[s-cars] RE: Wastegate creep

[QSHIPQ at aol.com]

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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