99 DX header + test pipe

I was wondering what can be gained from adding the header and test pipe to my dx?

is it just a increase in sound? Apprx. how many HP increase?

and does it save me more gas?

so overall, does it do any good?

and one last question... what is the diameter on a stock dx exhaust piping? 2.25", 2.5", 2.0"?

any help would be much appreciated. Thanks

Well, first of all, test pipes are not street legal in most states(if any) and they only give about one hp over a performance catalytic converter. So, it's really better to go with a performance cat than risk getting a ticket. But I would say with a header and a performance cat your looking at around 15+ to 25+ hp. I think the stock piping is 2" on a civic, but I'm not totally sure.