No dude, Bruce is 100% correct--there are sound laws in Cali that you're breaking and the CHP or whoever else WILL fine you for it, if they so choose.
Here's the problem--there's no standardized means for them to decide whether or not your car's exhaust is breaking the decibel law. It's basically up to the officer's discretion and then it's in your hands to take it to court and try to prove that your exhaust is legal.
So, if your car looks relatively normal and you seem like a decent guy, the cop's probably just going to go "it's a porsche, they're loud" and let you go without a ticket. If you look like a teenager and have neon and crap on your car, they'll be more likely to give you a ticket.
Ask me how I know!!!!!
