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Re: Dump Valve/Bypass Valve


Last June I installed a bypass valve (Stock S4/S6 Audi 034 145 710C,
~$35 Wholesale) on my 89 200TQ with the late MC dual knock sensor engine
with the K24 Turbo. Because I am using the 20V single pass intercooler,
I ended up using the stock 20V turbo exit rubber hose which has the
extra hose grafted for the bypass valve installation. 

I had the metal intake pipe modified with the addition of a 1" pipe
fitting (welded on) to allow connecting the other end of the bypass
valve to the intake side of the turbo. The vacuum connection on the
bypass valve was connected via a 6mm hose to a plastic tee fitting to
one of the small metal pipe fittings at the rear of the intake manifold.
It is important to note that you need to make sure you don't use the
small capped fitting which is next to the ISV hose connection. This ISV
and small capped fitting DOES NOT HAVE intake manifold
pressure/vacuum.If you connect the bypass valve vacuum connection to
this ISV connection, the Bypass valve will not work correctly as

Please also note, that the bypass valve REQUIRES boost pressure to be
applied from the intake manifold to this "vacuum' connection on the
bypass valve in order to hold the valve closed as the boost rises in the
intake system/manifold. 

I found this out, inadvertently while installing the bypass valve on my
car when I only had the one connection on the bypass valve connected to
the pressure side, without connecting up the intake side and the small
vacuum connection. I needed to run out to the parts store to get some
hose to finish the installation, and found the bypass valve would
immediately blow open as the boost went above 1.0 bar abolute. The car
make this very nice loud whoosing sound and basically barely ran as air
was blown out of the valve. (nice rich mixture with no boost)

With the bypass valve vacuum fitting connected to the intake manifold
vacuum/pressure, this system has worked flawlessly for the past 12,000

 The boost used to come up slowly during shifting before installing the
valve as noticed on the analog boost gauge. Now the analog boost gauge
needle immediately comes up to 15psi during shifting from 3rd to 4th. So
much so, that I was hitting the overboost cutout briefly, because the
boost came up faster and stayed up longer than the ECU overboost time
out would allow. Adjusting the WG spring tension down somewhat, elimated
this cutout. The boost was overboosting a little too much, and backing
off the waste gate spring fixed this problem.

Before installing the bypass valve, after installing the 20V single pass
intercooler with longer piping, I used to get a surge when briefly
letting off the gas hard, and then getting back on the gas. Now with the
bypass valve installed this surging no longer occurs. :-)

The Porsche 944 has a slightly different bypass valve, (different spring
and slightly different diaphragm). I have one that I took apart that
came from a 1989  944 Turbo, I should take another look and compare it
to the Audi part. The 944 Turbo has the Bypass valve boost connection,
at the outlet of the intercooler, and the intake routed very close to
the turbo suction side. The vacuum hose goes to the intake manifold as
expected. I assume this same setup could be used on the Audis, i.e.
connect to the exit hose on the intercooler but I have not tested this

I installed my setup, keeping in mind that I wanted to keep the length
or path of the hoses as short as possible to allow the pressure to be
more efficiently bypassed to the intake side of the turbo during
shifting as designed and used in the 1991 200TQ 20V and the S4/S6.

The only odd thing I notice with my bypass installation, is I can
occassionally hear a funny squealing noise as the boost slowly crosses
over the 1.0 bar mark as the bypass valve is open and then closing. This
only occurs when the throttle is slowing opened at cruise conditions.

I can't imagine any reason for installing a "DUMP" valve, which opens at
a particular pressure and vents this air to the atmosphere on a street
driven Audi. If you were using a dump valve, the mixture would likely
not be very rich during shifting or during deceleration, as on the CIS
systems, manual 5 speed,  with the decel valve bypassing air around the
air flow plate, the plate normally drops down to the bottom of its
travel during shifting/decel to lean out the mixture for emissions
reasons. I verified this using an oscilloscope connected to the CIS air
flow plate potentiometer while test driving the car.

If the DUMP valve was to open under full throttle, high boost
conditions, then obviously the mixture would get very rich as all the
extra metered air would be vented to the atmosphere and would raise the
CIS air flow plate higher than normally required.

Scott Mockry (no e)
SJM Auto-Technik
89 200TQ
89 80Q
85 4000 (no Q)
66 VW Bug (no engine)