[RC-Monster Mike]

Most of the info and advice in this post is pretty accurate. the xl3100 WILL run hot on 14 cells. Also, when you increase the cell count, you must gear the motor down some(smaller pinion), not the other way around as squeeforever mentioned. The 7020 will handle the xl2800 on 12 cells, and even 14 cells, but it will be pushing it. The 9920 is the better option. For 12 and 14 cell operation, I would choose the xl2800. For 14 cells, I would probably take the xl2400. All the basic xl motors are capable of the same power output, so any of these motors could be geared to achieve similar performance. The goal is to get the motor near its ideal rpm/efficiency range based on your cell count. Most brushless motors are most efficient somewhere between 30k and 40k rpms. I usually shoot for low-mid 40k rpm range myself. The 3100 is a great 12 cell motor, and can live comfortably on 14 cells, but it will be warm.