,
The EFR 8374 is a different story with 20psi of boost pressure landing near the peak efficiency island in the middle of the compressor map. The match of the EFR 8374 to this particular hypothetical engine compromises low-end torque for better mid-range and top-end power.
With the EFR 6758, you can see on the turbine map how high the turbine expansion ratio goes hitting a maximum of 3.25. The higher the turbine expansion ratio goes, the higher the resulting back pressure.
The EFR 8374 reduces the turbine expansion ratio at the top engine speed of 8000rpm from 3.25 for the EFR 6758 to 2.20.
The reduction of the turbine expansion ratio results in greatly reduced engine dP. At 8000rpm, the EFR 6758 has a dP of -19 psi whereas the EFR 8374 is only -2 psi. The bigger turbo has better engine dP versus the smaller turbo from 4000rpm to redline.
Of course, the compromise with the big turbo is the loss of low-end torque. The EFR 6758 has more torque below 4500rpm. For a 2.0L gasoline turbo engine, I like to use 300lb-ft of torque as a reference point for spool-up. As you can see, the EFR 8374 lags behind the smaller 6758 by about 1000rpm in hitting 300lb-ft. Note that the larger 8374 makes more torque than the smaller 6758 from 5000rpm to redline even though they are running the same boost pressure. Again, this can be attributed to the lower turbine expansion ratio and therefore better engine dP of the larger turbo.
