regulator-based MBC for 5kcstq, 15psi [long]

JShadzi at aol.com JShadzi at aol.com
Tue May 28 21:38:47 EDT 2002


>That does make sense.  I guess with the stock system, the WGFV will just apply more vacuum to the upper chamber to lower the boost to the mapped >target?

Yes

>
>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.

>Did you rethread the WG cap for the different thread pitch?

Yes, as I said, 9/16" 12 pitch.

>
>The bolt I got was a direct fit, per the specs online/archived.  I still need to cut the nut in half tho'.
>

Yes, I know the stock bolt fits, but it threads are too fine for practical purposes, took me about 10 minutes to drill and tap to the size I wanted.

>The washers I saw at the store didn't have a large enough hole in the center of them to perfectly fit onto the upper WG spring perch.  Do you drill out the plain old washers to fit the perch?
>

ya, sure, I suppose whatever works.  I don't think I've ever had a problem with ripping diaphrams either, I don't really use washers.

>
>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.

>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.

>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.

>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.

Javad



More information about the quattro mailing list