On the top of the suction housing right over the bearing, there should be a plate that tells you the model of your pump then the impeller cut. Most say JC-A Meaning its a JC pump with an A-cut impeller.
You mentioned cars. do you understand Torque Converters in Automatics?
Similar hydraulic principles apply here.
Unlike props, impellers all have the same 'pitch'. they do however have different lengths. Impellers are named A, b, C etc depending on how much is cut off of the backside of them, closest to the vanes in the bowl.
You should choose an impeller cut that allows your engine to operate in its peak TORQUE range.
Generally each cut of the impeller results in a 500rpm increase in engine speed.