The difference is fairly simple. Less weight. Less weight in a part that moves in the engine assy = Less rotating mass = Less energy required for movement.

The lighter your rotating assembly in an engine, the faster the engine will be able to rev up/down and in some cases the more power it can make.

 

HKS and GSC are solid cams, Stock are hollow.

 

So again does anyone make aftermarket Hollow Cams for the Evo 8?????

 

Adam at z1 said the Jun and Tomei are made using OEM billet cores, ergo the Jun and Tomei are hollow. Elemetary, my dear Watson.