Hi, i finally chose the option of go to a workshop and make a custom exhaust for the rex, buy i'm thinking about the piping size. The muffler that i want is the invidia that have an inlet of 60mm (like the invidia cat-back), So my first choice is the 60mm (2.38") piping, Buy i have read that for n/a tunes the max exhaust size for no torque losses is 57mm (2.25"). For additional engine info, I bought a toda rep exhaust header manifold (2.5" outlet), a Password:jdm carbon CAI (from civic ek) and a 200 cells sport cat and i'm converting to obd1 for a tune with a hondata s300. Regards, Noah.

 

Seriously? You're fussing over a difference of 3mm diameter? Use whichever size you want. With that small change it wont matter and you won't feel the difference.

 

So this 3mm doesnt matter. and for the sport catalytic converter, from the 2.5 header outlet, i need to buy a 2.5 inlet/outlet catalityc converter or one that have a 2.5 inlet and 2.38 outlet? Thanks.

 

Any exhaust shop worth their salt can shrink and stretch pipe to fit other similarly sized pipe (slip joint). Don't get caught up with small size differences. Buy the cat that matches your system diameter, which sounds like it's 2.5".

 

3"

 

If they use crush bends, I wouldn't worry about pipe size. If the use mandrel bends, a good rule is about 1" per 100HP