It depends. On my X4, I have the LRP Quantum Pro Competition, but you can use either Novak Spy or LRP MICRO. I have 8 cells on the X4 and 6 cells on the SP. Also, for receiver, I either use a Tekin FM Receiver ( a little hard to get), or HiTec FM Micro Receiver- $32.99 only.
If you want to see a 40+ mph Pro-Z, check out
www.latencyproject.com and go to the very bottom of the front page, on the left bottom, you will see an archive, click on 04/01 to 04/30.
Click on 40 MPH Pro-Z by NML. This is the same size as a Mini-Z.
I don't think a lot of people are aware of it. Perhaps when we do a tutorial on how to set it up, people will be less intimidated by it?