What are the best size pipes to run on a 1.8L tib that already has 4-1 headers??? I want to get a deep sound no harsh or raspy sound like most hondas. I also wanted to get a hi flow cat know where i can get one???

 

2.25 is the standard answer. You can go with the 2.5 which will likely give more peak hp and torque...but also more sound. What are your performance goals?
Thats an important consideration.

 

I got mine from Extreme Flow . It already have a O2 senso bung. It seems to really work good. I notice a lot of power gain on the higher rev range.

 

as far as exhaust piping go, bigger is not always better. 2.25 cat-back piping would be a good size to be at for N/A tuning. the biggest u want would be 2.5.

now if u go with 2.5, u are going to lose a bit of torque. but u are going to gain max hp. torque is create by back pressure(the right type of back pressure). with 2.5 piping, there is a bit too much free air flow space, therefore the pressure is not there to create torque.

a rule of thumb for any engine mod, is to ask yourself: what type of power am i looking to gain. hp or torque? to make it easier, torque is power from 0-about 3500 rpm and 4000-up is hp(which i'm pretty sure u already know that)
now if u are running turbo or supercharger of some sort, than 2.5 would be perfect.