Is they use cheap low flow silinods(SP?) and the jets are not as well built. Let me use a example A freind of mine is running a 100hp shot on a H22 honda it was a ZEX kit. I talked him into a NX kit and after changing at a 75shot he made 35more RWHP the 100 NX shot made almost 62 more rwhp then his ZEX kit. I like the Nitrious Express better myself but the NOS kits are good also.