BS Exhausts build a lot of cat back exhaust systems. A cat back exhaust is similar to a straight through but leaving the original Catalytic converter “Cat” in the car. A lot of people go for this option as it saves having to bolt in and out the cat at any stage if it was removed. If you leave it in it will keep the noise levels and emissions down in the vehicle and suits in a lot of cases. We build a lot of new systems from the Cat Back into twin pipes and tips with no silencer boxes only a dummy box for Test. These all pass nct with no issues. It also gives the perfect sound and all the looks.