Originally Posted by Miller_Time

I run the 20c 5s 5000 packs (third one you listed) on a Losi 2100kv system and it's overpowered. For racing I dialed the throttle end point back to 80% and it's still got more power than I need on a large track.

Given the small price difference between the 4s 5000 and 5s 5000 packs I chose to go with the 5s for a bit more efficiency. If you're racing you could use either 4 or 5, 6 would be WAY too much. The 20c rated Zippys are plenty good, no need for the 30c rated ones.