U-bend pipes in exhaust are sometimes there due to limited space, and sometimes due to the need to reduce noise, air flow, or air pressure. The u-bend pipe can sometimes be removed to improve many performance characteristics, but may make the exhaust tone notably different or louder.
The pipe section is referred to as a "U" bend because it makes a 180° turn, making the shape of the letter U. However, sections that are off by a few degrees in either direction may be referred to as a U-bend as well.
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