lol, that varies from location to location... But most things done to a car are at best in the gray area of the law, not technically legal, but who's going to check or care. You'd have to be an expert on local laws to be able to even guess what goes in your area. Best thing to do is ask a couple police officers in your area to find out what they actually care about. And if you have emissions inspections in your area, ask before any mod to see if it will effect emissions inspection/testing
remove bumpers subframe, use fiberglass-casted mock-up headlights ? some kind of plastic winshields and windows (not polycarbonate)? too loud exhaust/direct port exhaust? etc. I don't know, I'm not in the US. But just thinking about full-race cars, I think it's harder to make mods legal.... :P
Straight through exhaust with no cat. That's a common one. Nitrous, restrictions on certain wide body kits and spoilers, certain headlights, external lighting, flame kits, some places have certain conditions about dropping in a turbo and using slicks. It all depends on where you live.
live here in the philippines, we have laws on that but the land transportation office wont give a damn as long as it aint too loud or smokey.tuners here have a lot of wild engine combos and pass off as legal as long as they pass emissions tests