sergei wrote:That applies to internal. Flow dynamics is not simple subject.
But roughly speaking there is some venturi effect on the wastegate ports (from inside the turbine housing of course) when there is low pressure after turbine wheel .
See on the CT20b the ports are drilled at 90' into the housing, having also a narrowing in there and faster than designed gas flow would cause reduction in flow of through the wastegate ports.
Appreciate the link to Wikipedia, I kinda understand venturi effect already....... low pressure is a relative term. Do you think the pressure behind the turbine is lower than atmospheric pressure, or just lower than in the turbine housing?
The turbo doesn't spin at a constant relative speed to the engine either, it has an actuator which allows it to pump more air until a measured pressure level is reached before it starts the gases bypassing.
The only disadvantage I can think of putting a big exhaust on a GT4 is that you end up hearing 3SGTE noises.