I believe that it's the official specs they have on that website, which mean 18v. It should run best at 5-6s.

Reading back i noted that i was wrong. I originally though that you wanted to run it on 4s. That said if i was you i would use the mamba max on 4s and run 14.4v dewalt instead of the 18v. They are 820 sized too i believe(only .2oz of difference so they should be).

They are cheaper to start with too.
http://www.robotcombat.com/products/BP396505-21.html

Of course ohm law is ohm law and they should be a bit less efficient but it shouldn't more than 1 or 2% of difference.

some sparks do fly, but these motors handle 24 volts well.
@lincpimp get one. If you want brushed, get the dewalt.

I may still give the dewalt a try, but I think an outrunner will end up in the truck I am building. I have a 2826/12 axi that fits my gearing perfectly...

Ok, so I got a dewalt motor out of an 18v drill. How do I go about removing the pinion? Looks like it is pressed on, but the gear appears to be on as far as the front bearing. I guess I can grind the gear off on each side to release it...

Yeah, just grind off some teeth until the gear falls apart more or less, or grab a pinion-puller to remove it the 'proper' way... :P

You could try heating up the pinion. The metals are probably not the same, so will have different expansion rates and may be easier to remove...

I can try to heat it, but have no idea how to hold onto the gear. I could clamp it into the vice??? Then use a punch to tap the shaft out of the gear...

If it was me, i would stay away from any treatment involving heat. It might undo the heat treatment that the shaft probably received hence weakening it.

True, I ended up grinding the pinion on both sides and then split it with a pir of pliers. Easy when you know how...