It sounds like you just described "choked flow"[0], where reducing the downstream pressure no longer increases flow velocity. It's been many years since I lear

It sounds like you just described "choked flow"[0], where reducing the downstream pressure no longer increases flow velocity. It's been many years since I learned it, but I think this occurs at only about 2:1 pressure ratio across the venturi. If I'm remembering that right, just letting the air out of your tires would result in choked supersonic flow. [0]https://en.wikipedia.org/wiki/Choked_flow https://en.wikipedia.org/wiki/Choked_flow Edit: I got excited to reply before actually reading the wiki I linked. Looks like I was close, with pressure ratios between 1.7 and 2.05 resulting in choked flow, depending on the gas.