Can any one let me know, what downpipe use for?
I am thinking of changing the downpipe and the converter too. Please give me some suggestion.
Thank you.

 

huh?

 

the dp is where exhaust gas exits through then it passes through the cat then the b-pipe then the muffler then to the atmosphere

 

the DP is what a header would be on a car with cylinders...

I believe it is the pipe that "exits" the engine - the first section of your exhaust

 

Err...I believe there is a section of exhaust manifold prior to the DP. It's between the block and the turbos. Unless you consider the turbos as part of the engine, which I don't think is totally accurate.

 

Come on people, think like a new owner here.....

A downpipe is a section of aftermarket pipe that replaces the stock "pre-cat". A stock FD has 2 catalytic converters, the pre-cat and the main cat.
Some people DO replace the main cat with a mid pipe, but you are going to be very loud, very illegal, the car's exhaust will smell like hell, and you are likely to have engine management problems.

Now, the downpipe is a very good thing to do, getting one to replace the pre-cat. The pre-cat is heavy and retains a lot of heat, which it puts right into the engine compartment.

You COULD replace the stock main cat with a free flow aftermarket catalytic converter. This should net you some power gains and still be RELATIVELY legal. Keep in mind that some states have draconian exhaust laws, the literally read, 'ANY modification to the stock exhaust is a violation of _________ State Law.'