King, I've been studying the same situation for trying to make a hydraulic post hole auger. You also need to know the psi limit of your system (where the relief valve goes off) which is probably around 1600 to 1800 for an old tractor. Newer stuff and bobcats can go higher. You also need to know what rpm and how much torque you need out of your new hyd motor.
Surplus center has some formulas and deep into Baum hydraulics web site are some motor sizing formulas. It all depends upon your max gpm, max pressure, and then what you want for motor torque and motor rpm.
Torque is proportional to your pressure but since pressure goes up to your relief valve point, it remains fairly constant over the range of rpm. (something like a hyd post hole auger spinning in open air won't use the torque or cause the pressure to go up much but as soon as you start digging, the torque demand from the digging will drive the pressure up to meet demand up to the max the relief valve allows)
Rpm is proportional to the gpm of your pump. So you can regulate the speed of your motor by how much you throttle up your tractor.
The tractor pump only puts out the full 16 gpm at close to full throttle so you can idle it back and get less pump gpm and motor rpm.
Something else to consider is the size in cubic inches of your motor. Generally the bigger in size, the more torque you will get but it also takes more gpm to run a bigger motor at the same speed as a smaller motor. But then the bigger motors to maintain rpm, torque, and pressure then need more horsepower behind the pump which is why you have to throttle up the tractor to get more horsepower to run the pump.
If you don't need the torque, then a smaller motor would allow you to run your tractor at a slower speed and use less gas.
With my auger I'm designing, I'm more interested in torque than rpm but want to keep it under 100-105 rpm for digging. Slower than that won't affect its digging capacity that much.
If you want to come up with 540 rpm for a pto replacement, then that is more problematic.
You need to take it somewhere to flow rate the system as your 16 gpm pump probably doesn't put that out any more with age. With the right fittings, valve, guage, and a bucket you could probably do it yourself. But you need to size the motor to run steady at you max psi and gpm flow.
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