The Si is the top of the line civic, while as a "regular" ex. LX, EX, GX, Hybrid etc. is not. The "regular" civics have a K18 or a 1.8L motor, while the Si only had a 6 sp manual transmission with a K20 (2.0L DOHC i-VTEC) (rsx-s engine.) Si has more horsepower, and insead of having blue lights inside, it's red. Also has more options. Hope this helps. :]