There are a few ways to make your exhaust louder legally. One way is to install a larger diameter exhaust pipe. This will allow more air to flow through the system and therefore make it louder. Another way to make your exhaust louder is by installing a high-flow catalytic converter. This will also help increase air flow and therefore make your exhaust louder. Finally, you can install an aftermarket muffler or delete your current one entirely. This will drastically increase the noise of your exhaust, but it is not legal in all states so be sure to check your local laws before doing this modification.