If it has the typical 3.3:1 buggy diff ratio, I would say a 14 to start. (That's assuming you are running 2S LiPo.) Run the truck for 3-5 minutes and check temps.

cool will do i am waiting on my rx8 i already have the pro4 4000kv

You don't mention track size but a good place to start is 14/15.

i would say its a medium size track ,and thanks

i finally got my rx8 and my pro4 4000kv in my scrt10 and i must say i love it , i am getting the hotwire for it , then i will be able to set it up ,should i play with the timing or just leave it as is any infor is appreciated