I just realized something yesterday. Last year I had a new 2" exhaust installed. Glaspak muffler and no Cat. So basically a straight pipe. Stock KA24DE 240SX has 2" but with restrictive muffler and Cat.
With the supercharger, the 240SX websites say you need 3" pipe.
What are you Modified 1200 people using? (CA, SR, L etc.)
And what should I use?

Using 3 inch system with S type muffler, hot dog and cat (CA). Cat is a bit restrictive, reduces top end power. 2.5 inch tip. Without tip the engine is a bit noisy.

Do u have to run a cat legally?

yes. big fine if you dont. inpounded car if the cops in bad mood

2.5-3.0 inch exhaust needed for 150-200kW.

I'm guessing its all new exhaust for me this year.
Now I have to decide between 2.5 and 3 inch. I'm not to concerned about the noise. I just want good flow.

Don't s/c still need good exhorts velocity unlike turbos? If that is the case bigger is not better and I would think a straight through 2.5 would be a good size.