@trumpetb I will insist that "stretching" the spring will not improve performance. We may be able to squeeze a bit more life out of it, but if our stretch is NOT ABSOLUTELY STRAIGHT, the spring is damaged and will actually have worse performance.
Stretching a spring for more tension also assumes that the stretched wire will hold the stretch, that would have to be proven on a case by case basis.
I would insist that Hookes law does not apply here.
F = kx
“F” being the amount of push or pull is on the spring
“k” being a constant, indicating the stiffness of the spring
“x” being the distance the spring was pushed or pulled
Our distance that we stretched is a deformation of the wire and the "new" base for testing. Stretching the spring means that we exceed its elasticity, thus destroying all of the original parameters. What K is left after deformation depends on the spring material and tension. Now, if we retemper after stretch, we have a new animal. I have never read about retempering valve springs.