The way I specified the motors and decided what to get:
1. I wanted RPM to be similar to a geared 21.5T car motor. This is roughly 2:1 on a 1800kv motor. So that puts the kv requirement at 900kv.
2. I decided what power I thought would be needed, and looked for motors sized to be able to develop that power - that also had the required kv.
Most small outrunner motors suitable for these cars will be designed to run on 3S. In aircraft, that would give the best compromise of weight and efficiency while still giving reasonably high voltage - always the best route to efficiency and therefore performance. That doesn't mean that you can't run them on lower voltage, but to get the required power you will need to punch a bit more current through it. This wouldn't be a problem for most motors as the higher currents only flow in short spikes in cars.
The motor I am using is the Turnigy 3536/9. This has a 5mm shaft. For this application I actually turned the shaft down to 4mm and with the motor diff unit, don't use the original shaft at all. Why not give this motor a try? You're not exactly going to be much out of pocket if you don't like it!
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