DrCharles said

A 389# spring rate means that compressing the spring one inch will add 389 lb to whatever the valve seat load is. More likely you have around 0.5 inch valve lift, guessing about 120# seat which is then 120+(0.5*389) = 315# at max lift.

(The force on the cam nose itself will be multiplied by your rocker arm ratio, and some other factors like opening the exhaust valve against residual cylinder pressure, also. But these rules of thumb are usually noted for just the valve spring loads).

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