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Originally Posted by derass
Wouldn't that energy be used better in spooling the high pressure turbo?
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Sorry for the confusion here, I can see now what I was saying was not clear. When using a single-scroll low-pressure turbo, you can put both of the wastegates before it. The "primary" gate would then be the one that is controlling overall pressure the engine sees.
Quote:
Originally Posted by tb13
That setup is really cool, looking at pictured below this wastegate placement is interesting.
Although I have zero real world experience building crazy setups like that one, I think I understand why his setup works so well. I think most would expect a wastegate between the first and second turbo, like he is running, to send exhaust gases around the first turbo to the second once a certain boost pressure is achieved. This keep the first turbo from seeing insanely high shaft speeds and helps spool of the second larger turbo.
His second wastegate placement got me thinking though. I think most would expect a second wastegate before the second turbo venting to atmosphere, but with a motor seeing very high boost that would never work due to the limitations of flow through and around the smaller turbo. An engine seeing 40+psi of boost at high rpm is going to have an incredible amount of energy flowing through the exhaust, more than enough to spool a pair of turbos and keep them spinning. Having a second wastegate before both turbos allows for some of this excess energy to be bled off in the high rpm that would more than likely overwhelm the small turbo and wastegate between turbos. It also keeps back pressure low and the exhaust gases flowing smoothly through both turbos.
Correct me if I am wrong though, I would love to learn more about setups like that one. I really like the idea of using a pair of well sized turbos to keep great response, setup some wastegates and forget about it instead of trying to spool a massive turbo using something like Nitrous and extra tuning.
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What you're saying is correct, but also depends on setup and boost control strategy a bit. Having a wastegate between the two turbos does work, but seems a little less efficient to me. If you were to use a twin-scroll low-pressure turbo, having both wastegates before the low-pressure turbo isn't really possible though.
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Originally Posted by feito
Question: does the boost from the bigger turbo (once it starts pumping) force the compressor wheel from the smaller turbo to spin faster even if (the smaller turbo's) wasgate is open?
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No. The low-pressure turbo simply operates at whatever pressure ratio that the high-pressure turbo dictates, the lowest possible being 1:1. This means that the turbo spins fast enough to simply bypass the airflow from the high-pressure side, at least with any boost control strategy I can envision. This may or may not be the best way to do this in a gasoline engine.
You can do some math to figure out where on the map the low-pressure turbo is operating if the inlet pressure is above atmospheric, and Kevin Jewer (the guy with the setup pictured) does a decent job explaining the math in the yellow bullet thread, but I'm not convinced that his theories are right. He does have the most extensive pressure and temperature data logging setup of anyone with a compound setup I know though. Again, I would love to build such a setup with some serious temperature and pressure datalogging and run various boost control strategies to see how efficiency and power output are affected.