Answering Matt's statement that vapor lock has nothing to do with the
type of fuel: One reason why the FAA is reluctant to grant STC's for
mogas is because of its vapor-lock propensities. There have been
studies and plenty of discussion about the differences in vapor
pressure at the same temperature and pressure between avgas and mogas.
There was some discussion not long ago, iirc, in RAH. Google and you
will find.