Re: Bypass valves and turbos-long

[QSHIPQ@aol.com]

In a message dated 9/19/99 8:31:24 PM Central Daylight Time, 
scottmo@teleport.com writes:

> Jeff and Scott,
>  
>  The Dodge Turbo site mentions the use of a one way check valve which allows
>  vacuum to go to the bypass valve small fitting, but bleeds off boost to the
>  bypass valve small fitting. The reasoning is that the boost pressure acting
>  on the diaphragm will somehow "damage" the valve. 
>  
>  This " questionable" approach IMHO, will certainly cause the Bosch Bypass
>  valve to leak prematurely under boost, and is NOT how the factory bypass
>  valve is installed in the 1991 200TQ 20V and the 1992> S4/S6. Having boost
>  pressure acting on both sides of the bypass valve diaphragm as was done by
>  the Audi factory, one would think, would be less stressful on the
>  diaphragm, than having atmospheric pressure on one side, and boost pressure
>  on the other.

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")
>  
>  The OEM Audi Bosch turbo bypass valves REQUIRE boost pressure from the
>  intake manifold acting on the diaphragm though the small vacuum/boost
>  fitting in order to remain closed under boost. 

But, it really only has to be 1 pound more than the manifold pressure to 
remain closed.  Remember the spring is really what dictates when it opens and 
closes under vacuum (vs idle vacuum), not really to prevent diaphram 
cracking.  In absolute terms, linearly, 13psi on top of 13 psi should get no 
movement of the diaphram.

>  
>  As Scott J. mentioned, the stock 20V Bypass valve as used on the 1991 200TQ
>  20V and on the S4/S6 use a spring that will crack open at about 6 psi IF no
>  pressure is applied to the diaphragm via the small fitting. But of course,
>  the valve has boost pressure acting on the diaphragm to assist the spring
>  pressure.

Better get the saab part.  6psi + 7.35 won't keep ANY stock 20vt closed, nor 
a stage II closed under boost.  Thanks for finding the post information Scott.

om: "Scott Mo." <scottmo@teleport.com>
>  Subject: More Bypass Valve stuff
>  Date: Sat, 27 Mar 1999 11:50:03 -0800
>  
>  >Scott Justusson said:
>  >I measured several valves at my shop a while ago.  The valve with upper
>  >connection open to atmosphere (sea level) it will crack open at 6psi.
>  >The sport quattro valve will crack at ~7-8psi.  The sport quattro valve
>  >uses a slightly stiffer spring and has a thicker diaphragm.  It also
>  >costs almost twice as much as the stock 20vt. 
>  
>  Scott Mockry Said:
>  That jives with the two bypass valves I took apart, one from the Porsche
>...
>  
>...
>  
>  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. 
>  
>  There is also some additional help to keep the diaphragm closed from the
>  side of the valve opening which connects to the turbo inlet (slight
>  vacuum) which is 0.57 square inches. It appears from the above numbers
>  that the size of the diaphragm that receives intake manifold pressure,
>  is more important than the spring tension to keep the valve closed under
>  boost.

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.  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 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. 

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).  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. 

>  I have a diagram on my web site which illustrates the Bypass valve
>  installation from the 1991 200TQ 20V and shows the pressure applied to
>  the relative areas described above.
>  
>  http://www.sjmautotechnik.com/turbo.html#bypass
>  
>  HTH
>  Scott Mockry
>  scottmo@teleport.com
  
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.

Scott Justusson
QSHIPQ@aol.com