[tooski]

I did a search, read a lot, but no info for what I was looking for.
Which is: What makes a 60 psi pump different from an 80 psi pump?
I would surmise that the pump build specs would be different to produce different pressure specs. That would mean that the 60 psi pump should be unable to produce 80 psi. Otherwise why would there be 2 different pumps.
I have read that a washer can be placed in the spring housing to raise the pump pressure. I believe that only raises the point at which the relief operates. Why shim the spring to relieve at 70 psi, when the pump can only produce 60 psi?
Why does the 80 psi pump require a longer spring cup? Why not just use a stronger spring?