If you are in the US, laws are different in different states. Assuming that you aren't driving an ECO with a cat, you can put whatever you want on it so long as it doesn't exceed local and state noise ordanances. That said, given the number of ricers running around sounding like angry bees, I am sure you could get by with a straight pipe if you really want. My advice is the bigger, the better. I am seriously considereing putting a 4 inch pipe on mine. Really have no need for that big, but I have access to some 4 inch stainless pipe. If there is enough to do this car, I just might try it.