Vacuum alone is enough to lift the piston in our valve even with our stiffest spring so the valve will open regardless of boost pressure with any of the springs. The only issue comes in very specific instances when manifold vacuum and charge pipe pressure are both very close to atmospheric and the pressure differential between the two are so low that the valve will "flutter" open and closed as that differential cycles back and forth around the pressure needed to crack the spring. Nothign to worry about as the forces involved are very low. That being said, here is my cut and paste "spring" reply:
The springs are used to account for variances in atmospheric conditions, and the relationship between boost and vacuum that different cars will see. To simplify it greatly the more boost that you are running the stiffer a base spring pressure you will need to compensate for the pressure differential between the manifold and the charge piping right at initial boost onset. Cars with few modifications running at or near factory boost seem to respond best with the yellow or blue springs. Cars running a bit more boost the blue and a few shims. The red spring is best used on cars running upper teens or low twenties.
That out of the way let me quickly go over how to adjust the valve. I recommend using three different spring tensions and performing some real world testing. Say for instance the yellow, blue, and blue with shims. Drive normally for a few miles (part throttle shifting, stop and go, merging and passing for some wide open data) then do a few wide open throttle runs. Do this with all three setups and compare the results, with "seat of the pants" or with data logging. If there is no difference use the softest setup. If the middle seems best that is the setup to use, if the stiffest works best perform the same tests with the next stiffest and compare.
That is the more complex answer. The easy quick answer is "use the blue spring if the cars is modified, if the car seems sluggish add two shims to the valve". 95% of the cars out there using the valve are using the blue spring.
Hope this helps. Let me know if you have any other questions.
