stock piping is 1.8 or 2.0 (don't remember), evo is 2.36' and most aftermarket catbacks will be 2.3 or 2.25. The only 2.5 i know of is hytech and jdm. Same as the header, 2.25 collector unless you get a jdm header or custom kettler expensive header.

hollowing your cat out (test pipe)? Well it's illeagle, pollutes our air and you will be driving with a constant cel. So if your car really isn't running right, you won't know because now your cel is always on.

Stock Integra 218.3 cfm


Catco (metal core) 2.25" 223.8 cfm
Catco (metal core) 2.5" 271.9 cfm

Random Tech 2.25" 297.1 cfm

Catco (std. core) 2.25" 338.2 cfm
Catco (std. core) 2.5" 388.0 cfm

Car Sound 2.25" 342.7 cfm

Test Pipe 2.25" 407.1 cfm


cfm = flow capacity in cubic feet per minute

Just get a highflow cat.