I have a 70mm shroud, and 2600kv inrunner. The rotor should be a 5 blade for 4 cells, or 5 cells. I'm going to try a wemotech mini. All the 4 I've tried can't run, and at 11,000 rpm hit the duct...then the resononance goes away, and they will run up. This is obviously not the way to get a good effiecient fan unit. So I've decided on letting a pro balance them, either the HET 6904 or the Wemotech mini...and if I find another, maybe Lander. I asked Extreme RC about it.