Alex, what voltage drills are they?
if they are 9.6v, Id suggest 14.4v is the most you can reliably use (by reliable I mean they should last one event) I have run 9.6v drills at 21.6v, but they eat themselves very quickly.

If you are using higher voltage drills, as a rough guide, add 50% to their voltage, so run 12v drills on 18v etc. As I say, you can push harder, but it gets expensive to replace your motors every fight.

Out of interest, how have you wired your motors? Are they in series or parallel? they must be in parallel or they will only see 7.2v each. It may be a silly question, but it is more common than you think. Also, have you checked the electronize speed gain is turned up to maximum?

hope this helps.