More Bypass Valve stuff

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

>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 diaphram.  It also >costs almost twice as much as the stock 20vt. 

That jives with the two bypass valves I took apart, one from the Porsche
944T and one from a 1991 200TQ 20V. Both valves are identical except for
the diaphragms, I was wrong about my previous comment that the 944T BP
valve spring was different. Both springs are identical and provide about
6-7 lbs of force against the diaphragm. The Audi diaphragm is the same
size, but has the round metal plate riveted on both sides in the center
of the diaphragm as Scott J. has commented about before. 

The 944T diaphragm has no metal in the center of the diaphragm, it just
has the rubber, but one thing I did notice, is there is a hole through
the back plastic support which apparently adds some pressure between the
plastic support and rubber to help seat the diaphragm.

Bench tests I did on a new replacement bypass valve for the Audi S4/S6
(034 145 710C) where I applied boost pressure to the small vacuum/boost
fitting on the bypass valve, and applied the same pressure to the turbo
exit side, indicated that the valve would not open even with 2.7 bar
absolute (~24.5psi gauge) applied.

Because there is always a slightly lower pressure in the intake
manifold, compared to the pressure exiting the turbo, I always assumed
the spring was in the bypass valve to return the valve to the closed
position, and to help keep the diaphragm closed even when there is the
pressure differential. Pressure losses through the dual pass 5kTQ
intercooler and throttle valve drop at least a couple of psi at 14psi if
I remember correctly.

If you take the bypass valve apart, you will find that the area of the
diaphragm which connects to the intake manifold (vacuum/boost) is about
2.3 square inches, the area of the diaphragm which connects to the turbo
exit (pressure) side is only 1.2 square inches once you subtract the
center area of the opening. Pressure from the intake manifold that is
applied to the diaphragm "should" always win and keep the diaphragm
closed:

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 diaphram 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.

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. 

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

If that doesn't work, try my old URL at
http://www.teleport.com/~scottmo/turbo.html#bypass

HTH
Scott Mockry
scottmo@teleport.com