Good question.

Princeton dictionary lists dry heaving as a synonym for retching. It says that it is "an involuntary spasm of ineffectual vomiting".

Merriam Webster dictionary lists dry heaves as " repeated involuntary retching unaccompanied by vomit ".

Looks like there isn't a big difference.

And since my previous post in this thread was a declaration of my devotion to Doug apparently, I'll answer the question now.

I've never retched before v* but I have retched without v*.