Quote:
Originally Posted by spooled240
One major variable you're overlooking is the design of the turbine housing and internal wastegate valve itself. It's positioned in a way where faster exhaust gases will not flow through it efficiently. Wastegate position is very important in many long tube turbo manifolds where the gate needs to be positioned in a way where its partially facing against the flow of the exhaust. The same is true for turbine housings where the exhaust is flowing at an extremely high velocity in one direction. Its like trying to catch water from a stream with a cup perpendicular to the direction of flow. If you look at the EFR turbos, the internal wastgate is also facing the entry of exhaust gas for optimal wastegate flow.
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none of that matters when we hold the mass flow of the engine constant
If you have two identical examples with identical mass flows and turbine shaft speed, and you upgrade the downpipe or post turbine pressure drop efficiently, the only thing that will change is the wastegate fraction of ejected volume. The lower pressure means more exhaust can flow through (any type) of gate, whether internal or external or merged or divided etc... makes no difference, the flow rate will increase, which causes the gate to close slightly compared to previously, which means it is no longer "working" as much as previously. So if it was at 100% previously now it will be 99% or less, you gain gate effectiveness with no change to power, shaft speed, turbine behavior. Even though the post turbine pressure is lower the turbine never speeds up because we are manually controlling the shaft speed as a constant instead of a variable.
I get what you are saying, i.e. any change to the post turbine or pre-turbine will influence power, turbine shaft speed, rate of change, there are myriad variables that will wander around. But because there are innumerable molecules and pressure gradients in play in such a complex situation, for engineering examples, modelling, and classical physics in order to determine which variables are dependent and which are independent (If I wanted to create a dynamic equation and a matlab circuit to model this with a computer) we would be interested in mathing out individual examples while holding many variables as constants in order to see which variables influence which aspects. And so we lock the volume flow rate and external ambient temperature and pressure as constants and see that the only change in a post downpipe situation with an infinite number of potential downpipe sizes (i.e. 1.001", 1.002", 1.003", ... all the way to 5.0" or 10.0" of downpipe) will result ONLY with a change in wastegate potential volume ejection and not much else, no matter what type of gate or plumbing is involved. e.g. we don't need to guess whether 1" downpipe is better than 5" to understand that there will be some improved volume flow rate of any wastegate situation with the larger downpipe series.