well yeah i knew it was back pressure.... but why does higher back pressure yield higher low end torque?

 

Volume is the key and expeling those exhaust gases is one piece of the puzzle. I like to look at the extremes to understand that delicate balance at play, keeping in mind that the desired power band will drive where the balance falls. Lets say you have a 6" exhaust. Exhaust gas velocities are slow, larger viscous boundary layers at the walls, and a lack of fluid momentum. The low fluid momentum is a big key here, as the lack of fluid momentum does not "syphon" the headers/combustion chamber as effeciently. Now 1" exhaust. At this size the viscous drag of the tubing walls is enough to severely choke the engine. Excellent fluid momentum with astronomical fluid velocities, but not enough to compensate for the viscous drag. So the balance is finding the right fluid momentum and velocity to maximize escape of exhaust gases, but not choke the engine with viscous drag.