Like the smell of dirty burnt octane (tastes like burning). It's a very distinct smell. The only times you should smell it is when you're upwind to a stop, when you're behind someone with test pipes, or when you're in the garage huffin it up. But seriously, it's not too bad, just noticeable to 5-0. With the way things are now a days, I think cops are more educated on things like this, so I wouldn't chance it, unless you're in a state that doesn't care about cats. Just my $0.02.