How does the backpressure of the exhaust system relate to torque delivery? I know this is a simple question for many, but I'm hoping someone will take a moment to educate me, please. I read somewhere that some backpressure is needed to make low end torque, but that high backpressure decreases high end torque/power? Then, can too little backpressure be bad for power gains?

 

Back pressure to an extent is needed in a NA car however turbocharged cars do not require backpressure. Some possible "low end" torque may be lost however made up on the other end for a set up such as going to straight pipes or high flow exhaust.
a search will provide volumes about this.