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

[JShadzi at aol.com]

Agreed.
Javad
In a message dated Wed, 29 May 2002 11:19:43 PM Eastern Daylight Time, pjberr at rogers.com writes:
>
> The one advantage of feeding boost to the top side of the diaphgram is a
> reduction in pressure diferential across said diagphram thereby reducing the
> stress on it and keeping it healthier longer. A stiffer spring will put more
> stress on the diaghpram when keeping it closed and result in an earlier
> failure.
>
> My $0.02 cdn
>
> Peter
>
> 1990 200Q
> 1989 200Q - Parts
> 1987 Coupe GT
>
> -----Original Message-----
> From: quattro-admin at audifans.com [mailto:quattro-admin at audifans.com]On
> Behalf Of JShadzi at aol.com
> Sent: Wednesday, May 29, 2002 7:18 PM
> To: auditude at get.net
> Cc: quattro at audifans.com
> Subject: Re: regulator-based MBC vs. WG spring/preload [long]
>
>
> Ken, without getting too much into it, my assertion is that, basically
> adding x amount of pre-load to a spring is the same as feeding boost to the
> top side of the WG-housing.  Both prevent the spring from reacting until the
> difference between boost and preload is greater than zero.  I think what you
> are picking up on is that, realistically, it is easier to put x amount boost
> feed than preload to a spring.
>
> Either way will work, and it seems like you are looking for some huge
> advantages of one over the other, but I'm not sure the real world difference
> is that much.  If you can get 17psi with a spring, great, with the pressure
> mod, great, I just don't think either method will give you much of a real
> world performance difference.
>
> Javad
>
>
> In a message dated Tue, 28 May 2002 10:28:48 PM Eastern Daylight Time,
> auditude at get.net writes:
>
> >Hi Javad,
> >
> >Further comments and questions below.
> >
> >On 28 May 2002 at 20:38, JShadzi at aol.com wrote:
> >>
> >> > A concern I have about adding preload to the spring is that it may
> >> > decrease the total travel available for the WG.  It'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
> is left in the system (how big
> >is the passage) when the wastegate is functioning.  I'm not necessarily
> asking you Javad, but the question
> >popped up earlier in my mind.
> >
> >> > p.s.  Since 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).  So, unlike a simple stiffer
> >> > spring, the WG isn't partially open before target boost is achieved.
> >> >  With 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,
> reducing boost creep, sure.  But I
> >don't think it's "no different" than having equalized boost pressure on
> both 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
> will be zero cracking below the
> >boost the regulator is set at.  There would have to be, since no net
> pressure 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.  That may
> >> > allow me to run closer to the edge of detonation, since the WG may
> >> > be quicker(?) when biased towards pneumatic control.  Seems 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
> about 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.  So, it's
> >> > leaking "some exhaust" between 3psi and 15psi.  After 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.  So, 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
> ZERO.
> >
> >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
> pressure", 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
> "eliminate" or reduce boost creep, while
> >not also affecting max boost?
> >
> >I'll gladly entertain the idea that postponing the cracking of the
> wastegate is a bad thing, but that is not
> >what we are talking about here.  I'm saying that this regulator mod helps
> build boost faster than using the
> >WG spring and preload could.
> >
> >Or perhaps someone might explain that my understanding of how a pressure
> regulator 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
> output, below the regulated limit.
> >Then, if the input pressure is higher, the regulated output is never
> exceeded.
> >
> >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
> preload 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
> by the regulator, and maximum
> >boost can be controlled by the spring.  The pressure difference between
> these 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,
> most 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).  But
> >> > is the concept valid?  Quicker 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
> chamber, the wastegate will not
> >be cracked _at all_ until the upper chamber is exceeded.  From this boost
> level 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
> upper chamber regulator mod?
> >
> >If it's true that these two mods are the same with regard to the
> performance or boost, then at least the
> >upper chamber mod has the benefit of making the diaphragms life easier.
>  The only boost that "needs" to
> >be applied to the diaphragm is to offset the stock spring, if it's
> configured that way.  With the WG spring
> >alone, the diaphragm gets to make peace between the boost
> and the spring.
> >
> >Later,
> >
> >Ken
> >