does anyone know how to adjust the blow off valve to get that really cool "turbo sound?" If not then is there an aftermarket one that is recommended? thanks!

 

What year is your S40? There's some good options that I know of for the 2nd gen S40's.

But either way, your car doesn't have a blow off valve. It has a circulation bypass valve (CBV). It essentially serves the same function as a BOV but does it by venting the excess pressure back into the intake system so there's no loss of metered air as oppose to a BOV venting the excess pressured air into the atmosphere.

Post more info about your car and we'll get yo going in the right direction.

 

its a 2005 2nd generation. thanks for the insight. So based on what you're saying, there is no way to get the BOV sound/effect?

 

With your current setup that is correct. Though, you do have a couple options.
1. (my recommendation) You can upgrade to a Forge HD CBV. It's a cylinder style CBV instead of the rubber diaphragm. It's a little louder as it's an all metal unit.
The Forge CBV plus a metal intake with an open filter (ie.. K&N or Injen) will make the "pshhh" sound significantly louder.
Forge CBV CBV Upgrade Kit S40/V50 and C30/C70 - ViVA Performance
Injen intake Injen SP Air Intake Kit, Volvo S40/V50, C30/C70 - ViVA Performance

2. You can get a CBV block off plate and install a true BOV (hks ssqv) on one of the metal intercooler pipes. I know this works properly on a 6 speed manual, I'm not 100% about an auto. IIRC it could cause problems with the autos not cutting off fuel in between shifts and can cause rich spikes.
Block off plate CBV Blocking Flange, S40/V50 and C30/C70 - ViVA Performance

 

First, Turbos don't traditionally run any circulation style Wastegates, that is for Superchargers primarily. What you're talking about in a Turbo is normally a "Wastegate"
On cars that measure air volume via a MAF sensor and have a turbo from the factory, you will have a poor running condition when venting to atmosphere due to the ECU anticipating the surge of air in the intake. The surge is minimal but enough to effect how the car runs. Where as engines that read air density via a MAP sensor are not effected by this because it is metering off the pressure reading in the intake manifold itself. An example of the this is the 2nd gen 3sgte (89-94 MR2, Celicas, etc.) uses a MAF sensor so if you vent to atmosphere the car will run like crap but the 3rd gen 3sgte (94-99 JDM onry yo), which relies solely on it's MAP reading can be safely vented to air. I have the latter of the two in my MR2 and use an Apexi BOV. Even vented back into the intake the stock CBV (BPV in toyota world) still have a bit of a woosh when letting off the throttle but it's certainly nothing compared to a ricey SSQV.

 

^^^ I didn't say anything about wastegates at all...

 

which spring should i use with the CBV upgrade kit?

 

Depends on how much boost you're running.
You should ask the manufacture or seller of the CBV what the springs are supposed to be used with your boost level. If you bought it from ViVA, email George and ask. He's great about getting back to customers quickly.