Okay, I've waisted enough of my time asking my personal friends opinions. I'd like some help from you guys. If I run a lightly moded (I/H/E) civic, and chose to use 91 octane, in place of 87, am I doing more harm than good? I know I could find it on google, but I'm going to bed now, so it'll be easier for me to just get the answer from you guys when I wake up.

Thanks for the help.

Unless you've increased the compression level or added a power adder(turbo, supercharger, nitrous) then a higher octane really doesn't do anything but burn more effciently.

Just like Jackasssi said, unless you've messed with your compression ratio, or installed something that will, it's just a waste of money to run higher octane fuel than what is recommended.