Good post!The stiffer spring rate is a double edge sword. It makes sure the valve closes however if it is too stiff it will create a loss of HP on the top end. You only need enough spring pressure to control the valve, if the pressure is too low it will have “valve float”. If too stiff you will lose power as it takes more to turn the motor to overcome the pressure.
The main function of the spring is to control the motion of the valve caused by the cam. Some cams are very gentile with open and close rates which can take lighter spring pressure, while more aggressive cams will require heavier spring rates.
The trick is to find the proper rate to control the valves while at the same time not going too light to induce valve float.
Valve float is when the rocker loses contact with the cam at high rpm due to the spring rate not controlling the motion very well. This can be seen on the dyno as a flutter and loss of power.
The main reason for high rate springs is when you have forced induction with high boost levels. If you have a spring rate of 95lbs on the seat and you are running a turbo with 30lbs of boost you effectively have 65lbs on the seat, so you will often see turbo cars run very high spring pressures 125+ on the seat to combat this.