What is the difference between "Constant Speed - Pump Curve" and "Constant Speed - No Curve", for the transient pump type? (or "Constant Speed - Pump Definition" and "Constant Speed - Four Quadrant Characteristics" for CONNECT Edition Update 2 and greater)
"Constant Speed - No Curve" / "Constant Speed - Four Quadrant Characteristics" - A pump that operates at constant speed throughout the transient simulation, using a built-in 4-quadrant characteristic curve based on the selected specific speed (in the "transient" tab of the pump definition). Use this option if the constant speed pump can potentially operate beyond the first quadrant (positive head/flow/speed) or if a pump definition is not available (in which case you would only need to enter the nominal head and flow). If you're seeing unstable results near a pump that is set to "Constant Speed - Pump Curve", it could be that the pump needs to operate in the other quadrants - try changing it to "No curve".
"Constant Speed - Pump Curve" / "Constant Speed - Pump Definition" - This option is used to represent a pump that does not change speed during the transient simulation (no shutdown or start up) and operates only in the first quadrant. (no negative flow) It utilizes the pump definition selected in the pump properties (instead of a 4-quadrant curve) to adjust the flow produced by the pump in response to changing system heads at its suction and discharge flanges throughout the simulation period. Use this option if the constant speed pump will only operate in the first quadrant during the transient simulation (no reverse flow) and if you want the pump to exactly follow it's pump definition during the transient simulation. If the pump may need to operate in the other quadrants, it is best to choose "No curve", else the results may be unstable.
Source of the default Specific Speed for pumps and turbines
General HAMMER FAQ