Small body coil spring. If same spring rate how much difference in feel between a 10 in and 8 in spring

If the springs are the same rate there should be no difference in feel.

Driver says car doesn’t feel same. Helper spring has just a little pressure on it with 10 in spring

With torsion bars, the shorter the bar the quicker it reacts,so I would think it would be the same with coil springs.

Springs of the same rate but different heights will react exactly the same unless the shorter spring reaches coilbind where the longer one wouldn't. The shorter spring is likely lighter weightwise though, and a good percentage of that is unsprung mass, so there is a small theoretical advantage to running a shorter spring if it is lighter than a longer spring and you don't reach coilbind throughout your travel.

If the driver is saying it's way different feeling, run the corner through it's travel with the short spring on it and see if you're getting coilbind.

I’ll have to check and see if coil space is the same. I was told that the shorter spring would react quicker.

Coil spacing could be the same and you could still end up with coilbind going from a tall spring to a short spring. Only good way to check for coilbind is

A) Figure out your travel and put the spring in a spring smasher, compress the correct distance
or
B) Have a fat guy stand on that corner of the frame while you Jack up the Spring.

I am definitely no George Bignotti or Karl Kinser so take my opinion with a grain of salt, but everything I have ever read or been taught has told me there is no difference in the way those springs should react. As long as all ride heights, mounting points, etc. remain the same the car should act the same with the caveat of any changes due to unsprung weight.

The only other thing that comes to mind is that perhaps one of the springs is worn out and no longer rates what it's supposed to rate. Maybe you've ran the 10" for a few seasons and now it rates at 90% of what it originally did while the new 8" spring actually rates what it says on the box. Just spitballing ideas here.

10in about 15 races old. Not saying it might not test at 160

If helper spring has any load on it with 10" spring and not with 8" spring they are going to work and feel different.