As you can see in figure A, the 3" inch pipe that is mandrel bent measures 3" inches at the center of the bend.  True 3" inches throughout the pipe will give you 35% less backpressure.  Less backpressure equals increased horsepower, torque, and better gas mileage.
As you can see in figure B, the 3" inch pipe that is stock bent measures 21/2" inches at the center of the bend.  You don't  benefit much by having a 3" inch pipe with 21/2 inch restriction in each bend.  Your exhaust system is only as good as its weakest link.  Why settle for restrictive sub par bending.
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