There has been a lot of arguments on AS about DV vs DVs.
Lets have a think what is happening when you change gear in a turbo engine:
1. Under acceleration the engine is producing lots of exhaust, spinning up the turbo and supplying pressurised air (more oxygen by volume) to the inlet
2. Now you lift off and depress the clutch to change gear
3. The exhaust pressure and flow disappears and the turbo is spinning like a flywheel
4. We now have a positive pressure between the inlet turbine and the engine and a lower pressure between the inlet turbine and outside atmosphere. Thus air will try to flow backwards through the inlet turbine, which will stall it (because the exhaust turbine has no force acting on it to keep it spinning)
5. To prevent stalling the turbo, the DV exhausts pressure in the inlet manifold either to atmosphere or back to the inlet side of the turbo.
6. A 'Diverter Valve' exhausts to the inlet side of the turbo, which is open to the atmosphere, so the best it can do is equalise the pressure across the inlet turbine
7. A 'Dump Valve' exhausts to atmosphere, meaning that the best it can do is equalise the pressure across the inlet turbine.
Thus, there appears to be little difference in actual effect and I would conclude that manufacturers dump the excess into the inlet side of the turbo to reduce the noise. Basically, the air inlet side of the turbo IS atmosphere (with a pipe on it).
Rally cars use the brute force method of stopping compressor stall, they simply explode some fuel in the exhaust, keeping the turbo spinning and the pressure across the inlet turbine positive.
