twstdmtl

5&6 ports are opened by actuators that that hook to your cats via a nipple or a bung. When you reach a certain RPM @3800-4,000 you have enough back presure 1.2-2 PSI to open the ports. If you replace your cats you need to weld a nipple or a bung into the pipe so you can connect your actuators up. Otherwise they do not open. Or you can do the unspeakable which is leave them open all the time which takes away from low end torque... YUK!