Lifters?

Hydralics have a more aggressive ramp that solids but suffer from lifter pump up at high RPM. They reason they have more aggressive ramps is that they do not have to have the gentle initial ramp designed to take up the clearance (valve lash) that is in a solid setup.

You can get 425hp out of a 340 with an agressive hydralic and still have the maintainance free design.