Dump valves/wastegates are not always fitted but are normally required to shut down the boost then the vacuum in the manifold rises due to taking your foot off.
"The Lotus Esprit does not come fitted with a Dump Valve for the turbo. This is unusual because, especially in high boost applications, the back pressure caused by lifting off the throttle can cause the compressor of the turbo to stall. In extreme cases, the compressor can actually shatter.
After a dump valve is fitted, you will notice an immediate change in feel to the car, especially after changing gear. This is because the turbo is kept spinning during closed-throttle conditions and is therefore able to provide full-boost quicker."
To take advantage of the spin up lag a different/additional dump valves can reduce the pressure more rapidly and therefore decrease the load on the compressor thus decrease the spin up time for when you put you foot back down
Dump valves and wastegates are different:
Waste gate is on the exhaust side and opens at a predetermined boost pressure to bypass some of the exhaust gas and hence limit the boost pressure and turbine speed.
Dump valve is fitted in the air side between the turbo and inlet manifold and opens when the throttle is shut to vent the air and stop the turbo stalling, as has been said.
There is little point in having a dump valve on a diesel engine because you don't have a throttle and aren't therefore in danger of closing off the inlet manifold suddenly at high boost pressures, and stalling the turbo.
see wat i mean??
All club meets will be held at the Woolpack pub on the second Sunday of every month from 1pm onwards. See you there, *Trouble*
Subject: dump valves on diesels 
Wed Feb 03, 2010 8:26 pm
Wow, thanks for that Rixy, great info dude.. 