[s-cars] RE: Wastegate creep

[Kirby Smith]

One other point that I recall was raised in the past: A WG controller,
whether factory or otherwise, has to be able to increase and decrease
the pressure on one or the other side of the WG diaphram more quickly
than the dynamics of the engine can change.  As far as I can tell for my
Stage 1 chip and otherwise stock application, the A'pexi and its valving
is fast enough, but I don't know if either the A'pexi or the factory WG
control valve are fast enough for every conceivable giant turbo setup.
It might be necessary to get a higher flow WG valve and larger
hoses/larger ports in the WG.

I suspect two WG with two controler valves driven by a buffer circuit
could be adapted to by the A'pexi's learning mode, but don't expect to
be in the position to find that out soon.

kirby


QSHIPQ at aol.com wrote:
>
> --
> [ Picked text/plain from multipart/alternative ]
> Comments inserted below
>
> SJ
> > 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."
>
> Be careful here.  A high manifold pressure means you need a bigger wastegate,
> not more preload.  A WG is designed to bypass the exhaust pressure driving
> the turbine blades.  ANY variable that affects exhaust pressure (including a
> big/small turbine) could "max" the WG ability to bypass exhaust going to the
> turbo.  You could put a k30 turbo on the I5 with the RS2EM, and still control
> the turbo, it's a WG problem.
>
> > <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.
>
> No.  The pressure in the EM is CONTROLLED by the WG.  The problem is
> controlling that pressure, not an 'exducer' increasing or decreasing it.  The
> non WG event is that the pressure buildup sooner = faster spooling turbo, but
> the 'trim' of that spoolup is the WG job.  I'll reiterate that what's more
> likely happening is the WG control isn't enough to reduce the pressure in the
> manifold.  That can be from two problems, WG control interface (feedback loop
> to the controller isn't fast enough) or the WG itself is too small.  Here
> (Hapersize theory) I think it's the latter.
>
> >
> > 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.
>
> Sure it can, pre 91 turbo cars.  Without a WG spring (or even with one), if
> you put 100% boost pressure on the top of a WG vs 100% on the bottom, the WG
> stays shut.  If you put 80% of boost pressure on top of a WG and 100 on the
> bottom, the WG will open, at some point (ideally 0) with no pressure on top,
> and 100% on the bottom, the WG will be fully open.  A stepper motor (or
> valve) controlling the % of boost on the top of the WG chamber can control
> boost as effectively as the lower chamber.  FYI, the reason audi went bottom
> control is to fix the WG creep due to the slow feedback loop of the top
> controller system.  You have no WG creep if you allow no boost to go to the
> bottom of the WG.
>
> >... 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'll agree with that.  Piggybacking boost controller with stock is a recipe
> for disaster.  Pick one.
>
> > 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).
>
> Bigger bypass valves aren't a solution to surge, they only affect pressure
> pre T-body in off boost (not part throttle) situations.  Big turbos can
> overcome the bypass valve the same way as they overcome WG, you have a given
> amount of air that can be bled.  If the turbo sizing can exceed that, the
> bleed isn't big enough.  For a dual bypass valve arrangement for instance, at
> closed throttle the pre T-body p=o, if it's not, then the BPV isn't big
> enough.  BPV don't need to be "bigger" necessarily, it's a cross sectional
> thing, 2 smaller BPV can do the same thing (I argue beter) that one big one.
>
> >   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.
>
> Not sure I agree with the conclusion Bob.  Turbo Maps and specific engine
> mods overlayed on that map is going to dictate what needs to change.  I argue
> if you don't have them overlayed, you are wasting energy more often on a
> small displacement motor with big turbo, than otherwise.  Yes EM pressure can
> cause problems, but I'd also argue that the same EM pressure can affect
> different turbines based on a variety of factors.  The BEST, specifically
> FIRST way to address that is at the WG.  Take the default position in turbo
> theory, that you are just using what's wasted anyway.
>
> No doubt in my mind these "surging" problems are indications that WG are too
> small, or (IMO) turbos are too big and you are trying to ramp them too
> quickly.  No doubt in my mind either that in quests for all out HP, part
> throttle situations are going to become bigger issues.  Many of you have gone
> so far into the "wild" that you forget the basics to turbo theory,
> specifically what a WG function does, it's designed to affect turbine speed.
> If it's too small to do that, you will have pressure problems on the cold
> side.
>
> HTH
>
> Scott 'turbo nerd' Justusson
> QSHIPQ Peformance Tuning
>
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