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

[Djdawson2 at aol.com]

In a message dated 1/12/2006 9:03:58 PM Mountain Standard Time,  
tedebearp at yahoo.com writes:

we agree  that in order to increase boost, the ECU actuates
the WGFV to reduce  pressure in the WGFV-WG hose.
 
 
Yes, that's accurate.
 
 

if i  understand you right, you're contending that rather
than just bleeding off  boost, actuating the WGFV causes
a vacuum to be created in the WGFV-WG  hose, which helps
to hold the wastegate shut.
 
 
Yup... at a certain % duty cycle (I don't know what % that would be), a  
vacuum is created.
 
 
 

in other  words, rather than just reduced positive pressure,
you're saying there is  negative pressure in the WGFV-WG
hose when the WGFV is  actuated.
 
 
Yes... but to pick nits, since we're getting more specific than the  original 
simple explanation, there's no such thing as negative  pressure...  When it 
comes to our wastegates, the bottom side is simply  having a pressure applied 
to it that is less than atmospheric... AKA a  vacuum.  Because the pressure on 
the top of the diaphragm is higher than  the pressure on the bottom of it, you 
are enhancing the pressure applied to the  WG valve to keep it shut.
 
 
 

i have  no opinion on that point, and i don't disagree with
you.
 
 
Cool.
 
 
 

you  said:
"If the ECU decides that all conditions are good for high boost  operation, 
the
WGFV allows the vacuum source to pass through it to the WG,  and this
effectively increases the WG's ability to remain closed.  Net  result... the 
WG
remains closed to a higher boost level than would be  attainable with the 
spring
alone."

agreed.
 
Great...
 
 

"If the  ECU decides that conditions are not good for high boost operation, 
the
WGFV  begins to cycle the boost port open, allowing boost to pass through it  
to
the WG, and this effectively decreases the WG's ability to remain  closed.  
Net
result... the WG remains closed to a lower boost level  than would be 
attainable
with the spring alone."

my understanding is  that the boost port of the WGFV is always
open, and so is the wastegate  port.  the only port being
controlled by the WGFV is the intake port,  which is normally
closed.  when the WGFV is actuated, the intake port  opens and
is connected to the other two ports.  i verified this by  
testing with a 12v source and blowing through the ports.
 
 
Yes, you're correct... however, I elected to NOT state  it that way.  The 
last time I did, we couldn't get past the concept that  "somehow" we could get a 
vacuum from a boost source.  So I decided to try a  different path.
 
 

i would  describe it thusly:
"If the ECU decides that conditions are not good for  high boost operation, 
the
ECU begins to reduce the duty cycle of the  WGFV,
cycling the vacuum port closed, reducing the vacuum and  
increasing boost passed through it to the WG, and this effectively  decreases
the WG's ability to remain closed.  Net result... the WG  remains closed to a
lower boost level than would be attainable with the  spring alone."

would you agree to that?
 
 
Absolutely... and is what I've been trying to describe... and I'll add a  
note on this to the end of my post.  Technically, the WGFV isn't cycling  the 
vacuum port closed, it is cycling the exhaust port closed.  The port  that goes 
to the turbo inlet is simply an exhaust port in this little vacuum  generator.
 
 
 

this is  an important point, because your earlier phrasing
implies that actuating  the WGFV (increasing its duty cycle)
reduces the wastegate's ability to  remain closed (and thus
causes the wastegate to open).  i believe that  increasing
the duty cycle of the WGFV increases the wastegate's  ability
to remain closed, and this is consistent with your  later
observation that unplugging the WGFV (reducing its duty cycle
to  zero) causes the wastegate to open at low boost.
 
 
 
Implied... maybe.  But I was trying to stay away from talking about  duty 
cycle.  What you've said is what I believe to be true.  I tried  to make my 
explanation as simple as possible when we first started this  thread... simply 
because I've tried to explain this before, and failed to gain  any buy-in.  That 
is why I elected to call the WGFV a "switch."  And  really, that's exactly the 
way it works.  It switches the "signal" to the  WG... and that signal can be 
anything from full boost, to the max vacuum allowed  by the duty cycle sent to 
the WGFV.  I was going to steer clear of even  discussing duty cycle... and 
simply stick to the concepts of either boost or  vacuum being delivered to the 
WG.
 
But, now that we're here...
 
Boost CAN flow through the WGFV at all times.  It serves as a  safety net... 
bad WGFV = low boost, and therefore, no chance of engine  damage.  The WGFV 
can direct boost in 2 directions... either to the WG (low  boost), or to the 
turbo inlet (high boost).  The ECU determines the duty  cycle based on a variety 
of sensors... and that duty cycle controls how  much boost we ultimately get.  
More duty cycle allows more boost to return  to the turbo inlet.  When the 
flow of boost through the WGFV is great  enough (to the turbo inlet), it begins 
to generate a vacuum in the line to the  WG.  Many inexpensive shop tools use 
this type of configuration to generate  a vacuum with only compressed air.
 
That's really it...
HTH,
Dave