First of all, read this.....like most US sites, its not 100% correct
Here's a rough diagram of how the BOV looks inside.
It's better than an aftermarket one because it's designed as a safety valve. It's supposed to open with excess boost, an aftermarket one won't.
It's also open at idle.That's why vent to atmo makes them stall, it's got nothing to do with metering air and dumping it making it go rich, it's because air goes in through the BOV, bypassing the AFM in turn dropping the Hz signal to zero, making it stall.
The pics have gone but there's a link here to another site.http://www.bimmerforums.com/forum/showt ... hold-boost
There's 4 chambers in it, 2 will try to keep it closed with positive pressure, the other 2 will try to open it with positive pressure.
So, the spring plus pressure in A will hold it closed
Positive pressure in C will hold it closed.
Positive pressure in both B and D will force it open.
Lets say we get the boost up to 13 ish PSI. that's with respect to atmospheric pressure.
In A there's 13 PSI plus the spring.
In C there's zero.
In B there's a bit less than 13 PSI.
In D there's 13 PSI.
A and D are equal and opposite so cancel leaving just the spring.
C is always zero.
That leaves just B, C and the spring.
As the boost rises, so does the pressure in B which eventually overcomes the spring pressure so it vents.
The pressure in B will always be between the pressure in C and D due to leakage/vent hole.
Lifting the throttle, the pressure in A drops and the pressure in B and D overcomes A and the spring, it opens.
Likewise, when the engine is idling, THE VALVE IS ALWAYS OPEN. it's very important.
Reversing it works(C to the Y pipe, D to vent) , so long as you've got 9B or 13G turbos that hold less that 13 or so PSI to the red line.
The spring + boost in A plus boost in C will hold it closed.
Atmo in B and D, it won't leak.
Lifting the throttle, boost at C will still try to hold it closed as will the spring, vac at A has to be sufficient to open it. Again the pressure in B will be between C and D, that determines whether it will open or not. Higher boost = it's harder to open to the point where it won't open at all until the boost drops.
So that's how it is from the factory.
Next step. Modification.
There's the tools needed to mod it easily
Looking back at the diagram, all you need to do is to block the vent hole between B and D (RTV).
Then make a hole in the plastic divider between B and C.(melt it with a hot welding rod)
( Alternatively you could make and external hole so that it looks like the GReddy)
Then it'll be the same as most aftermarket valves with 3 chambers.
B and C will always be at the same pressure so they cancel out.
That leaves just A plus the spring fighting against D.
A and D will always be at the same pressure so long as the throttle is open, they cancel out leaving just the spring to hold it closed.
You can see the tester I made, I got a modded one up to 60 PSI(air compressor and air line) without leakage and the hose blew off.
If you're feeling bored and want some entertainment at the expense of retards, check this out.http://www.3sgto.org/threads/3007-Open- ... d-Loop-BOV
quote from barryboys.
Yes, yes, yes, that might be so, but you're clearly missing the bigger picture.....