Just to add a bit to this -
Depending on which county you live in, your cars may or may not be subject to periodic emissions testing.
The heavy majority of our CA members live in the Bay Area or Socal where testing is mandatory on a bi-annual basis, however if you happen to live in mono county, or have the ability to classify your vehicle as a "farm vehicle", then you are exempt from smog testing. (there are some rules obviously, you may be subject to bar tests).
Though there are these loopholes, the heavy majority of auto enthusiasts tend to igonore smog rules and modify their cars as they see fit to do. My car is by no means smog legal, but I have a large cabinet in my garage where I store all of the factory emissions equipment in case I need it.
There is definitely a risk though. A policeman can inspect your car at any time if he suspects that you have removed mandated emissions equipment, and he can issue you a fix-it ticket, or require that the car not be driven until said issues are fixed. This is usually pretty rare, though it does happen (mostly to idiots who annoy cops).
Generally speaking though, so long as you dont draw attention to yourself, cops will mostly leave you be.