Im putting together a custom 2.5" exhaust for my hybrid. Is a flex pipe a necessary addition? what is it's purpose?

 

are you talking about a flex joint with the look of like a weave of metal in the center and it increases in diameter on the outside, just it is nec. if youd like the exhaust to last, lets the engine move and the exhaust without binding very well worth the minor extra cash. now if your just talking flex pipe that bends thats bad, leakes, it rusts, and its just a bad idea. if your running a header that is still stock size collector, just take your stock honda flex joint flange with the two bolts with the springs inbetween and use that, and it would make it easier to take it apart.

 

yup thats what Im talking about

are you talking about a flex joint with the look of like a weave of metal in the center and it increases in diameter on the outside

 

Where does the flex pipe go? In between the header and cat or after the cat?

 

it goes between the header and cat.

It is there and is important because it allows the motor to shake and flex without moving the rest of the exhaust. You can imagine if everything under your car moved w/ the motor, then you'd have some things flyin off when you got on it!