This has been covered 1,000,000 times, but I was wondering because I already have some aftermarke parts- meaning I think my stock exhaust mani is holding me back.

I have a B18c1 with a new fuel pump (walbro 190 lph) and rewired it straight to the battery with 10 aug wire (more lph), I have a catback 2.25 exhaust, and an open Blitz filter.

My question is how much hp gains will I get and which header is the best? I'm thinking some kind of 4-1 like that jdm dc or something. Please tell me what you think I should get.