Hi Mike, your spring selection will depend on how much vacuum your vehicle draws at idle. You want to run a spring tension that's slightly stronger than your idle vacuum. We attached a instruction sheet to the product page showing which spring to select based on your idle vacuum.

Hi Justin, you generally want the BOV spring a little bit weaker than however much vacuum your car pulls at idle and/or with the throttle closed. This ensures that when you let off, the vacuum created by your engine will overpower the spring and force it open.