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

[JShadzi at aol.com]

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
>