What is the purpose of fitting two valve springs to each valve?

The purpose of fitting two valve springs to each valve is primarily to reduce valve bounce. In high-performance engines, especially those with high RPMs, valves can experience significant forces, particularly during rapid opening and closing. When a valve closes, if the spring is not adequately strong or if it has insufficient damping, the valve may not settle into its seat correctly, leading to what is known as "bounce." This bounce can adversely affect engine performance by causing poor sealing and potentially leading to engine damage.

By using two valve springs, the system increases the overall spring pressure and improves the damping characteristics. The inner spring, having a different rate or a tighter fit, can help control the motion of the valve more effectively, ensuring it remains in contact with the seat after closing and reducing the likelihood of bounce, especially under high-frequency operation conditions. This setup allows for a more reliable and efficient engine operation, sustaining high RPM without compromising performance due to bounce-related issues.