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No matter what design is used the vertical head and gpm determine the amount of energy you can get out of the system. A big head lets you build a big wheel for more torque and gpm increases the size of the buckets increasing the wieght (torque) that came be converted to hp.

With turbines and enclosed pipe more head equals more psi. High gpm values increases the potential to apply the psi to a larger surface area. If you can only supply a 1 inch pipe not so much power. A 12″ pipe is moving a lot of energy.