FSM gives injector crack pressure for a '78 930 as 2.1-3.2 bar. So ~31-47 PSI are the lower/upper limits of spec.
Things to check for a CIS injector:
- injector opens at correct crack pressure
- Spray pattern is a fine mist in an even cone shape
- check valve works (doesn't leak or dribble)
Any of these tests fail - replace.