Originally Posted by Boosted Performance

As the exhaust back pressure increases (with RPM's) the plunger at the bottom of the waste gate is exposed to it. This works with the spring and opens the plunger/waste gate more as RPM's rise. The spring in the waste gate also gets hotter as exhaust temperatures rise, increasing spring elasticity, making it easier to compress.

This is why we have the boost controllers. As the timing was increased the drop in boost increased a bit as well. This is just the nature of a turbo system. The boost controller also helps in spooling the turbo a bit faster. You will find that most cars out there with a turbo system do have a boost controller of some sort.