exhaust

my brother said I would loose about 20 hp if I put a 2.5 inch exhaust to my civic hatchback cx is this true because I don't think it is true.

You wont LOSE hp, but you probably won't gain any. Stock engines need *some* backpressure in the exhaust in order to perform properly, a 2.5 inch exhaust isn't that big, but it will take away some backpressure. This may lead to a "lag" at low RPM (the car making very little power) - however, high RPM (and we're talking 6500+ rpm) performance benefits greatly from a larger exhaust (higher volume exhaust gases can escape faster).

well do you think I would get HP more down low and up high if I just add a header and a cat

Truth be told, exhaust upgrades should only be done after major engine performance work is done (i.e. forced inducion/performance cams). With a stock engine (espically with your Y7 head), freeing up a little space in the exhaust manifold and getting a bigger exhaust pipe isn't going to help the gases escape much faster. The regular exhaust configuration is tuned specifically for your engine, when you add a 2.5" pipe to the an engine that was running fine with 1.7" piping, it wont change much. Headers are the same story.

However, if you wanted to add forced induction, this is a completely different story. The higher pressure of a turbo/supercharger warrants the use of upgraded exhaust components.

Hope this answers your question, now if a nice sounding car is appealing to you, then by all means please throw in the cat, they do wonders for the sound.