[nated]

Thanks for the link, this is what I found as the exception to one of the rules(27156)

h) This section shall not apply to an alteration, modification, or modifying device, apparatus, or mechanism found by resolution of the State Air Resources Board to do either of the following:

(1) Not to reduce the effectiveness of any required motor vehicle pollution control device.

(2) To result in emissions from any such modified or altered vehicle which are at levels which comply with existing state or federal standards for that model year of the vehicle being modified or converted.

From that it sounds like you can modify anything as long as you are not making it less effective. This also sounds like you can make changes to mufflers because they have nothing to do with pollution. However, I have a feeling the part I found is only a small section of the CARM laws.