Originally posted by Darth Fiero
Ok, first of all, GM calls it a boost solenoid, boost control solenoid, wastegate solenoid, depending on what publication you are looking at.
You are correct that it does not regulate boost pressure under WOT throttle. Therefore changing the spring inside would have no effect on performance other than determining when the bypass valve will close in relation to manifold vacuum. So in other words there is no point in changing that spring unless you want the bypass valve closing earler (in relation to throttle position/load). At WOT, the PCM is still going to command 100% duty cycle to the solenoid unless you are getting close to the factory top speed limiter as set in the PCM.
I did a test on my friend's 99 Regal GSE and found that his boost duty cycle began to fluctuate above 95mph which starter reducing boost. It felt like the car was surging but you could plainly see in the data stream that boost solenoid DC% was fluctuating. If you remove the vacuum line that connects the boost solenoid to the bypass valve actuator, the PCM will no longer have any way to cut boost. The actuator will still work as intended during decel and cruising modes due to the vacuum side still being connected.
Again, normally under WOT the bypass valve is closed, therefore allowing full boost. If you are experiencing some fluctuation in boost pressure you might be experiencing SC belt slippage.