regulator-based MBC vs. WG spring/preload [long]

auditude at auditude at
Tue May 28 20:20:23 EDT 2002

Hi Javad,

Further comments and questions below.

On 28 May 2002 at 20:38, JShadzi at wrote:
> > A concern I have about adding preload to the spring is that it may
> > decrease the total travel available for the WG. =A0It's almost like
> > the orifice is being made smaller (at WOT/target boost) by reducing
> > the travel.
> Theoretically that is a concern, but realistically, with about 1" of
> compression, this has never been a problem for me.  It would be easy
> to see, though, boost should be rock solid, if it fluctuates up or
> down then the scenario you describe could be true - like I say,
> though, never happened to me yet.

I was thinking a question that could be answered would be how much travel i=
s left in the system (how big
is the passage) when the wastegate is functioning.  I'm not necessarily ask=
ing you Javad, but the question
popped up earlier in my mind.

> > p.s. =A0Since I'm posting again, I was thinking about how now that I'm
> > running the SchrapnelKnobben mod on my car, I can switch back to the
> > stock WG spring.
> >
> >My idea is that it would provide the following changes/benefits:
> >
> > One feature that I like about this mod is that, I believe, the WG
> > doesn't even begin cracking until the boost (lower chamber) exceeds
> > the regulated pressure (upper chamber). =A0So, unlike a simple stiffer
> > spring, the WG isn't partially open before target boost is achieved.
> > =A0With a spring type mod, you are "chasing" boost as exhaust leaks
> > past the WG before max boost is achieved.
> Not true, if you crank down the spring 3 more lbs, then its no
> differnet than the upper chamber mod.  The nice thing about cranking
> down on the spring, you greatly reduce boost creep because of the
> higher initial pressure on the valve.

I heard that the WG spring is half deflected at half the max boost.  Do you=
 disagree?  Is it deflected at all
below max boost?

Adding preload or a stiffer spring would put more pressure on the valve, re=
ducing boost creep, sure.  But I
don't think it's "no different" than having equalized boost pressure on bot=
h sides of the wastegate
diaphragm, in addition to whatever spring stiffness and preload is there.

With the regulator routing limited boost to the upper WG chamber, there wil=
l be zero cracking below the
boost the regulator is set at.  There would have to be, since no net pressu=
re is pushing on the diaphragm.

> > With a stock spring and Schrapnelknobben mod, the WG closing force
> > is more pressure-based than mechanical/spring force-based. =A0That may
> > allow me to run closer to the edge of detonation, since the WG may
> > be quicker(?) when biased towards pneumatic control. =A0Seems like it
> > would, since I would be putting out higher pressure on the upper
> > chamber than currently, so the WG would stay totally closed until
> > that higher limit.
> I doubt it, that is stretcing the theories a bit, practically
> speaking, you will experience detonation because of your timing
> maps, maybe fuel if its running in just the right mixture, but if
> you boost comes on 1/10th of a second sooner or later, that just
> won't matter wrt detonation.

I see your point regarding timing maps.  In my situation, using a TAP ecu, =
I may not want the boost to
come on too much earlier in the rpm range than usual. I'll have to think ab=
out my spring and regulator
settings as a combination to see if I can optimize it.  There's that boost =
referenced retard that MSD or
somebody makes, I suppose.

> > In other words, I've got boost pressure trying to push the WG open
> > at 3psi, and the WG is holding the additional 12psi. =A0So, it's
> > leaking "some exhaust" between 3psi and 15psi. =A0After swapping in
> > the stock spring, I should have no boost trying to push the WG open
> > until about 9psi, then the spring will take care of the additional
> > 6psi. =A0So, boost may build up quicker between 3 and 9 psi after
> > going back to the stock spring.
> Ken, boost is pushing at 3psi, and in either case, you have 3psi
> more initial pressure, there is NO difference.

Really?  At 3psi boost in that second scenario (stock weak WG spring, boost=
 controlled by the regulator),
how much pressure is being exerted on the lower wastegate chamber?  I say Z=

With the cranked up or stiffer WG spring holding the WG close without help =
from the regulator, you would
have 3 psi acting on the lower WG.  To me that's different.

Maybe you're talking about leaving the stock WGFV in place in your scenario=
?  Is that why there's no
difference?  Maybe that's why I don't see how they are they same?

You're saying you can crank down the spring to regain that 3psi "cracking p=
ressure", but doesn't that also
add 3 psi to your max boost?  Or, am I misunderstanding what you mean above=
 about being able to
control max boost with spring preload/tension?  Is there a way to "eliminat=
e" or reduce boost creep, while
not also affecting max boost?

I'll gladly entertain the idea that postponing the cracking of the wastegat=
e is a bad thing, but that is not
what we are talking about here.  I'm saying that this regulator mod helps b=
uild boost faster than using the
WG spring and preload could.

Or perhaps someone might explain that my understanding of how a pressure re=
gulator functions is not
correct.  I think of it as a like a voltage clamp, or a low pass crossover,=
 to use some unrelated terms.  I
think the pressure is equal on both sides of the regulator, input and outpu=
t, below the regulated limit.
Then, if the input pressure is higher, the regulated output is never exceed=

Now, if this is wrong, and the regulator is more of a proportional type of =
situation, "below" the regulated
limit, then that's different.  Then maybe a stiffer WG spring and/or preloa=
d is "the same" as the regulator
way.  In this example, you set the regulator to limit to 10psi, and at 5psi=
 input it's output is something
less than 5psi.  I don't think that's how they work tho'.

My "theory", if it has to be one, is that minimum boost can be controlled b=
y the regulator, and maximum
boost can be controlled by the spring.  The pressure difference between the=
se two values is the max
boost that is generated by the (same) spring if it were by itself.

For example:
[My way]
Regulator set at 10 psi, 7psi spring.  Wastegate stays TOTALLY closed until=
 10 psi, THEN starts
cracking.  Max boost is limited to 17psi.

[WG spring only]
17psi spring.  Wastegate starts cracking at some pressure below 17 psi, mos=
t likely even below 10(?)
Perhaps at 8.5psi it's half open?  Max boost also limited to 17psi.

> > I realize this all happens at the same time, and I can't attribute
> > specific pressures to one or the other (spring vs. regulator). =A0But
> > is the concept valid? =A0Quicker boost buildup, due to reduced WG
> > leakage (higher WG cracking point)?
> No, its all about creating a higher opening threshold through
> increased pressure on the mechanism holding the wastegate shut - the
> same thing in either scenario.

In "my" scenario, where boost pressure is acting on the upper wastegate cha=
mber, the wastegate will not
be cracked _at all_ until the upper chamber is exceeded.  From this boost l=
evel on, the spring is being
compressed and exhaust gas is flowing through the wastegate.

Was there a reason that the WG spring and/or preload is "better" than the u=
pper chamber regulator mod?

If it's true that these two mods are the same with regard to the performanc=
e or boost, then at least the
upper chamber mod has the benefit of making the diaphragms life easier.  Th=
e only boost that "needs" to
be applied to the diaphragm is to offset the stock spring, if it's configur=
ed that way.  With the WG spring
alone, the diaphragm gets to make peace between the boost and the spring.



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