Water wheel calculations are fairly straight forward. Potential Power (HP) is (H*Q*g)/746, or in this case (5)(8)(32.2)/746 = about 1.75 HP. Actual calculations would be a function of waterwheel efficiency and actual mounting location (your H value could change significantly depending where you mount it).
Torque then is your HP / (RPM/5252).
RPM can be calculated from w=r*v/r^2 where V is the velocity of the falling water, w is the angular velocity of the wheel, and r is the radius of the wheel. Velocity squared = H*2*g, using our same H value we get SQRT(5*2*32.2)=17.9 ft/sec. w=(2.5)(17.9)/(2.5*2.5) = 7.16 rad/sec. RPM = 7.16*60/2*pi=68 RPM.
So your maximum theoretical torque is 1.75/(68/5252) = 135 ft-lbs. This will again vary widely based on your efficiency, design, and location of your water wheel, but it gives you a place to start if you find a torque curve for the compressor.
