turbo exhaust manifold questions
Discussion
I'm building a motor with a twin scroll turbo and about 4 inches between engine and turbo inlet for low lag. I'm designing the manifold in SolidWorks and plan to have it investment cast in stainless. The turns from cylinders 1 and 4 will have some relatively tight turns, but no where as tight as in the stock log type manifold and I can have a smooth transition in size and shape from engine to turbo.
The motor is a GM 1.4 and the inlets into the log style stock manifold are quite a bit larger than the outlet ports on the head. I assume this is for anti-reversion.
Since I'm using a twin scroll it doesn't seem like I really need to worry about anti reversion, at least in terms of reflecting pressure waves, since there are never two exhaust valves open at the same time in the two halves of the manifold. Matching seems like it would reduce turbulence and maintain smooth higher flow velocity into the turbine impeller.
Can I remove the step by making the inlets to the manifold match the outlets on the head?
The motor is a GM 1.4 and the inlets into the log style stock manifold are quite a bit larger than the outlet ports on the head. I assume this is for anti-reversion.
Since I'm using a twin scroll it doesn't seem like I really need to worry about anti reversion, at least in terms of reflecting pressure waves, since there are never two exhaust valves open at the same time in the two halves of the manifold. Matching seems like it would reduce turbulence and maintain smooth higher flow velocity into the turbine impeller.
Can I remove the step by making the inlets to the manifold match the outlets on the head?
Back here: https://www.pistonheads.com/gassing/topic.asp?h=0&... I asked pretty much the same question and got some replies, but I guess I'm still kicking it around.
Right now I'm about to just go with matched.
Right now I'm about to just go with matched.
"backwards" steps in the exhaust are really not any restriction during the blow down event, as the flow is highly turbulent anyway. Generally, unless you are using a lot of CFD/test data to optimise cylinder scavenging (ie F1) then you don't want deliberate LARGE steps. But, small steps, say of 1 to 3mm are really not going to make any difference to the flow losses, and allow you to have some sensible amounts of alignment (static & thermal) built into the design
OK, small steps it is.
I measured the ports in the head and they are around 36.5 x 20 mm. The exhaust manifold openings are larger, but not all the same size. You might think it's manufacturing variance, but the lengths vary form about 38 to 42 mm, or by 4 mm. OK, that's not a lot, but the gasket is quite precisely made and those opening vary in length from 43 to 49 mm and in more or less the same pattern as the manifold.
It's possible that this is due to the particular way the manifold is located and clamped to the head. The manifold is located front to back on the engine by a vertical slot that is a fairly tight fit on a stud near the center of the manifold. The rotation and height are located by two horizontal slots in the manifold that are at either end of the manifold. All the rest of the studs go through significantly oversized round holes. The gasket is similarly located and has hard metallic surfaces on both sides.
All of the nuts that clamp the manifold to the head bear against a thin smooth plate that sits on machined faces on the manifold. My guess is that the design assumes that the slots locate the manifold, but as the head and manifold expand and contract with temperature the two slip so that a seal is maintained, but there won't be forces big enough to crack the manifold.
Comparing dimensions the horizontal step around the side of the port opening varies from around 1 on ports 2 ad 3 up to around 2 or 3 mm on ports 1 and 4. vertically all the steps are around a one mm. I guess these aren't too much more than you might expect for tolerances and allowance for thermal expansion. Maybe with the accuracy of investment casting I can go a little tighter, but it doesn't sound like there is any significant performance available here.
I measured the ports in the head and they are around 36.5 x 20 mm. The exhaust manifold openings are larger, but not all the same size. You might think it's manufacturing variance, but the lengths vary form about 38 to 42 mm, or by 4 mm. OK, that's not a lot, but the gasket is quite precisely made and those opening vary in length from 43 to 49 mm and in more or less the same pattern as the manifold.
It's possible that this is due to the particular way the manifold is located and clamped to the head. The manifold is located front to back on the engine by a vertical slot that is a fairly tight fit on a stud near the center of the manifold. The rotation and height are located by two horizontal slots in the manifold that are at either end of the manifold. All the rest of the studs go through significantly oversized round holes. The gasket is similarly located and has hard metallic surfaces on both sides.
All of the nuts that clamp the manifold to the head bear against a thin smooth plate that sits on machined faces on the manifold. My guess is that the design assumes that the slots locate the manifold, but as the head and manifold expand and contract with temperature the two slip so that a seal is maintained, but there won't be forces big enough to crack the manifold.
Comparing dimensions the horizontal step around the side of the port opening varies from around 1 on ports 2 ad 3 up to around 2 or 3 mm on ports 1 and 4. vertically all the steps are around a one mm. I guess these aren't too much more than you might expect for tolerances and allowance for thermal expansion. Maybe with the accuracy of investment casting I can go a little tighter, but it doesn't sound like there is any significant performance available here.
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