the EXUP controls the flow of exhaust gas.. lower rpms, u want reduce the airflow, at higher rpms, u want better airflow... the exhausts without any form of control of the rate of airflow, will 'mainly' be power only at a certain range.. u reduce the diameter of the pipe, u get better power in the lower range(at higher rpms, the engine cant 'exhale' very well). increase it, u get power in the top(lower rpms, little backpressure, causing lean mixtures/power loss).. but as u should already know, engines need a certain backpressure to prevent itself from running lean.. too much backpressure, u'll be spitting soot.. when u're talking about aftermarket pipes, especially full systems, these manufacturers dont make them just to specifically replace ur original system.. they are mostly built with racing in mind, not just replacing the original systems(explaining why their targets are mostly horsepower/torque).. however i have seen/heard of a few manufacturers who made their exhausts which actually increases mid-range and lower range.. some even to soften the power deliver.. one example that i do know of, is the fmf powerbomb for the KTM 525.. the bike is too torquey, so they actually made it so that it becomes more driveable.. which actually means they reduced the torque/power for the low/mid range.. correct me if i am wrong.
