I'm afraid I have no idea.
I will say that the only ab work I do is 3 sets of weighted crunches twice a week on a ball, in the course of my regular workouts, and I'm starting to be able to see my obliques. I think it's from all the other exercises I do, not particularly from the crunches. Those are tiny little muscles and I'm not convinced that they need their own special exercises. Personally, I like the compound exercises that work many muscles all at once. It's much more efficient and you don't find yourself doing crunches all day long.