Why does the MIT Racecar not use the build in ESC?
All ESCs that are designed to work in an RC car use the same pulse width and (roughly) frequency.  It's the RC non-standard (as in, everyone uses it, and has since the 1970's, but there's no standards document).  50-60Hz frame rate, a 1ms positive-going pulse is full reverse (zero throttle on a plane), a 2ms positive-going pulse is full throttle (plane or car).  Different car ESCs may have different pulse widths for zero speed -- you'll need to dig around, or just find out what it is for your ESC by experimentation.