Dual Port Internal WG Actuator for Bewstzzz

No worries. Radial turbines are fascinating and frustrating. They don't behave in an easy to grasp intuitive way...and to get that intuitive grasp you have to play around with the full maps (including efficiency) and real test data for the engine you're working with. Even then it's still easy to make mistakes with turbine wheel and housing selection.

It's rare to find any published turbo test data, turbine or compressor, from any non-OEM turbo company. I am not trying to knock ATP at all; they do great work with the resources they have. It takes mega millions to set up a lab that can easily test turbos on gas stands reliably and generate maps from data. That's why you rarely see any maps at all from the smaller companies that aren't Tier I automotive suppliers.

It's kind of a moot point anyway since you'd need a full engine dyno cell to get all the data you need from your engine to do a proper turbo match even with the exact maps. That's why turbine housing and A/R selection is more of an empirical thing. Make some educated guesses, choose a housing, test it out on your engine, then evaluate and iterate if you're not happy...

It may also be academic, but your engine will flow more than the choke flow curve from the turbine map, even after un-correcting back to physical flow. Any wastegated exhaust flow adds to the turbine flow. So at any operating condition where the wastegate is open, the turbine should be right at choke, and the total exhaust flow will actually be up above that curve.

We've been pretty happy with the turbo, except for the WG issue. How is the flow through the stock Garret t25 internal WGs, what is the diameter on the WG opening?

I guess the other option is to use an external WG somewhere in the system if there's boost creep issues.

culberro said:

We've been pretty happy with the turbo, except for the WG issue. How is the flow through the stock Garret t25 internal WGs, what is the diameter on the WG opening?

I guess the other option is to use an external WG somewhere in the system if there's boost creep issues.

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The as-cast port diameter is just under 25mm standard, and it's at 90 degrees to the flow. This housing dates back to the SR20DET and RB26DETT OEM applications so likely undersized for what you'll be doing with it. You could port it out as is commonly done with MHI turbos.

The v-band 0.57 and 0.72 A/R housings are nice and modern, non wastegated, but flow with the 0.72 is lower than the older 0.86 A/R wastegated housing as you've seen. If you need the flow of the 0.86 you could always remove the swing valve, weld in a plug, and run a larger external gate.

This would be neat, so long as ATP makes them dimensionally similar to the original.

https://www.atpturbo.com/mm5/mercha...roduct_Code=ATP-HSG-043&Category_Code=GTHGT28

That is a nice solution, especially for packaging an external gate with a stock style manifold.

Turbine Housing offers FULL T3 flow! Based on GT30 geometry and the .72 A/R is between .63 and .82 A/R GT30R.

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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...

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?

culberro said:

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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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.

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.