When to appply

Hello, I’m just searching for a short answer, which I can’t seem to find…
I’m going to apply to UT Austin (Cockrell school of engineering), but my SATs are preety low (660m,600r,640w) and I’m taking the ACT on September 12th. I’m wondering WHEN to apply to UT Austin. I know they have rolling admissions, but should I wait to take the ACT, get the scores at the end of September, or should I apply NOW and then send them the scores? The problem I see is that with the SAT Scores I have now, I’m afraid they might deny me by the time I get my ACT Scores back and send it to them.

Anyone?