Compressor Map For 4/71 and 6/71 Roots

BDS info states that a 4-71 on a 231ci engine OverDriven 20% will make 18 PSI
That is the max limit for their 4-71 blower.
BDS 6-71 on a 289ci engine at 1:1 drive ratio produces 21PSI

These are rough estimates. So if you run a 6-71 at 5% UnderDrive on a 225ci engine, you will be around your 20PSI goal.

The PSI for the Roots blowers are controlled by the throttle position.
Mash the gas pedal from idle and see an instant 20PSI, Roots boost is linear. 20PSI at any RPM.

Several centrifugal maps show the "island" being between 60 and 70% adiabatic efficiency. With a wide range of HP to drive the blower 20 to 40 HP.
The 6-71 will eat at least 75HP and have less than 50% adiabatic efficiency.