The formula is Hp.= GPM X PSI divided 1714 x % pump efficiency.
The 2-stage pumps are 85% efficient.
When a 2-stage pump down shifts it will continue at approximately 1/4 it's rated gpm.
The formula is from Northern's catalog.
When motor recommendations are made they figure the maximum rated psi plus allow for actual continual hp. rather than advertised hp.
I checked the factory specs for the 2-stage pumps and found the 16 gpm and smaller pumps drop to 1/4 the rated gpm at 650 psi.
The 22 gpm pump drops to 1/2 the rated gpm. at 650 psi.
The 28 gpm pump drops to 1/3 the rated gpm. at 650 psi.
Now that I found some new specs I'm in trouble . Picked up a 22 gpm pump thinking I might be able to push it with a 9hp. Robin using the 1/4 theory. Guess it's time to look for a bigger engine.
I was very surprised when I found those specs and I,m hoping it was a misprint.
I have been working on building log splitters since I retired ,as a hobby. I'm on version number 5. The goal is a splitter that is relatively inexpensive yet has speed and all the bells and whistles.
If the 9hp motor and 22 gpm pump setup don't work I'll put the 16 gpm pump back on. Sell it and start on version #6.
Damn the wife is going to lock me out of the work shop . Oh well!