i got a hks exhaust and i need to know what down pipe and catalck converter is compatable with the cat-back?

 

Down pipes are only for turbos.

Catalytic converter, most go with the High Flow cat. I cant remember what the name of it is.
If you get aftermarket headers, you can go with many different routes. Here are a few examples:
Catless headers with stock ypipe and keep the stock bpipe with resignator
Catless headers with aftermarket ypipe and keep the stock bpipe with resignator
Stock headers with aftermarket ypipe and keep the stock bpipe with resignator

But neither will give you any real noticeable HP, unless you dyno it.

Just ask if you need more info

 

so my exhaust system is pretty much completed. i am going to put a turbo in the long run. i am just building up my motor to equip it.