Thrawn

As I understand it, the physics behind a loss or gain in torque have to do with the volume and velocity of air that a given exhaust provides with an engine throughout the powerband. Lets say the stock exhaust allows for a certain velocity and volume of air to pass through it with the stock engine. If aftermarket exhaust A allows for a greater volume of air throughout the powerband, but at a loss of air velocity during the lower RPMs, then you'll see an increase in top-end power due to increased volume, but a loss of torque down low due to a loss in velocity. If aftermarket exhaust B provides an increase in air volume over stock, but not as much as Exhaust B, but also gives an increase in velocity over the stock exhaust, and an increase in air velocity in the lower RPM range over Exhaust A, then there should be an increase across the entire powerband, w/o the loss down low as seen w/ Exhuast A. But that increase will not be as large of an increase at the top end as Exhaust A, due to the loss in volume and therefore velocity in that area of the powerband. That’s because the engine is pumping out exhaust a lot faster in that area of the powerband, and can therefore handle and make use of (in terms of air velocity) the greater volume offered by Exhaust A. So, its not just how much air an exhaust will allow an engine to pump out, but also how quickly the air can go through the exhaust throughout the powerband given its shape and diameter. This only applies to N/A cars, however. When it comes to forced induction, bigger is always better. The super or turbo charger needs volume when it comes to exhaust, the velocity is always there.