Well, first off a TiAL will stay closed under any boost you want even with a 7 PSI spring. So you can go as light as you need. What you NEED to do is go as light as it takes to reduce that "flutter". That isn't the BoV it's the compressor surging, which is bad. The question really is, how much vacuum do you pull at idle? If you pull 15in of mercury then that is -7.3PSI, you will need a 7 spring. If you pull 18in of HG it's -8.8PSI you should go with the 9. And if you pull 22in/hg that's -10.8PSI you should have the stock 11 spring. The larger the cams you put in the less vac you have. The less vac you have the slower your BoV will be to react, or some times it won't at all. This is where part throttle flutters come from, which can kill a turbo
Marcus has found that by running the 7 spring and shimming it you can get it to stay closed at idle and still open fast enough to not surge. And that is what is important."
http://www.lancerregister.com/showthread.php?t=255956
http://www.tialmedia.com/documents/w3_tial_bov50_sp.pdf
как я понял надо вакуум замерить и подобрать пружину исходя из значения