Dump valve question

[Azman]

Morning again guys,

Just brought a cheap dump valve to see if my problems are from the stock one, a metal type forge piston replica (have to admit had one on the old coupe which worked really well to be honest), question is it comes with three springs!! Which to use?

Stock Green (up to 19psi)
Black (up to 25psi)
Silver (up to 35psi)

Thoughts guys, thinking of the black one but what do you think?

Cheers
Az

[urquattro-island]

hmm

i am running this, with red spring and two shims, and still get the woossooosoossshh sometimes. its not holding shut, ??? thought this was supposed to hold???

[Azman]

Hmm Might just dump the silver one in then

[EspenW]

Test it with a pump and homemade adapter

It probably holds quite a bit more pressure if you route the pressure hose into the piston side of the DV, and not into the side port.

[Rusty]

My forge runs 1.7 bar spring with Homefries, no probs in a year. IMO run the weaker spring. If it leaks then replace it, you'll know cos the car will run a bit toss.

[S2central.net]

Az - I say put the Black one in... The green should be fine on standard ECU but when you get fried up then you'll need at least the Black one as ~21psi on full boost will start to open it (if I understand the way those piston type valves work correctly).

[Rusty]

No Paul, you're a bit off there, the vacuum connection to the top of the piston means that there is no pressure (or very small) pressure difference across the valve, these are called twin piston type. The spring is for idle and for post-woosh closing with vacuum on the top. The spring does not need changing for increased boost, purely for the pressure difference at idle.

[S2central.net]

uhhhhhhhh - let me think about that... Got any diagrams of one ?

If spring is only to counteract pressure difference at idle, then why have three flavours of spring. The penny hasn't dropped yet

[Rusty]

The vac cinnection is from the IM, the other side of the piston is just after the turbo, net pressure difference is only the pressure drop of the IC and pipe work so pretty small, say 0.2bar at a guess.

The spring makes up differences at idle when there's <<P_Atmosphere in the inlet and approx P_atmosphere at the turbo, the spring governs the rate at which the piston closes after venting and before more boost is generated.

No diagrams but I've got a new twin piston DV in front of me and the installation instructions too... linky

[EspenW]

When pressure at the intake manifold equals pressure at the turbo outlet, the spring isn't needed. However, during the time required to reach this state, the spring prevents loss of boost pressure. In addition , there will always be a slight pressure difference from turbo outlet to manifold, from pressure loss due to friction (IC, hoses, etc)

[Rusty]

Yeah, sounds like we agree there EspenW.

Some older turbo cars like Ford RS have single piston DVs with no spring, just vac connection, see Forge DV001, these open at idle, hence why you need a twin piston on a car with air flow meter. Not sure how they stop them from leaking under boost, never really thought about it much, I guess it doesn't matter that much or something as it'll be pretty much constant.

[S2central.net]

OK - I think I get it now - thanks...

[Rusty]

Here's a fag packet sketch.