Not sure if this has already been posted but it's helpful. Apparently adjustable spring BOV's will work better.
Posted by Phatron
PhatBotti Tuning - Tuning FAQ & Misc Info - NASIOC
Blow Off Valves (BOV) & By-Pass Valves (BPV)
By-Pass Valve - does exactly what the name suggests. when you let off the gas and the throttle closes, the BPV opens to BY-PASS the turbo and send the air back to the intake. This has 2 benefits...it prevents compressor surge and at the same time utilizes that pressurized air to keep the turbo spooled when shifting.
Blow Off Valve - Blows off air to atmosphere at throttle lift. Bad for MAF based vehicle since you are releasing air measured by the MAF...this makes the car run rich and blow fireballs when shifting. BOV's only prevent comp surge and the turbo will slow more while shifting compared to a bpv since the air isnt recirculated to keep the comp spinning.
In both cases when you lift of the throttle the pressurized air now takes the path of least resistance.....if you put a 30 psi spring in your bpv and you're running 20psi and you let off the throttle....the spring is going to be too tight to open so the air has to go somewhere....which is through the compressor backwards. Thats compressor surge. You're shoving air through the compressor backwards.
BPV's and BOV's dont do anything at WOT...unless they are leaking. Their main functions (preventing compressor surge and utilizing the pressurized air to keep the turbo spooled while shifting) happen when you let off the gas.
I recommend the forge race bpv with spring kit or a Tial bpv with spring kit. The spring kits add adjustbility that stops them from leaking at idle. The HKS SSQV is the worst valve on the market...stay away at all costs.
.....Its not only the HKS BOV.....its most on the market. All the bov's come with chitty weak springs that leak at idle. Basically i like any bpv/bov that has a spring kit and is adjustable.