The stock RB26 camshaft has a 240/236mm duration with 8.58mm lift (intake) and 8.28mm exhaust. Hiro says if a cam has a lift of 9.5mm or larger or the cam has a base circle larger than the factory 32mm, modifications to the head are mandatory. There's also a catch when determining mods. An aftermarket camshaft that has a smaller base circle usually means it also has a higher lift such as the HKS Step Pro L 280/280mm camshaft we plan to use. The Pro L set has a 10.7mm lift (intake) and 10.3mm lift exhaust cam profile with only a 29mm base circle. This camshaft has a smaller base circler but because of the lift, it will come in contact with the head if not modified.