Whats a down pipe and where does it connect???? If i buy a cat-back i don't need this right???

 

down pipe is part of a turbo manifold, id you dont have a turbo, you dont need one.

 

sometimes the pipe going from the cast iron part of the stock manifold to the cat is called a downpipe. if thats the case, then yes you need ONE, but not anythinng aftermarket. it would come as a package with the whole header anyway. but chances are, youre thinking of the DP for a turbo, so no, you dont need it.

 

sometimes on an n/a setup its referred to as a mid-pipe.