[s-cars] Waste gate creep

[Richard Tanimura]

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RE: [s-cars] Waste gate creepDr. Hap and Dr. Bob,

Thanks for those inputs. This stuff is finally starting to come together. My
turbo knowledge has gone from "creeping" to just barely "afloat". Sorry for
the mixed metaphors :-)


Rich
  -----Original Message-----
  From: Robert Pastore [mailto:rpastore at animalfeeds.com]
  Sent: den 12 mars 2003 17:36
  To: 'CaptMagu at aol.com'; Richard at Tanimuras.com; s-car-list at audifans.com
  Subject: RE: [s-cars] Waste gate creep


  Hap:

  A few questions, comments, & probably useless opinions, FWIW:
  <snip>
  "Another related issue for my car has been WG float. This particular
problem
  comes from using a significantly larger(Happersize tm.) turbo and still
using
  the RS2 EM. The pre-turbine pressure from using this larger turbo can
build
  to a very high figure. The float manifests itself as a 2-4 psi boost
pressure
  pulse that can be eliminated with a higher WG spring preload."
  <snip>

  I completely agree that the rs2 EM is too small for the CFM you are
pushing.   I do not know what your hot side trim is, but it sounds to me
like the hot side of your turbo is too small also.    If you have a TEC
turbo, I recall they are partial to a t3 stage III, which has an exducer
bore only marginally larger than a rs2.  Innovative, on the other hand,
typically specifies a t3 stage V for most high HP I-5 Audis.   The symptoms
of an undersized turbine section seem to match perfectly with your set of
problems:

  I've forgotten most of what I learned in Physics class, but I think it was
Boyle's law that said "P1*V1=P2*V2".  Basically, as you force a certain mass
of air into a given volume, the pressure will increase as volume decreases.
Pretty simple stuff.   So if you have already measured pre-turbo pressures
significantly higher than the stock pressure ( please share the data, as I
have never measured either and would love to get an idea of what a factory
baseline is), I think the data is telling you that the exducer bore is too
small, with the result will be spooling up the turbo very early, and then
over spinning it at higher engine rpms.    The ramifications on the cold
side will be surging at low rpms, and poor efficiency range at higher rpms
as the compressor is spinning too fast.

  Since the s4/s6 uses boost pressure in the lower chamber acting against
spring pressure in the upper chamber, this early, strong, boost "attack"
could easily cause WG creep as the stock WGFV is probably not up to the task
of bleeding off boost from the lower chamber fast enough. A top chamber
boost controller doesn't solve the root problem, but should help mask the
symptoms.  But I am not sure I agree with or maybe I don't understand your
statement about "limiting versus controlling boost" with the AVC-R.   The
top chamber can only help keep the WG shut, but can't do a thing about
opening it.  It is the pressure differential across the WG diaphragm that
determines whether the WG is open or closed.   On the bottom side, you have
EM pressure pushing against the face of the valve (probably a small factor
since the areas is not that large) and boost pressure in the lower chamber.
The stock system uses a WGFV to regulate the amount of boost seen in the
bottom chamber. If you are using a top chamber boost controller, you should
ditch the WGFV and let the bottom chamber see full boost.   The bottom
chamber pressure acts against the WG spring pressure + atm. pressure only
(in the stock system), and WG spring plus regulated boost with an
aftermarket boost controller. I don't think you can set up an aftermarket
boost controller and have the benefit of the higher pressure differential
across the WG diaphragm, while retaining the WGFV, which is the only tool
the ECU has to provide the "override protections".



  I also recall you've invested in/investigated several solutions to cure
what you thought was a compressor surge problem --i.e. a big bypass valve,
dual bypass valves, and streamlining the intake tract before the turbo (as
per Julian Edgar).    My guess is that you properly identified compressor
surge, but maybe didn't couple the surge with the high EM pressure to
conclude that the turbine was over spinning because the hot side is too
small.   With a properly sized hot-side, the EM pressure will be lower (and
so will temps), the cylinders will breath better, and the WG will be used
much less -- i.e. more energy can go into spooling and driving the turbine,
and less "wasted" through the WG and out the tailpipe.


  Bob



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