[s-cars] Waste gate creep

[Kirby Smith]

IIRC, the A'pexi setup includes full boost pressure on the bottom of the
WG.

kirby

Robert Pastore wrote:
>
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> 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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