2011tdiproject
Veteran Member
Hello, fellow automotive enthusiasts. I'm starting this thread to analyze the possibility of using the oem turbo as the high pressure turbo in a compound setup. I apologize if I missed any threads where this was discussed, I looked for several hours, but came up rather short.
Specifically, what I am wondering is will that little oem turbo flow enough on the exhaust side with the vanes fully open?
In the diesel pickup world, compounds using the stock turbo as the high pressure turbo are relatively common, including applications where the stock turbo is a VNT or VGT setup.
I've seen a few variations of this. Stock VNT turbo, no additional wastegate, no wastegate on secondary turbo. Stock turbo, additional wastegate (on the common rail VW, the high pressure EGR tap on the manifold would probably suffice) with no wastegate on the secondary turbo. Or also with a wastegate on the secondary turbo. Or no wastegates, with both turbos being VNT controlled.
Each one of those setups behaves a little differently, and can be tuned to operate a little differently. Of course, the most tuneable would be 2 VNT turbos, that way the pressure ratios across each compressor could be mapped in 3d, for the sweetest spot at any operating condition. That would be incredibly cool.
At this point, I am theorizing that if the stock turbo cannot flow enough with the vanes fully open, I might not want to use it. My intuition tells me an additional gate in parallel with the stock turbo would be less efficient, or create more pressure in the exhaust manifold, than a larger VNT primary turbo that wouldn't need an additional gate.
Same thing if a non VNT secondary turbo is sized perfectly to achieve the pressure ratio increase desired without a wastegate, it will create less backpressure than a smaller one using a wastegate. However, I'm guessing it will be more laggy also.
What I would be trying to accomplish with the design is quick spooling at low rpm, while progressively biasing the pressure ratio increase toward the larger turbo as the rpm increases. I don't know what my target boost would be between the stages exactly. But say the stock one is tuned to a peak of 20lbs, but say by 2700rpm it's already going out of its efficiency range. I'd progressively open the vanes and drop the boost across that turbo, and have the secondary make more, to make up for it. The more the motor would rev, the more the pressure ratio would be biased across the larger turbo. So I would not necessarily be able to hit 50psi of boost at 5000rpm, but it should still broaden the powerband quite a bit vs stock.
That's how I'd like it to work, anyway. Reality may be a whole different thing.
I won't be surpised if no one here knows specifically about this, as I have not been able to find a single compound setup using the oem turbo on a common rail vw, but I thought I'd ask anyway.
Specifically, what I am wondering is will that little oem turbo flow enough on the exhaust side with the vanes fully open?
In the diesel pickup world, compounds using the stock turbo as the high pressure turbo are relatively common, including applications where the stock turbo is a VNT or VGT setup.
I've seen a few variations of this. Stock VNT turbo, no additional wastegate, no wastegate on secondary turbo. Stock turbo, additional wastegate (on the common rail VW, the high pressure EGR tap on the manifold would probably suffice) with no wastegate on the secondary turbo. Or also with a wastegate on the secondary turbo. Or no wastegates, with both turbos being VNT controlled.
Each one of those setups behaves a little differently, and can be tuned to operate a little differently. Of course, the most tuneable would be 2 VNT turbos, that way the pressure ratios across each compressor could be mapped in 3d, for the sweetest spot at any operating condition. That would be incredibly cool.
At this point, I am theorizing that if the stock turbo cannot flow enough with the vanes fully open, I might not want to use it. My intuition tells me an additional gate in parallel with the stock turbo would be less efficient, or create more pressure in the exhaust manifold, than a larger VNT primary turbo that wouldn't need an additional gate.
Same thing if a non VNT secondary turbo is sized perfectly to achieve the pressure ratio increase desired without a wastegate, it will create less backpressure than a smaller one using a wastegate. However, I'm guessing it will be more laggy also.
What I would be trying to accomplish with the design is quick spooling at low rpm, while progressively biasing the pressure ratio increase toward the larger turbo as the rpm increases. I don't know what my target boost would be between the stages exactly. But say the stock one is tuned to a peak of 20lbs, but say by 2700rpm it's already going out of its efficiency range. I'd progressively open the vanes and drop the boost across that turbo, and have the secondary make more, to make up for it. The more the motor would rev, the more the pressure ratio would be biased across the larger turbo. So I would not necessarily be able to hit 50psi of boost at 5000rpm, but it should still broaden the powerband quite a bit vs stock.
That's how I'd like it to work, anyway. Reality may be a whole different thing.
I won't be surpised if no one here knows specifically about this, as I have not been able to find a single compound setup using the oem turbo on a common rail vw, but I thought I'd ask anyway.
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