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Old 11-12-2019, 08:26 PM   #26
Duder
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That is a nice solution, especially for packaging an external gate with a stock style manifold.

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Turbine Housing offers FULL T3 flow! Based on GT30 geometry and the .72 A/R is between .63 and .82 A/R GT30R.
To me this means that they chose the throat width to match the GT30 turbine wheel, so it'll be a bit oversized for a 28 and undersized for a 35. Throat width is a cast dimension so it's about economies of scale I would guess.

I don't have any personal experience with that exact housing but I can ask them for the scoop if anyone's seriously interested...
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Old 11-13-2019, 02:13 PM   #27
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To me it seems like the oversized throat width would cause the 28 series turbo to be a bit slower to get moving, but have higher overall flow capacity.
Is that a correct assumption?
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Old 11-13-2019, 06:53 PM   #28
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Originally Posted by culberro View Post
To me it seems like the oversized throat width would cause the 28 series turbo to be a bit slower to get moving, but have higher overall flow capacity.
Is that a correct assumption?
I'm not an aerodynamic expert, but essentially it is un-shrouding the wheel a bit, and increasing the nozzle flow area which will slow down the gas velocity.

The nozzle is the thin little ring between the volute (scroll) and the wheel inducer (major diameter). Throat width is the axial width of the nozzle, along the direction of the shaft. The nozzle area would be pi*(inducer diameter^2)/4 x (throat width), approximately.

I think what happens with response is that the housing will act like a bigger A/R than it physically is, because the flow is not being accelerated as much in the nozzle. Un-shrouding the wheel a bit means more of the wheel is acting as inducer; it extends past the corner of the blade to include some of the contour area. The wheel wasn't designed to work that way so it will likely reduce turbine efficiency. Velocity vectors at the inducer will be "sub optimal." Flow velocity at the turbine inlet would be reduced, and the amount of power you can make from a turbine is directly proportional to inlet velocity...so you'd now need a higher turbine pressure ratio to make the power your compressor needs.

The measurable effect would be higher exhaust manifold backpressure vs. what you'd get with the same A/R but smaller throat width that matched the wheel inducer width.

Overall flow capacity most likely won't be affected. Usually the turbine stage chokes in the volute - i.e. the only thing that will increase flow is a larger A/R, up to some really large maximum A/R where it will start to choke in the wheel.
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Old 11-13-2019, 07:33 PM   #29
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Throat width is represented by the little yellow line I drew in the turbine here...it's a twin-scroll but same idea, just ignore the divider wall.



And then a nice simple diagram I found on a University of Cambridge site to illustrate what I'm jabbering about. This is a cross section through the turbine centerline. Flow is coming "out of the page" in the volute but spiraling in towards the turbine inducer. The volute itself is off-screen up at the top. Throat width is the left-to-right width of the inducer (section I) plus the clearance, and the shroud is the little bit of housing that follows the wheel contour as you move downwards and rightwards, along "meridional length S." Increasing throat width basically just removes the inlet part of that little shrouded area.

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