What I see looking at on line hydraulic motor calculators is:
For:
5 ci/rev 1500in-lbs motor that turns at 750 rpm. requires approx 15 gpm at 2,000 psi
23 ci/rev 4500in-lbs motor which turns at 150rpm requires approx 16 gpm at 2,000 psi
In other words, I do not see a lot of difference between the two motors....... they consume the same gpm while turning at a speed difference ratio of 5 which happens to be the ratio of the motor displacements.
Tell us more about your pump driven by the 9 hp motor. Your operating pressure too.
Have you asked about the 23ci motor because you have one?
Are you open to buying a different motor?
A calculator to show the HP required to drive a hydraulic pump producing 15 gpm at 2,000 psi is 21 HP.
This is real HP not what a gasoline engine is rated at.
It may turn out that you do not have the engine power needed to drive the motor you want and further, you are not driving the existing motor at the speed you think you are.
Have you done anything to establish exactly what speed the 5 ci motor is running at.
Dave M7040