I'm sorry, but I have to disagree with drusba. You CAN get into engineering with a 26 on the ACT and a rank in the top 5%. How do I know? Because I am a living proof of that.
I got a 26 on the ACT. I took it four times. If I studied for the ACT, maybe, I could have done better. I'm just not good on standardized tests. I'm not stupid. English is my third language, and even now, I don't understand it completely. For example, my friends can't talk to me slang because I won't understand them. My friend asked me "What's up, dawg?" I glared at him and asked "Are you calling me a b-word?" Then, he laughed and explained what that meant. Little by little, I'm learning slang.
Anyway, that's off topic.
So, um...I got a 26 on my ACT and I was also in the top 5%. I didn't think I would get into engineering. Even now, I'm surprised. It's a miracle.
I remember worrying like crazy in the being of my senior year about my scores. Somehow, it all worked out, and thank god too.
To answer your question, there is no definite answer. I think UIUC looks more than just scores. They're not like "Okay, students with a 32 and above on the ACT, and top 1% of the class are automatically admitted. Everyone else...who cares about them...throw their applications away."
To be honest, I think UIUC looks for personality. Don't let anyone tell you can't make it. Many people told me that, and I cried and worried so much. But guess what? I did it. I got into UIUC Engineering this year. When I look at other people's stats, I'm like "How in the world did I get in?" because my scores aren't as nearly as good as theirs. I'm lucky.
Try your best in your classes and on your tests. Don't be lazy or have too much pride to study for the ACT and etc. (I was lazy.) Don't let anyone tell you that you can't do anything. If you tried your best and your results aren't as good as you wanted them to be, then work really hard on your essay. In your essay, tell them your passion for engineering. Show them why they should choose you over a person who has a 36 and is number 1 in his class.
To be honest, I didn't work on my essay until what seemed to be at the very last minute. I didn't focus on UIUC essays much because at that time, I was into medicine. So, I focused my time on the medical program more than I did on UIUC. Things happened in my life, and I decided UIUC is a better choice than the medical program.
Sorry for the long post. Good luck! Actually, if you are going to study for the ACT, summer is a good time to study.