Cross posting and bypass valves

[QSHIPQ at aol.com]

Since you brought this up (and with my name) here's my take on this
crossposting, and in the subject matter presented:  Bypass valves.  The only
list IMO that shouldn't be getting a post on Bypass valves is the v8 list
(for now:).  Euro urq's have them, US urq's should have them, S cars have
them, 200's have them, ALL TURBO CARS SHOULD....

Over the years I've posted several times on the bypass valve vs blowoff
valve, and believe it should be in EVERY archive.

Now, wrt the thread itself, I'll add this tidbit...  IF you have a leaking or
faulty bypass valve, you CAN get a power increase just replacing it, in fact
it sure looks like that was the problem.  The neu S4 has 2 of them, and they
fail quite regularly, many times at the expense of the turbocharger/s.  If a
bypass valve is leaking the turbos need to blow that much more air to
overcome the leak, which reduces blower efficiency>power.

Aftermarket bypass valves can be very effective especially if you play with
the preload.  My own experience would tell me that you only want the valves
totally open when the throttle plate is completely shut.  The audi bypass
valves have no adjustment, under any vacuum they open.  Most aftermarkets
allow you to adjust this opening so that you must see a higher vacuum before
they open, IMO a good idea.  Adusting this preload does help in the respool
(the lag between the valve open and valve closing) of the turbo during
shifting.  I also believe that aftermarket bypass valves installed
can/will/do/should affect your audi factory warrantee.  Proceed with caution
when adding boost, but I will say that the audi bypass valves (none of them
including the sport q, btdt) take 17psi> consistently without failing.  The
bypass valves from audi/bosch are of the square type, and it's been my
experience that under hi boost conditions, they don't always drop straight
down on the tube, which can cause leaking as well.

Glen P does have a good point, leaving the throttle plate partially open
during shifts will give a faster 1/4mile, dyno reading than having any bypass
valve in the circuit.  There is a lag between open and closed bypass valve
that does affect power output.

IMO, crosspost away, get bypass vs blowoff valves in every archive.  A lot of
misunderstanding/misinformation is out there.  Punting the thread from
relevent lists appears to add "missed application" to the problem as well.

My .02

Scott Justusson
QSHIPQ Performance Tuning
Bypass valve aplenty
'87 5ktqw
'84 urq
'83 urq
'87 4runner turbo
'85 FJ1387