Guys, I'm thinking of doing a straight pipe build on my Lancer Ck4. Do you guys happened to know what's the best size for my exhaust is supposed to be? I'm thinking for reaching high speed but also needs to have better torque too. My Lancer spec is ported, open intake, and the engine is 4G92 SOHC.

 

An NA build is all about reducing restriction to let the engine breath.
Honestly, a good place to find info on what provides the best gains NA is honda forums. Some of those guys get really really into it.

That said, I probably wouldn't bother unless you are planning some high dollar NA build. There will be basically no gains especially without a tune and it will just be loud for no reason then.