Yippers®

IS atmosphere. The fact that there is a panel filter at the end of the pipe (capable of flowing enough air to satisfy 1.8 liters of volume @+/- 7000 rpm does not mean it is a closed loop system. If the vacuum at both the intake hose and the manifold behind the throttle plate were equivelant, why wouldn't AUDI have saved three feet of hose, a casting detail in the manifold and the brass fitting and just plugged the BPV vacuum hose into the intake hose along with the dump valve itself? Ignoring the boost side, by leaking vacuum across the diaphram of the BPV this is exactly what happens. The intake manifold and the intake hose vacuums and pressures try to equalize.

dingster

Under vacuum there is no significant pressure difference between the left side and right side. I am not saying this is a closed system. The whole system has only one opening, the intake and I believe the size of the MAF is the bottleneck. When you're driving whether the engine is underboosts or under vacuum there is always vacuum between the intake and turbo. In there it is never atomphere pressure unless your engine is off. Once you start sucking air in you're introducing vacuum.

The only time(under vacuum) that you can alter the overall system vacuum to this system is to create another opening to the atomsphere which does not happen in the case of a torn diaphram.

hurricanesteve

At idle:

There is a level of vacuum at the intake manifold...that has to 'suck' against the pressure drop of the intercooler and piping to get air from the turbo. So pressure is higher at the BPV (high pressure side) than at the manifold because of the pressure drop through the piping.

If the diaphragm ruptures allowing the high pressure side to "short-circuit" the intercooler path, you'll get an increase in pressure at the manifold (slight, because the vacuum line is small)...AND my boost gauge is T-d into that line. So you get flow up that vacuum line to my boost gauge, short circuiting the standard intake path.

EdP

I will open one up to better explain what I mean by a vac leak.

I am not talking about a leak across the diaphram from one port to another, but the portion of the BPV that "contains" the vaccuum that operates the diaphram itself. that too can rip, as it's connected to the diaphram as well. that would be an internal tear that would prevent the diaphram from operating.

I guess I topic jumped too far :/

Yippers®

drops. Vacuum on the intake side (read air filter side for ease)of the throttle plate is at maximum at WOT. When you introduce a leak into the intake manifold at idle (leaking manifold vacuum hose of any type but BPV here) you are decreasing the vacuum behind the throttle plate. I know it sounds screwy but vacuum is screwy .. especially when you rotate plates across a throttle body or carburator.
There is a reason that manifold vacuum lines are used instaed of lines located at other places in the intact tract. Take a look at this URL for a quick and dirty about vacuum at, below and above a throttle plate.<ul><li><a href="http://www.emsgas.com/vacuum1.htm">http://www.emsgas.com/vacuum1.htm</a</li></ul>

dingster

When my bailey melted, it melted at the bottom of the piston(if you orient the BPV with the nipple pointing up). The leak in this case is only from the high pressure side of the BPV to the low pressure side. in this case manifold pressure shouldn't be affected.

On a stock BPV, it's not a solid piston like the Bailey, rather the piston/diaphram has "skirts" on the side of the piston. It that skirt is torn then there would be a leak from the intake hose to the manifold and it could potentially affect the manifold vacuum. in either case a leak is not good.

This is why my vacuum was not affected when the Bailey failed.

Yippers®

Probably bored the crap out of everyone else but .. lol.. who cares.

EdP

I'll try and take pics. It's amazingly simpe, and sparse in components - the plastic piston, one very small o-ring at the top 1/2" of the plastic piston, and the case. Thats it. The o-ring cannot hold manifold vac - that o-ring is TINY, and manifold vac on A4's is pretty high for any engine - you can see 30 Hg. This also worried me in that that same o-ring holds back boost pressure from diluting vac as well - so I am now not so big on baileys, esp for big turbo applications. IMHO of course.

Yippers®

pressure is "sealed" via a ball shape on the bottom of the piston sealing against an angled face on the valve body. Using my mouth I can blow air past this "seal" but not past the o-ring. It isn't enough flow to be an issue with losing boost.

I can also open the piston by sucking [no suck start a Harley comments please : ) ] so I know I am getting a seal on the vacuum side.

My issues with the Forge are that it appears prone to failing to reseal once drawn open. I think, in spite of the centrally located spring, the piston can **** in the cylinder. A secong o-ring would diminish this. I also think, to be effective, the piston ball/flange seal interface needs re-thinking, as the seal area is very narrow.

I like the OEM design in that, unless and until the diaphram tears it provides an excellent seal between the intake hose and the compressor outlet hose. I installed the N OEM valve and I think I will stick with the Bosch part.