Hi guys.
I've been thinking ... (and thinking again ... reason for the edit)
It seems to me that the valve is designed to be closed all the time except when the pressure in the intake plenum is lower than that on the other side of the butterfly. When I'm on boost and the butterfly closes there is less pressure inside the intake plenum than on the other side of the butterfly. The low pressure from inside the intake plenum acts on the BOV diaphragm and it opens. The pressurised air in the piping can then escape via the bypass and back down to the intake just after the air filter.
My car runs 20 inches of vacuum at idle but as the pressure is the same on either side of the butterfly the BOV remains closed.
Now picture this ..... I am at around 5,000 rpm under light throttle ... my vacuum gauge shows between 0 and 5 inches of vacuum. Therefore, at this stage my valve is closed, because the spring rate keeps it closed when the pressure on either side of the butterfly is the same. If I lift off there will be a pressure drop (increase in vacuum) inside the plenum but because the butterfly is not closed that pressure should be the same on both sides of the butterfly.
We know that it needs 3 inches of vacuum to commence opening the BOV and it is fully open at 8 inches. (Note 5 inches of vacuum is not much) But what would happen if by lifting off the throttle there was a slight pressure change of between 3 and 8 inches created, wouldn't that mean that the diaphragm would be mid-way between open and closed ... being drawn up by the vacuum and pushed back down by the spring ....and cause a flutter.
In the picture below the valve is closed, as you can see. The high pressure entering the side of the body is behind the valve seat and acts with the spring to keep the valve closed. If the pressure side was from the bottom then it would act to push the valve seat up and crack it open.
I'm with the side feed and bottom exhaust crew.
AND so is the author of
http://www.s4biturbo.com/art-dvtests.php. Mike, who runs
http://www.vastperformance.com/ has emailed me to say
>On every 1.8T, 2.7T, 4.2V8 Turbo, 2.0TFSI turbo w/mechanical conversion,
> I've ALWAYS mounted the boost inlet on the SIDE of the valve, just as
> you were describing in your email. This is the orientation that VW and
> Audi uses from the factory and it's always worked great for us. The
> other reason I don't put the boost inlet to the bottom of the piston, is
> because if there happens to be a lower pressure on the vacuum actuation
> side/nipple, I don't want the valve cracking open with the boost pressure.
>
> On the contrary, if you do orient the valve in the opposite fashion of
> VW/Audi, you must make sure that the actuation signal/nipple sees the
> same exact pressure as the inlet so it's less likely that it will crack
> open and bleed off boost.
>
> I hope this helps....if I were you, just use the VW/Audi convention that
> I'm used to and you should be ok.
