Re: Bypass valves: More, also long

[Scott Mockry <scottmo@teleport.com>]

A few more comments, sorry to nitpick this topic:

>Scott Justusson said:
>Scott, I think if you look at the diagram again, you will see only one ck 
>valve listed in the key, but there are 2 in the system.  However, I do agree 
>with your summary, but for a different reason. The site says about half the 
>boost, but in reality, in order for that to be true, the ck valve/s would 
>have to be effectively the same spring pressure as the bypass valve or the 
>bypass valve will open early (cuz it has more resistive force from the 
>opposing boost side, than the ck valve, effectively causing a non linear 
>definition of "half") 
 
Yes, I noticed the two check valves, but the number of check valves and/or
the use of the bleeder orifices are un-necessary IMHO, as my main point was
that you don't need anything but a simple piece of vacuum hose connected
between the intake manifold and the Bypass Valve small fitting to have the
OEM Audi Bypass valve (made by Bosch) to work correctly. 

The contention of the Mini-Mopar web site article (Gus Mahon setup) is that
you can not send boost pressure directly to the Bypass Valve, utter
nonsense, IMHO If that were true, someone should contact Audi and have them
recall all the thousands of 1991 200TQ 20V and S4/S6 cars to have that hose
from the intake manifold to the Bypass valve removed and have the Mahon
setup installed.

Here is the quote from the Mini-Mopar Web site:
"The Bosch BOV, used on the SAAB 9000 Turbo, is smaller but flows better
because of the 7/8" hose connections. Unfortunately, this BOV has a weaker
spring and is forced open at about 9psi.  The BOV can be held shut by
supplying the vacuum nipple with some boost.  Gus Mahon has a vacuum bleed
configuration that can accomplish this.  You cannot send all of the boost
to the BOV because it will rupture the diaphragm, which is why his design
bleeds off about half of the boost, while supplying it with full vacuum."

>Scott Justusson said:
>Better get the saab part.  6psi + 7.35 won't keep ANY stock 20vt closed, nor 
>a stage II closed under boost.  

I have never tried the Saab part, but I don't know why anyone would not use
the OEM Audi part for boost pressures below 20psi. 

Regarding the size of the Bypass Valve internal Diaphragm and how many
square inches receive vacuum/boost from the intake manifold:

>>  Scott Mockry Said previously:
>>  For example, 14psi intake manifold pressure times 2.32 square inches
>>  equals 32.5 lbs force, plus the spring pressure of 6lbs equals a total
>>  38.5 lbs holding the bypass valve diaphragm closed, versus the turbo
>>  exit side with lets say, 16 psi times 1.2 square inches equals 19.2 lbs. 

>Scott Justusson said:
>BINGO.  That's because the spring is NOT a boost pressure device, it's a 
>vacuum device.  It is set to stay closed under idle vacuum, but open under 
>decel vacuum.  The reason most aftermarket bypass valves (Greddy, HKS, etc) 
>have a spring preload.  Bottom Line: the diaphram dictates that it will stay 
>closed under boost.  <SNIP>

Actually the OEM Audi Bypass Valve is wide open with the engine idling. You
can easily test this by placing your hand on the bypass valve, and
disconnect/reconnect the vacuum/boost hose, you can feel the valve operate.
You can also take a spare Bypass Valve and connect vacuum to it, it will
open at ~6in Hg. This theory of operation is detailed in the Service
Training books on the 1991 200TQ 20V. Perhaps you were referring to the
operation of the aftermarket valves.

>Scott Justusson said:
What you will find (esp with CIS), is that a weak spring 
>will cause a lean mixture or stall the car all together (btdt - cis).  The 
>adjustable bypass valves mean that you could put 2 sequential Greddy/HKS 
>bypass valves in, one that opens during mild closed shut throttle (say 25in 
>Hg), a second one that opens at full closed (29in Hg)

The Bypass valve provides an additional path for the air to flow into the
engine, the air either flows through the turbo when the valve is closed
under boost, or the air flow is split and flows through the bypass valve
"and"  through the turbo when the engine is idling. The OEM Bosch valve
shouldn't affect the mixture at all, you can connect and disconnect the
valve with the engine idling with no change in idle behavior. 

>>Scott Mockry said previously:
>>  The dynamic behavior of this valve is likely more complicated because of
>>  the difference or time delay in pressure rise between the turbo exit and
>>  the intake manifold, but it does give some insight to how this valve
>>  "should" work. 

>Scott Justusson said:
>The time is really irrelevent, an easy test for this.  Put total manifold 
>boost pressure on top of your wastegate with a 1psi effective spring in it.  
>You will blow up the turbo in one run (same concept, I'm not convinced that 
>the diaphram sizes are all that relevent here, cuz equal size diaphram 
>exposure top and bottom doesn't change WG behavior). 

Well, a better test would be to connect the lower chamber of the WG
diaphragm  to the turbo exit boost pressure, and connect the top of the WG
diaphragm to the intake manifold boost. It would be interesting to see if
the waste gate would pop open, BTW: the turbo should not blow up if your
ECU overboost fuel pump cutout is working. The data acquisition values
recorded from pressure sensors I have installed at the exit of the turbo,
at the throttle valve port, and connected to the intake  manifold, show a
definite delay in boost pressure rise at the intake manifold. How much the
time delay affects the operation of the Bypass valve, would need to be
further investigated.

>Scott Justusson said:
>A pre vs post IC 2 
>stage sequential bypass valves might be relevent because of pressure drop 
>across the IC, but a pressure drop of more than 6psi is a cause for IC 
>hardware, not bypass valve hardware concerns. Interesting, that the S cars 
>use pre IC charge air for the high pressure feed, post IC charge air for the 
>nipple feed. 

The S Cars do use pre IC charge boost air for the high pressure feed to the
Bypass Valve, but they use the same connection to the "intake manifold" for
the Bypass Valve nipple feed, not a connection to the post IC charge air.
This is the same connection scheme as was used on the 1991 200TQ 20V. The
Bypass valve won't open and work correctly if the nipple feed is connected
to the post IC connection. 

>Scott Justusson said:
>Thanks for the post scott.  Bottom Line:  This "dodge" mod won't work on 
>stock audi bypass valves (or the sport) and might work on the saab valve.  
>Best just get an aftermarket and not worry about it at all.

Or just use the OEM Audi valve with applications under 20psi?
I have not seen any failures after installing the OEM Audi replacement valve 
034-145-710C on the 10V I5 Turbos, or on the 1991 200TQ 20V or S4/S6t
vehicles running 18psi. Just because the original Bypass valve fails after
4-8 years of use, and 50-100k miles, doesn't mean it can't be used for a
replacement. 

Scott J. mentioned the failure of the "sport q" Bosch Bypass valve within a
year, when he used it on a high boost application, but has anyone who runs
over 20psi boost with the 20V Turbos had failures using the superceded
replacement Audi 034-145-710C bypass valve? I would be interested in seeing
the failed valves if anyone can send one to me. 

HTH
Scott Mockry