HV pumps simply have longer gears with the same tooth pattern and number of teeth as the stock pump. There are no springs except the pressure relief valve (which is present in the stock pump, and can be compressed between your fingers). The extra volume of oil moved is at the same pressure unless the spring is changed - you can have more volume with less maximum pressure and vice versa.
1. An HV pump provides more pressure than a stock pump at idle and low speed before maximum pressure is reached (regardless of the pressure spring).
2. An HV pump reaches maximum pressure at lower engine speed than a stock pump.
3. Once the bypass pressure has been reached both pumps deliver the same rate, and the extra from the HV goes straight back to the pan.
An HV pump does place extra load on the pump shaft, which is driven off the cam.
If you have high mileage bearing wear and low oil pressure at low speed it helps. About 10 psi per 1,000 RPM is enough. If increasing oil viscosity slightly brings the pressure up, just do that (don't run 70 wt.!).
If not you don't need it.
