When a discharge starts outwardly blast wave in the exhaust pipe, after the blast wave will form a negative pressure, so-called exhaust gas back pressure.
determines the optimum back pressure of the engine exhaust gas field, if the greater the back pressure at low engine speed due to the blast wave front by the blast, the sooner the negative pressure is sucked out of the blast wave front, i.e. the better the effect of the exhaust gas, but when the engine reaches at high speed, the pressure in the exhaust pipe can not only quickly sucked out of the next wave of the blast, but the discharge of each blast wave caused considerable resistance, and influence each other between each blast wave, resulting in poor exhaust cis, thereby reducing the performance of the engine.
That is, when the engine speed is low, a certain back pressure can contribute to the increased engine power. But in the high speed region, resulting in exhaust back pressure over the General Assembly is not smooth, resulting in decreased power, so it should be appropriate to reduce back pressure.