I think that the main readon is valve float, once you get to a certain point the springs on the valves cant close the valves quick enough, so that is one way the redline is determined. I was reading an article recently about a new valve technology, it uses spheres with slices cut out of them, when a whole part of the sphere is over the opening, the valve is closed, when part of the slice is over it, the valve is open. They tested this on a ford v8 from a lincoln that previously dynod at 240 hp after the new valve technology was installed, the engine made 576 hp and could easily rev to 17krpm. The power increase is from the lack of drag from the valvetrain, the rev limit is from the directnes of the valves, no springs to limit the rpms, i think that it was called radial sphere valve, but i could be wrong