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Andy M's problem

http://www.saabscene.co.uk/ubb/ultimatebb....3/t/000761.html
got me wondering: is there a problem with running a dump valve (of whatever type) that has a 'strong spring'?
Provided the system is still able to release pressure in-extremis, is this actually a good way to run a car - allowing boost/pressure levels to stay as high as possible for as long as possible, before releasing only as much pressure as is neccessary (with a small PHSST) to ensure no turbo stall?
And what of running a spring too soft?
Is this the 'hard' and 'soft' setting of adjustable valves? And if it is, why would you want to be able to change?

Carl
 

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Hmmm...

Think about the way a dump valve is configured- it effectively sits across the throttle butterfly, and opens when a pre-determined pressure differential is reached across the butterfly. With charge air pressure on one side of the dump valve piston and inlet vacuum on the other, the point at which it opens will then be governed by the spring. Too strong a spring will mean that a much higher pressure differential has to be reached- or in other words, the turbo will have to try to spin longer in to a closed circuit, resistance to flow increasing all the time.

Equally, I would say that if the spring is too weak, then running in to an "open circuit" the turbo will simply lose all pressure and speed too quickly.

There's a balance to be struck depending on the size of your turbo, it's inertia and the boost pressures you are running. As an example of that, my DV will operate before the inlet charge pressure even exceeds 0 bar if I take my foot of the accelerator sharply enough as it is larger than normal and has more inertia.

So, I would say that both too strong and too weak a spring will give problems with dropping the turbo speed too much, but for different reasons.

I don't see that the strength of the spring has anything to do with controlling leak under high boost either- this is down to the quality of the seal around the piston on the DV.
 

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Hi all

my problem was cured with a spring. I called forge and asked of potential breakages etc that could be caused by the higher spring load:

answer - No breakeages will occurr although you will notice a large amount of turbo lag (reduced spool up times etc). When the new softer spring arrives check that it isn't too soft(ie it will seep air due to too much pressure in the intake).

I guess all this depends what model you run and how much pressure your turbo makes. again mine is only making about 6.5psi. (feels like 1psi) Going to check the pressure later due to lack of air from the dump valve.

cheers
 
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