Originally Posted By: ZraHamilton
Exhaust systems do not need any backpressure at all to run right. Look at the video made by Engineering Explained. Exhaust VELOCITY is needed for low-end torque, not backpressure. However, to get higher exhaust velocity, the diameter of the pipe must be decreased, and a negative consequence is increased backpressure. When tuning an exhaust, you have to find a balance between sufficient exhaust velocity for low rpms, and the least backpressure possible for high rpms. If removing the cats doesn't decrease the velocity, then it wouldn't lose any low-end torque or mpgs.
Agreed, it's not the back pressure itself, but the velocity of the exhaust gas (which is, of course, dependent on the back pressure). Think of trying to blow through a straw that's too bid - the air velocity will be quite low. I believe that the exhaust system is designed around the cats such that exhaust gas velocity is maximized for idle and low RPM. Without the restriction of the cats, the exhaust gas velocity may be compromised. Make sense?