b18c cam upgrade

if i was going to upgrade my cams from stock to zex cams would i need to change my retainers and valves and if so what else would i need to change thanks and can someone tell me what might be the best cams for my gsr and im gonna be shooting 50 to 75 shot of nos not all the time but once in a while

Depends on the cams if you need retainers/springs. Some are mild enough that you can keep your valvetrain stock, and the beefy ones need new retainers/springs.

what about the zex cams stage 2 cause i seen on a web site they added it to their gsr and it made 19 hp and it didnt say anything about retainers and valves but toward the bottom it said it did so