Valve spring pressure is the force a valve spring applies to the valve, measured in pounds at two points: seat pressure with the valve closed at installed height, and open pressure at full lift.
Installed height and spring rate together set seat pressure, so height gets measured from the retainer to the spring seat and corrected with shims before anything else. The targets are cam- and lifter-specific. ISKY’s Nolan Jamora, quoted by Dragzine, put a flat-tappet limit near 145–155 lbs seat and 400 lbs open, a stock-lifter hydraulic roller around 135 lbs seat and 300–350 open, and a hydraulic roller on HPX lifters at 185/535. Use your cam grinder’s numbers for your combination, not those. Springs fade with heat and cycles, so what matters is the drop from where yours started. NHRA notes an on-head tester lets you confirm springs are holding installed load; Pro Stock checks after every pass, a bracket engine maybe once a year.
A spring that has quietly lost pressure lets the valve float at RPM, and a floated valve usually meets a piston.
Read the full post:
When to Replace Rod Bolts and Valve Springs: The Parts That Fail Without Warning