pressure is more related to valve weight (size) and
RPM range. I now have conical springs and retainers
for the old style big stem valves, stock valve and big valve
heads both. They have less spring and retainer weight, and
way better valve control, and typically they can rev 1000+ RPM
higher with no other changes just the ticket for all the V6'ers
looking for a big HP boost for cheap! The better springs cost more
of course, but if you are changing anyway it is not that much more.