picclock,
Goto your local auto-truck junk yard and find some old swash plate,
AC cylindrical compressers, they made them in 3,5,7,9, piston units.
They shorten and or lengthen the stroke to change the cubic inch's
on the fly depending on the load needed. Vicker's also made hydraulic
swash plate pumps with a hand crank on the front to vary the speed
using the same as above.

Ken.