Intake manifolds?

[MiracleMax]

Spring pressure (for obvious reasons) plays a big part in the longevity of that stuff too. Heavy spings at low engine speeds mashes the needle bearings flat, at higher engine speeds there is less of a sustained load on the bearings so they dont tend to flat out as quickly or at all.

I dont know you finacial position or your willingness to try new technologies (if available for your application, but titanium retainers and conical valvesprings might help in this area.

The idea behind the two is a reduction in mass and a spring that is more resistant to harmonics which allow a light sping with the same or better rev capability.