It follows original rules that goes with con modifier. With cha modifer save dc would be too high to beat.
You can google for it around web but here's one quote "An elemental savant gains the speed and movement modes, natural attacks, special attacks, and special qualities of a Medium elemental of the type appropriate to her elemental specialty, as noted in the Monster Manual, except that the save DC against her elemental attack form, if any (whirlwind, burn, or vortex) is 20 + her Con modifier."