They are illegal if they are on a car that doesn't have one from the factory. It is the same with most turbos as well. If it isn't carb exempt and it is on a car that doesn't have it from the factory, it is illegal. Check out the carb laws on CARB.com and you will see that there are certain setups on certain cars that are illegal. For instance, my car is smog exempt with the basic Greddy kit w/o intercooler or BOV. Once you add any of those items, the smog exemption is void.