They are illegal if you drive on the highway. That's why they are called off road X pipes. They are used on race cars and dragsters. Now, most states don't have a sniffer to sniff emissions during inspections, but if the inspector crawls under the car and sees you don't have Cat Backs (Catalitic converter) you will fail the inspection. They are not really worth the effort and expense in my opinion. You can get a throaty rumble by using mufflers designed for that sound.