[lincpimp]

What kind of batteries?

If you are going with the flightpower 3700 25c pack, you would have 92 amps continuous. At 18.5 volts you have around 1700 watts.

On 3s 5000 25c packs you have 125 amps. At 11.1v you have 1400 watts.

Plus the 9xl on 5s will be at a better max rpm, around 34k.

The 7xl on 3s would be around 26k max rpm, which would require taller gearing, thus causing more load and heat. The 7xl is a good 4s motor, when geared down.

So the 7xl 3s combo will require more power, and the battery will have less ability than your proposed setup. Plus the mm will not like that either.

Your cheapest way out would be a 4s setup with an 8xl. Some trakpower lipos, or a 4900 4s flightpower would be optimal to power that setup. A mm with an external bec would work with that.

5s is a big step up from 4s, much better temps and power, plus less load on the batteries. Amp spikes will be less too.