Turbo Bypass Valve, Dump valve? long

["Scott Mo." <scottmo@teleport.com>]

Dave,

>From: "Dave Aukerman" <mhauk@ccsdana.net>
>Subject: dump valve help
>Hooking up the dump valve on bros car.  Tee'd into one of two vacc. lines
>at back of intake manifold but it now creates such a vacc. leak at the dump
>valve that the car dies on any off throttle.  Valve is installed in port of
>Samco hose(inline with intercooler/intake.  Tried installing valve both
>directions same effect.  Any ideas would be greatly appreciated.
>Thanks,
>Dave

I did not see any mention of the model of "dump" valve you are using, but
if you are using the OEM Audi Turbo "Bypass" valve (034-145-710C) which is
the replacement part for the 1991 200TQ 20V and the 1992> S4/S6, it won't
work as a dump valve with one end open as you have installed. 

I have no experience using any of the aftermarket bypass and or dump valves
(TAP etc) so I can't comment on their specific differences. Possibly others
can comment on the TAP valve as to whether it was designed as a bypass or
dump valve.

The stock 20V Bypass valve is designed to be open when the engine is idling
and intake vacuum is applied to the Bypass valve diaphragm. This operation
is detailed in the Audi Service Training book #WSP 521-209-00. "The New 20V
Turbo Engine for the Audi 200 Quattro" (1991 Model year, US)

The book states: 

"The bypass valve is used to reduce boost pressure in the intake air duct
when the throttle plate is closed, such as, at idle or when decelerating.
The use of the bypass valve serves another function, which is to maintain a
higher speed of the turbo with the throttle closed. This improves the
response characteristic of the turbocharger when accelerating. "

It further states:

"The bypass valve is operated by intake manifold vacuum. During idle or
deceleration with the throttle valve closed, the bypass valve is opened by
vacuum against spring pressure. When the valve is open, intake air is
recirculated and the turbo is free to spin but doesn't develop any boost
pressure. This maintains a higher speed of the turbo and improves the
response of the turbo." end quote

These valves will begin to crack open with as little as 6 in Hg vacuum and
will be fully open by ~12 in Hg vacuum. The I5 Audi engines normally have
16-18in Hg at idle. Your connection with one end open to atmosphere is
allowing un-metered air to enter the engine and bypass the CIS Air flow
metering plate or the Hot Wire MAF in the 20V engines.

The other thing to note, is that this valve will blow open under boost
pressure if the bypass valve diaphragm is torn and leaking OR if the small
vacuum hose going to the diaphragm is leaking or not connected. This bypass
valve requires boost pressure acting on the internal diaphragm in order to
hold it closed. 

When tested on the bench, if no pressure is applied to the small hose
fitting, these valves will blow open with any boost greater than ~1.0 bar
absolute, i.e. above atmospheric pressure when pressure is applied to the
large hose fitting that is 90 degrees from the small hose fitting.

I found this out during the installation of the bypass valve on my 89 200TQ
10V when I went out to get some parts to complete the install, and
temporarily had the turbo inlet end open to atmosphere and did not yet have
the small vacuum line connected. The valve blew open any time the boost
gauge needle moved past 1.0 bar (absolute) and you could hear the boost
pressure  being purged out the open end of the valve. 

I use the 20V intercooler, so I was able to use the 20V turbo exit hose to
install the boost pressure side of the bypass valve, and I had a CO list
member weld a fitting onto the metal turbo intake pipe to connect the other
end of the bypass valve. I connected the Bypass valve small hose to the
intake manifold hoses at the back of the manifold.

If you add pressure  to the large boost side fitting and at the same time
to the small vacuum fitting, the valve will remain closed with over 2.7 bar
(absolute pressure) applied. (~25psi gauge pressure)

I have a copy of a diagram that shows the bypass valve installation on the
20V turbo on my web site at:

http://www.teleport.com/~scottmo/ecu20v.html#bypass

My 89 200TQ has a small hose fitting near the throttle plate (under neath),
but this fitting provides vacuum only when the throttle is open  slightly
(ported vacuum) and does not appear to be a good place to connect the
bypass valve line.

HTH
Scott Mockry
SJM Auto-Technik

http://www.teleport.com/~scottmo/