case, the only thing that can overcame
the spring and open the WG is pressure of the exhaust inside the
manifold.

Tom

On 1/11/06, Djdawson2 at aol.com <Djdawson2 at aol.com> wrote:
> In a message dated 1/11/2006 6:42:23 PM Mountain Standard Time,
> tmullane at gmail.com writes:
> Teddy is correct.  Unplugged, or without voltage from the ECU, the
> WGFV allows boost through to the WG, opening it.
>
> I agree, and stated that in my response to Teddy.  Yep, default mode... if
> something isn't working right, the ECU (and the mechanical design) default
> to a setup that allows very little boost.
>
>   When voltage is applied, the valve opens the third port, allowing boost
to
> both the WG and into the intake boot.  This bleeds enough pressure off so
> that the
> WG does not open.
>
>
> So, are you suggesting that "bleeding off pressure" is the only way the WG
> remains shut?  If that is the case, you're suggesting that the WG spring,
> and only the spring, is responsible for holding the WG shut.  That being
the
> case, how would you explain the fact that we are capable of running higher
> boost levels than the spring alone (no WGFV or ECU input allowed) can
> maintain?
>
> Yeah, Mark is right... this has been beaten to death.  But I'm not quite
> sure how anyone makes the logical leap between a WG spring that will only
> allow 14 or so psi by itself... and the fact that we can run double that
> with the WGFV functioning as designed.
>
> The ONLY way the spring can be assisted, from the bottom of the WG
diaphragm
> to allow more boost than the spring alone, is by introducing a vacuum.
That
> is simple fact.
>
> Dave (and I'm smiling about this age old topic)
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