Dump Valves (boost Control)

[T3k 0]

After doing some reading, it seems the best way to control boost levels and to stop a turbo ceasing to spin on liftoff due to raise in pressure after compressor, is to use a dump valve of some sorts.

 

What i dont know is the advantages/disadvantages of the recirculating type over the atmospheric type, i really dont want the noise of an atmospheric, (i want people to wonder why a standard gti killed them ), but if its better, then thats what ill do.

 

Also i can only find installation/application guides for atmospheric valves, does anyone know how to fit a recirc, or know of any documentation that shows how?

 

Ta.

 

Tris.

[jacobs53 0]

the only difference between a recirculating and atomspheric dump valve, is that a recirculating uses the excess boost (when butterfly is closed) and pushes it back into the the turbo, through the cold air inlet pipe. The closer to the turbo the pipe the better.

 

The only difference is when off throttle or gear changing the blades still rotates as air is being pushed into the turbo cold inlet, but it only lasts a short burst. It actually does make a huge difference on high boost cars with high lag turbos, as it keeps the blades spinning instead of stalling them, or slowing down momentium.

 

cheers lee

[adrian_gti 0]

i'm after a dump valve too, anyone know what fitting kit would git the TT conversion and is it easy for a monkey like me to fit?

 

cheers

[cybernck 402 1 Cars]

it's advisable to use a recirculating one on AFM-equipped cars anyway,

as they'll be running rich when because you're dumping metered air.

 

you only really need a dump valve when running close/over 1 bar anyway.

 

 

what does "boost control" in your title has to do with this though?

 

i thought i will be a question about mechanical and digital boost controllers.

 

i've changed it slightly now.

[adrian_gti 0]

hmm i was hoping the dump valve would sort the lag out bcos its quite noticable.

 

Any other TT owners experience heavy lag?

maybe it wont be so bad when the 2 stage boost is fixed?

[T3k 0]

Cheers lads, but two things that have just occured to me...

 

I read that the recirc dump tube should be as close to the throttle as possible, not the turbo?

 

Also, if im re dumping compressed measured air back into the system and im off throttle, surely the car will run lean, which is more dangerous than overfuel right?

 

Tris.

 

Edit, 1 other thing, the "vacuum reference tube", where is it fed from-to? is it before afm, to the ecu's vacuum sensor?

 

is it to measue the amount of pressure by measuring the vacuum as opposed to the pressurised side?

 

Sorry if these questions seem remedial, but i would like to understand it more for my rebuild.

 

Thanks.

 

Tris (again;))

Edited February 19, 2004 by T3k

[T3k 0]

and to answer your question cybernck, i was under the impression that you could control the boost levels by letting some of the pressurised air escape.

 

it does sound crude to me, is there some way of doing it other than this, and other than wastegate (which IMO is crude too).

 

Tris.

[Wiggs 0]

and to answer your question cybernck, i was under the impression that you could control the boost levels by letting some of the pressurised air escape.

You're thinking of a bleed valve. A dump valve is used to stop the turbo from stalling during gear changes. A bleed valve will leak air when it reaches a certain pressure, usually controlled by an adjustable spring-loaded mechanism. On a standard TT car though, the boost is controlled by the wastegate on the turbo.

[T3k 0]

that probably is what im thinking of, thx.

 

whats the best way of controlling boost then? wastegate/bleedvalve?

 

Wastegate will put less pressure on turbo, cuz it wont ever run at full bore (if set not to) where as bleed valve will not control turbo, only boost pressure into cylinders, right? so wastegate would be best for longer turbo life?

 

Tris.

[cybernck 402 1 Cars]

oh my... what a confusion!

 

 

bleed valve goes into the hose between the inlet manifold and wastegate.

 

if you want to raise the boost over what's set with using the wastegate, you

bleed some air/pressure from that hose out (hence the name bleed valve)

so that wastegate "thinks" the boost is lower hence closes a bit to raise it

to the new (higher) level.

 

 

you don't want dump valves to leak! :-O

[Guest g-spot]

dump valve sound annoying mine pisses me off after a while much prefer the sound of me mate wastegate chatter on his pulsar

[jacobs53 0]

I read that the recirc dump tube should be as close to the throttle as possible, not the turbo?

 

well thats very incorrect! as when the butterfly is closed (lifting off gas) the excess boost pressure will hit the closed butterfly, and do nothing, probabily slowly stall the turbo as it has nowhere else to go.

 

Where did you hear that from!

 

cheers lee

[T3k 0]

cybernck, now uve completely confused me, i thought the wastegate was the almost "valve like" thing inside the turbine housing side of the turbo that under certain circumstances allows some of the exhaust gasses to bypass the turbine wheel, thus reducing output boost pressure?? they are on opposit circuits are the not (bleed valves and wastegates)??

 

Tris.

 

Am working on finding the webby that told me to put inlet from recirc nearest as poss to throttle, history has gone unfortunatly so might take a while.

[blue1.9 0]

The wastegate is the valve in the exhaust housing allowing the gasses to bypass the turbine. The wastegate valve is actuated by a rod connected to a wastegate actuator. The wastegate actuator is connected (usually by a length of silicone tubing) to the output side of the compressor housing and as the boost pressure increases the actuator opens the wastegate slightly to allow the gasses to bypass the turbine and therefore limits the boost pressure that the turbo can produce.

 

Boost pressure can be permanently altered by adjusting the length of the rod that couples the actuator to the wastegate (so that, for example, a higher boost is needed to open the wastegate by a certain amount - raising the oerall boost).

 

Boost pressure can be controlled by adding a bleed valve into the tube between the compressor outlet and the actuator to 'bleed' off some of the pressure. So when the compressor is is producing 1bar, the actator is only 'seeing' say 0.8bar (the other 0.2bar is lost through the bleed valve). Adjusting the amount of boost bled off before the actuator can control the boost without adjusting the rod length and is a bit easier.

 

A dump valve is, as stated above, a device to dump excess boost when the throttle is snapped shut, for example during a gearchange. This stops the compressed gas between the throttle and the compressor trying to expand back through the turbo and causing the compressor to stall. It makes sense to have the dump valve near the throttle housing, but it isn't essential. The dump valve is actauted by the sudden vacuum in the manifold which pulls a piston up to unseat the valve so the pressurised gas can escape. Not too sure about the recirc side of things, but you are trying to get the compressed gas away from between the turbo and throttle, so I guess feeding the compressed air back into the inlet side of the compressor might be worth a try.

 

Hope that helps

Alex

 

Phew....

[adrian_gti 0]

some excellent info here guys, anyone know what size kit i should get for a dump valve?

 

cheers

[blue1.9 0]

I think the pipe that you are going to fit the kit to has a 50mm internal diameter.

 

Alex

[T3k 0]

jacobs, ive just re-read ur post and mine and realised i was unclear about the recirc's feed back to the pipework.

 

I meant it was as close to the throttle housing as possible BUT between the throttle and the compressor housing now does that make better sense?

 

Does anyone know where the Vacuum reference tube is fed from and to? is it the uncompressed side of the intake to some sort of meter for the ECU's use?

 

Tris.

[M3Evo 0]

With a recirc dump valve, the valve itself goes near to the TB and the pipe it vents to gets fed back to the air filter or pipework near it.

 

The vacuum tube for the ecu goes on the engine side of the throttle plate so as to give a linear reading to the ecu i.e. when on light load, the ecu reads large vacuum, when under full load, the ecu reads little vacuum and when under boost, the ecu reads negative vacuum (positive pressure).

[cybernck 402 1 Cars]

that's right M3Evo... but what puzzles me is why are the MAP sensors

connected to the boost hose before the TB instead to the inlet manifold

on my T16 .

[M3Evo 0]

Mmm, that is bizarre! Does it have an AFM too?

[cybernck 402 1 Cars]

no, no AFM... but it does have two MAP sensors .

 

probably as a safety measure but it gives a 5k rpm hard-cut rev limiter when one MAP sensor dies .