Dump valves

[JohnW]

Hi,

I watched with interest on Saturday as Phil was fitting several sets of dump valves to the RS4s.
I understand that the OE plastic ones tend to suffer from split diaphrams, and even the Audi dealers I've spoken to say changing these is recomended.

What I'd like to know though is
(1) where is the best place to buy these, and
(2) which spring should be used (I noticed there seemed to be more than one spring supplied - 3 different colours ?)

Cheers guys,
John.

[PhilT]

Try this....

[Jon]

i've inheritted the forge ones on mine. don't know what springs are in them. forge tend to be "de rigeur" these days.

hope this helps

[runrowsam]

Call me stupid, but exactly what difference does the change in springs make? I'd be worried about using a tougher spring, as naturally it'll increase the pressure within the unit on lifting off [img]images/graemlins/crazy.gif[/img] and pressure is the reason I don't like turbos!

Rock on
Sam [img]images/graemlins/thumbs.gif[/img]

[Jon]

the spring regulates the blow off point - bit like a pressur cooker

[JohnW]

Thanks guys.

I gues that'll be my first mod to the car when I get it [img]images/graemlins/wink.gif[/img] [img]images/graemlins/1syellow1.gif[/img]

So from reading your link Phil, should I assume you are all running with the Red (strongest) springs ??

Cheers,
John.

[Dippy]

JohnW,

Remember that DVs are a compromise, so if you start messing about with the spring tension then you must accept the disadvantages with the advantages.

The purpose of the DVs is to keep the turbines spinning by keeping the air flowing after the throttle is closed. This reduces lag (because the turbine does not need to spin up again) but also reduces back-pressure on the turbo which stresses it.

So as soon as the throttle closes, you need your DVs to open quickly. Opening is done by the ECU by feeding a vacuum from the engine to the diaphragm/piston. This works against the spring to open the valve. So a weak spring is an advantage here.

However when you open the throttle again, you want to get the DVs closed as quickly as possible so that the boost rises again quickly and forces air into the manifold. The only thing doing this closing is the spring, so here it needs to be a strong spring.

So you can see that there needs to be a balance between opening rate (and degree of opening) and closing rate. The trouble is that this balance is different for tuned cars. Whilst the DVs are open, air from the turbos is rushing past the diaphragm/piston, so this gives an extra resistance for the spring to work against.

Thus it is useful to have a range of springs with different tensions to suit different cars with different boost/air flow rates.

The thing to bear in mind is:
1) If the spring is too weak, then the DVs might not be fully closed when they should be. Since the S4/RS4 ECU works on a control loop based on measured boost, this means that the turbos will be flowing more air (and spinning faster) than they should be.
2) If the spring is too strong, then the DVs may not open as quickly as they should. This could result in excessive pressure spikes in the system, and of course result in some turbo lag.

Comments/corrections gratefully received.

[JohnW]

Okay,
I can see now that it is a trade-off.

From what you are saying it is possible I'd need different springs for an otherwise standard car compared to a chipped car.

so has anyone done comparative checks/tests using the different springs ?

My driving tends to be around 40% twisty A & B roads, and 60% hammering (err, sorry - 'cruising' [img]images/graemlins/grin.gif[/img]) down the motorway, if that makes any difference.

Cheers,
John.