I use HPI Race vented CB on my Hyper 8 pro and Hyper 8.5 pro but I think the smallest they make is 12 or 13 teeth. I use the 13 teeth on my Hypers. I all so use the HPI Race vented CB on my Savage and MT. If you want more punch or bottom end power you want to go smaller on the CB or bigger on the spur. For top end it is just the opposite.