My friend has a Bailey dump valve on his Saab Turbo'd GBS Zero and it was a 2nd hand 'bargain' from ebay, but we think it might be broken..

When idling, the dump valve is 'open' i.e. the piston has moved outwards and you can see through the vent holes out the other side of the DV and you can see a spring. It always whistles on part throttle, not in the usual DV 'tisssch' but more of a high pitched whistle which makes us think maybe it has a leak. It's plumbed in before the throttle flap and the vacuum line of the top of the dump valve goes to the inlet manifold after the throttle flap.

So does it sound like it could be broken? Should you be able to see through it during idle? No airflow meter so it'd still run fine regardless of air leaks etc.

Any thoughts gratefully received!

Thanks

Sounds like it's plumbed in correctly, but it definitely shouldn't be open unless it's venting excess boost. Does it have a spring preload adjustment that you could tighten?

Some of the old sierra dump valves whistle like mad. Look on YouTube.

Sounds normal for a single piston valve. If it's a dual piston valve it's got issues. This is why I don't run a single piston valve on my engine - unfiltered air.

Was gonna post a long explanation on this. Waste o time I realised :-).
Short answer: Bin the crap valve. Refit the OEM Saab setup .
Clearly there is a massive amount of knowledge missing.

[Edited on 11/11/11 by Bare]

Whether knowledge is missing or not, if you can help the op in any way, you probably should

Open on idle is fine in certain setups, it just means that the turbo is gently spooling while the high vacumn in the inlet is pulling up the valve, okay you are spilling some filtered air, this is only an issue if you have a MAF in front of the turbo as it will be metering more air than is being delivered to the engine, you now need a recirculating valve.

If you are drawing in air then this is very bad so replace.

This is why it's generally bad to replace a recirculating BOV with an externally vented one when using a production EFI setup as it will have a MAF sensor straight after the air filter.

If you just have a MAP sensor then all is good infact a slight advantage as spooling times are slightly lower and you get a little boost the second you touch the throttle.

Have a read of Corky Bells book if you doubt me.

Regards Mark

[Edited on 11/11/11 by mark chandler]

I've a bailey twin piston DV fitted to my scooby setup which has a MAF so therefore needs to be closed on tick over to prevent unmetered air from entering.

Below is a link to a guide which explains the basics. Not sure if you may have a problem with the spring but have a look at the guide and see what you think.

BTW 10/10 for enquiring and learning

linky

[Edited on 11/11/11 by Mr C]

Helpful as ever Bare...I dont know why you bother posting most of the time. Never seen anything helpful or positive posted by you. If you have something helpful to say, if not say nothing.

Anyway...

You dont have a MAF so dont worry about any of that stuff.

An open valve at idle is correct, its held open by the idle vacuum. Even with a dual piston setup.

Revving the engine with no load wont close the valve as theres no boost, just slight vac or atmos.

Depending on the strength of the spring fitted it may dump quickly or slowly which will affect how it sounds. However you need to match the spring to the boost level you are running.

[Edited on 11/11/11 by flak monkey]

If you want to know the workings it's like this;

There's a piston inside which can move up and down, and a spring to help it stay shut. On closed throttle you have vacuum presented to the top and atmospheric pressure on the bottom, so at idle the imbalance either side of the piston causes it to open.

As soon as the throttle is moderately open then the pressure either side of the valve is equal and now, due to the spring helping slightly, it will shut.

When you come off the throttle there is an immediate vacuum on the top of the valve as the throttle butterfly has shut and at the same time boost pressure on the bottom, this moves the piston up, venting the boost. As soon as the pressure before the butterflies released it will close, or at least move toward the closed position, but as you are experiencing, the vacuum overcomes the spring and it hovers partially open.

This is not a problem if you have no air flow meter, only in that you are getting unfiltered air into the engine in small quantities.

The dual piston valve works on the same principle only it has a small piston inside a larger one. The vacuum only pulls the inner piston at idle and the outer one keeps the valve shut, only when simultaneously presented with vacuum and boost on either side will the second piston be "blown" open. This is the type to use if you have a maf before the valve.

If you want a look inside one just split it, takes 2 secs and you don't need a seal to rebuild it or anything, there's an o-ring in the top.

Hth
Stott

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Originally posted by Bare
Clearly there is a massive amount of knowledge missing.

[Edited on 11/11/11 by Bare]

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I was sure I saw a MAF on my friends 2.3T 9000, my 2.0T defo has a MAP sensor though, which engine has he got?
Also.. Cheers for the new word MakeEverything, "fora", had never heard it before - no idea if it's real but I like it,

Great, thanks to (almost) all for your comments - it was the fact it was open I found odd due to unfiltered air being drawn in as people have said, but it doesn't sound like its broken then, although perhaps the weird high pitched whistle is perhaps due to a weakened/old spring.

We would have used the original but must have thrown it out as we can't find it, and the boost pipes take such a convoluted route around the engine bay there's nowhere to plumb in the vent outlet from a recirculating one.

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Originally posted by GRRR
Great, thanks to (almost) all for your comments - it was the fact it was open I found odd due to unfiltered air being drawn in as people have said, but it doesn't sound like its broken then, although perhaps the weird high pitched whistle is perhaps due to a weakened/old spring.

We would have used the original but must have thrown it out as we can't find it, and the boost pipes take such a convoluted route around the engine bay there's nowhere to plumb in the vent outlet from a recirculating one.

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Originally posted by flibble
I was sure I saw a MAF on my friends 2.3T 9000, my 2.0T defo has a MAP sensor though, which engine has he got?
Also.. Cheers for the new word MakeEverything, "fora", had never heard it before - no idea if it's real but I like it,

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Even with a dual piston setup.

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Originally posted by coyoteboy

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Even with a dual piston setup.

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Not true, a twin piston setup holds the secondary piston shut at idle with a bias spring - the secondary only opens up with boost applied at the inlet when vac is applied at the vac line. That's the point of it, so MAF cars can idle.

Most turbos produce a enough flow to blow air out the single piston valve at idle, but under light load they'll suck.

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No need to take it apart had a look at a film clip.

On idle the "inner" secondary piston is closed preventing any unmetered air from entering the intake pipe, or metered air being lost. As the boost/revs pick up the outer primary piston closes as well covering the secondary piston and the vents in the body of the dump valve. when the throttle is closed both pistons simultaneously retract uncovering the holes and dumping the air once the air is dumped and pressures equalise the secondary piston closes again as the engine settles to an idle.

Film clip available via my build blog

V-Storm Build Diary



[Edited on 12/11/11 by Mr C]

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Clearly not everyone's knowledge is as in depth or complete as yours, hence why some seek the answers on enthusiast fora such as locostbuilders. It would be helpful and contributory if you actually shared your wealth of knowledge and experience rather than pissing on someone's fire.

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On idle the "inner" secondary piston is closed preventing any unmetered air from entering the intake pipe, or metered air being lost. As the boost/revs pick up the outer primary piston closes as well covering the secondary piston and the vents in the body of the dump valve. when the throttle is closed both pistons simultaneously retract uncovering the holes and dumping the air once the air is dumped and pressures equalise the secondary piston closes again as the engine settles to an idle.

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Fairly sure I said that

(Having stripped and rebuilt a few of them, and modified a couple to suit my own needs!)

No offence taken, just wasn't sure where it was aimed Hope that somewhere along the line we've cleared up some info for the OP!

Hmm, seems I was wrong about the dual piston valves, apologies.

I was pretty sure mine used to be open at idle (Forge dual piston) but closed under load. Was a long time ago now though.

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Originally posted by coyoteboy

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On idle the "inner" secondary piston is closed preventing any unmetered air from entering the intake pipe, or metered air being lost. As the boost/revs pick up the outer primary piston closes as well covering the secondary piston and the vents in the body of the dump valve. when the throttle is closed both pistons simultaneously retract uncovering the holes and dumping the air once the air is dumped and pressures equalise the secondary piston closes again as the engine settles to an idle.

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Fairly sure I said that

(Having stripped and rebuilt a few of them, and modified a couple to suit my own needs!)

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Interesting topic as I'm currently building this part of my supercharged project... I went with a 50mm tial type BOV.
Now I haven't had the engine running yet but is the springs function supposed to keep the valve shut at idle? and the different PSI spring rates are actually nothing to with the boost level at all but are linked to the vacuum the engine creates at idle and should be set just above....? The vacuum created at higher rpm throttle shuts will be greater and overcome the spring and allowing to vent..
Or have i got it wrong again LOL!

Assume all vacumn pipes are disconnected and its running 10psi of boost, set the spring a little above this so its not open when running on boost, however if you ge an overboost situation then is will bleed off pressure.

Regards Mark

But the vacuum line is an integral part of the valve operation??? The vac line turns to boost when on throttle and this is what really keeps the valve shut and the spring just adds a bit of extra pressure to tip the balance..?

From the forge website

The quoted spring pressure (Psi) relates to the un-assisted clamping force of the spring and should NOT be taken as an equivalent to boost pressure when determining suitability. Once a valve is fitted to a car it is subjected to various pressures that act upon the diaphragm or piston to aid its respective ability to seal or hold boost. An example of this, would be when using a FMCL007P valve on a 1.8T engine as found in the TT 225hp. The boost pressure on a "chipped" car can peak at 26psi. The recommended spring would be the Green or Yellow

The green spring is a 6psi item..

Also found this online...

First thing is first: The purpose of the spring in a BOV IS NOT to hold your BOV closed under boost pressure! Selecting a spring based on PSI is incredibly confusing/meaningless and it makes us wonder if the people who pioneered aftermarket BOV design in the first place even understood how they work.

All BOVs have a reference line coming into the top of the BOV from your post-throttle body intake manifold. Under high boost, the force holding the BOV closed is BOOST! The pressure coming through the reference line is equal to the pressure under the BOV piston. Therefore a VERY mild spring will hold it shut just fine under these conditions.

Rather, the purpose of the spring in a BOV is to hold the BOV closed when your throttle plate is closed, IE during idle and deceleration conditions. Under these conditions, the pressure in your intake manifold is much lower than the pressure in your intake piping (therefore under the BOV piston) so the BOV has a natural tendency to spring open. Being open isn't even a problem if you are recirculating your BOV into your intake. However if you are NOT recirculating, it can either allow unfiltered air into your intake or push metered air out of your intake which can be a problem.

Therefore, your goal when selecting a BOV spring and adjusting the BOV should NOT be to select a spring based on your boost level; that makes no sense. Your goal is to get the BOV to stay CLOSED during high intake manifold vacuum/closed throttle plate conditions, using the very smallest amount of spring energy possible. Using less spring energy will allow the BOV to snap open as rapidly as possible when pressure release is necessary.


I'm no expert I tell theee but that makes sense to me..

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Originally posted by Craigorypeck



I'm no expert I tell theee but that makes sense to me..

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