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

[urq]

... You are correct in theory of course, but I would wonder if there would
practically be any difference in the longevity of the WG diaphragm ...

I hacked away most of the thread on this one ... but from what I've read so
far there is one issue which is not being discuussed.  The interesting thing
is that I saw a note in one of Ken's posts which shows that it should be a
real concern ... you give up any possibility for backing off the boost upon
knock with the mods being discussed.

I really like Audi's (Bosch's, whomever's) solution of having the WGFV and
the knock sensor.  They deliberately used a weak spring and let the WGFV
provide the extra boost via the upper chamber of the WG.  This allows the
ECU to be able to cut boost back if excessive knock is detected.  That said,
I also am a big fan of the Schrappnell Knobben (as I like to refer to the
"Timmerman mod").  I have yet to follow through on my proposed solution ...
simply having the reprogrammed WGFV profile from the QLCC chip gives me
enough improvement ... even with the stock WG spring.  What I have wanted to
do was to have a pressurized reservoir (supplied with manifold pressure and
a check valve ... or some other pressure source like a CC pump) set up to
provide a pressure source for the WGFV to select from.  This solution might
also benefit from a replacement WGFV which has a higher flow rate ... I
don't know this for a fact that this is necessary, there isn't going to be a
high volume flow.  The pressure reservoir averts the one negative I've heard
of the Schrappnell Knobben ... that you have to wait for the boost to kick
in to get the stiffer spring.

One thing I suspect is that you'd get a significantly stiffer WG "spring"
simply by removing the vacuum line from the top of the WG and capping it.
Assuming this is the case, you could adjust the spring simply by attaching a
length of hose with a plug in the other end ... effectively changing the
volume of the upper chamber, and changing the pressure vs. WG displacement
function.  Has anyone ever tried this?

Steve Buchholz
San Jose, CA (USA)

----- Original Message -----

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.