What's MANDREL BENDING?

Consider a straight plastic straw that you would use to drink a soda.  If the straw is completely straight, it is very easy to blow air through it, as there are no restrictions.  However, if you decide to bend the straw ninety (90) degrees into an "L" shape, blowing air through the straw will become very difficult, as the opening at the straw's elbow will have changed from a circle to a very narrow slit.

This is exactly how the pipes in your car's intake and exhaust system work.  Air is drawn through the pipes, which when compression bent, become very restrictive to the flow of air.

Mandrel bending is the solution to this problem.  Mandrel bending works much like the flexible straws that you can buy in a grocery store.  When the straw is bent, a flexible section of the straw expands to maintain the round opening no matter how much of an angle the straw is bent at.

When an exhaust or downpipe is manufactured with mandrel bends, the steel is allowed to stretch on the outside of the bend and compress on the inside of the bend, maintaining the nominal pipe diameter.  Exhaust gas follows fluid dynamics.  You want to eliminate backpressure while maintaining exhaust gas velocity.  Mandrel bending allows the hot exhaust gases to flow with less turbulence.