I have the opportunity to get a stock exhaust from a 99 Civic Si for free. Would there be any benefit for me to put this system on my car? I have a 96 CX hatch with a JDM B18C1 motor. I don't want to spend money on an aftermarket exhaust at this time since I will get a complete header-back system once a new motor is finished.

 

i think the diameter of the si exhaust is larger over the cx.

 

It is and it isn't. Towards the header, the si piping is bigger. The more towards the muffler the si piping gets, it goes down to the same size as the cx piping. The only use for an si b-pipe is for doing a b-series swap and using the si mid pipe so you can use a b-series cat and header.