no back pressure is great....its not enough exhaust velocity. ie...if u were to take a paper towel roll and blow into it and take a 3" pipe a blow just as hard into it....which one is moving faster..the faster the air moves through the better the gas get scavenged....but then u get to a point that it begins to back up cause its too small...so u have to find that happy median where the gasses flow fast but dont begin to back up high in the rpms.
