Trent - glad to hear that beauty is actually back together and running!

Tial BOV's are kind of a pain in the ass, honestly, particularly when they're being ran as the only BOV. The higher the spring pressure, the more force holding it shut against vacuum when it's idling, etc, but the more resistance it has to opening when you need it to. If the 12psi spring is the cause of the idle issues, that means the vacuum is overpowering the spring so the 6psi spring would be even worse at idle.

But with the 6psi spring, less force is required to get it to crack open at throttle letoff, so it'll open more quickly and more easily and generally do a better job at preventing compressor surge. But in turn it'll likely cause even more of a problem at idle than you're currently seeing.

If you wanted it to work with the current 12psi spring, try adding a restriction to the vacuum reference connected to the BOV. The downside is that any restriction added there will also slow BOV response at throttle letoff.

For most purposes, particularly in single-BOV setups below 500whp, I honestly prefer the HKS SSQV to just about anything else. Since the SSQV's valve operates perpendicular to boost pressure, you don't need to play with different spring pressures to prevent boost from pushing the BOV open, as you need to do with the Tial, TurboXS, Greddy, etc.
They're also responsive and well made, though I've come to prefer the sound of it without the silly whistle-fin installed.

 

i happen to know that even with the right springs the Tial BOV's stay open a little bit at idle, my thought is that maybe this is causing air to leak in past the MAF? try unplugging the vacuum line going to the BOV and capping it on the intake side to see if that helps your idle

 

Wreckless, I actually was thinking of adding the 6psi spring in addition to the 12psi spring. I was told that could be done but again, like you said, it will be more difficult to open. The Tial is actually mounted using an adapter to the HKS flange, so I could easily put the HKS valve back on there and sell the Tial, but I'd love to keep the Tial. Oh well, may just put the HKS back in.

 

Remove the vac source and mark the side of the piston that moves so you can tell if it does. Next start the car and take note of how much the piston cracks open. Install enough shims springs whatever to keep it shut at idle. If both springs fit then trim one as needed. Set it a bit tighter so it stays shut during decel when the car pulls a bit more vac. Then leave it alone dont tune for sound or surge as you cant once it is set. I run a hks racing bov and a blitz bov. For years it was just the blitz different piping, throttle bodies, intercoolers, etc. all make the same bov sound different. Personally when running a bov i perfer one with a adjustment screw on the back

 

Have you try putting a check/one way valve in the line to keep it close at idle?