In a wastegated turbo, a certain percentage of the exhaust gas will bypass the main turbine wheel to limit how fast it spins (reducing or limiting boost pressure).
It is imperative that a large open dump pipe be used in this system to promote mixing of wastegate gas, and turbine gas.
As most wastegates open, it blows directly into the path of the turbine flow. Resulting in turbulent gas that will not flow readily in the one direction it is being sent.
What happens if you open the dump pipe up?
If you open the dump pipe up, it helps get this turbulent gas mixed, and flowing well down the rest of the exhaust system.
What happens if you have a narrow or small dump pipe?
If you have a very narrow or small dump pipe, the gas will continue to be turbulent and continue mixing much further down the system. Lowering overall flow.
The ideal solution is to have a divider in the dump pipe that straightens the wastegate gas out, and points it in the right direction before it has to start the mixing process.
Some people play around with divorced dump pipes. The problem with this is, it really limits the size of the pipe you can use for either/or.
Why large bell mouth dump pipes work the best
Off boost you want as much flow as possible for the main turbine, and on boost you need a lot more for the wastegate. So limiting your pipe size one way or the other will only benefit one condition (more flow on boost or more flow off boost). This is why large bell mouth dump pipes work the best under all conditions.
VNT/Variable Vane Turbo/Variable Geometry Turbo
The boost control of these turbo’s is achieved effectively by altering the size of the exhaust housing through use of ‘vanes’ (to direct the exhaust gas at different areas of the turbine) to spool the turbo.
The best analogy I can use for this is a large bicycle wheel.
It is important to note with a VNT turbocharger, as no wastegate gases need to be mixed in the dump pipe. It can effectively be made much smaller than a wastegated turbocharger with no ill effect to performance.