Dude, car mods a mostly legal. Any performance mod is legal as long as it passes emissions standards for the year in which the car was built and meets local noise laws. Visual mods like bodykits and such are perfectly legal, with a few exceptions. If it blocks your vision, it's illegal (window tinting, tall wings). If it makes your car too low, it's illegal (body kits, lowering kits). Headlights must be properly aimed and DOT approved, the same with tires.

Now, ALL of this is subject to your local laws. Every state and county has different laws concerning cars. For example, in California you can't do much with the strict emissions laws. Here in Wyoming, we have no emissions testing. You can do anything to your car, within reason. Loud exhausts and trucks with too high lift kits are about the only thing ticketed around here.

Anyways, my point is, most car mods are not illegal. Check local laws, call the sheriffs office or ask the next cop you see at the donut shop.

As for the original question about the loud exhaust, if you have proof you installed it before the law went into effect, you should be completely legal. If not, keep off the throttle around that neighborhood.

 

You guys do know that with like Magnaflow custom exhaust setup...it's cheaper and you get to choose a good high-flow muffler (something that resembles stock) that happens to be high-flowing. They have the import styled exhaust...you know...the long coffee can exhausts, with the blue hue tips.