[smoked240]

Originally Posted by PoorMans180SX

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.



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.



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.

Lots of great info as usual! I'll be starting the build this winter on the compound setup.