you need to know the flow rate of the engine too get this use

L x rpm x VE x pr
--------------------- = CFM
5660

to convert from cfm too lb/min (which most compressor maps are in multiply by 0.07)

L = engine capacity in liters
rpm = i hope you know that one
VE + volumtric efficiency
PR pressure ratio


boost +atmospheric pressure
------------------------------------ = pressure ratio,
atmospheric pressure