The S-type, G-type, M-type and straight-out type of the universal drum 18 all mean that there is nothing wrong with the exhaust form, but in essence, there are only two of the above four forms: one is back pressure and the other is straight-out. Back pressure tail drum includes s type and g type. Why do you want to call them back pressure tail drums? The reason is very simple. Back pressure, as its name implies, is the pressure to go back. How did this pressure come from? In fact, the direction of exhaust gas becomes complicated by the bending or stretching of the pipeline, thus reducing the discharge speed. In this way, when the exhaust pipe is filled with exhaust gas, pressure will be generated because the exhaust gas cannot be discharged quickly. At this time, the exhaust gas in the exhaust pipe will resist the combustible mixture in the cylinder, which is what we call back pressure. It is easier to understand the point of direct exhaust. There is no complicated pipeline in the pipeline for direct exhaust, so the exhaust gas can be directly and smoothly discharged outside the vehicle, which is called direct exhaust.