Kyosho X-Speed has 2 major types, they are X-Speed for AM type and X-Speed for 2.4GHz. If your Mini-Z is 2.4GHz, both type can use, and X-Speed for 2.4GHz is faster than AM one. If your Mini-Z is AM, and you want to use X-Speed for 2.4GHz, it better to do the FET upgrade.