My "friend" removed his cats and put it in a straight pipe (N/A). However, the possible issue being, the pipe is a different size than the ones it connects to (see pictures). Shown is the front (left side of first picture) and rear welds of the pipe, but the diameter is uniform throughout the pipe (which you can see almost all of).

Is this an issue?
Will it hurt performance?
Etc.

 

The tubing was crush bent, which reduces area by about 1/3. Crush belt tubing will hurt performance a bit, but it's probably not worth replacing with mandrel bends unless your looking for maximum performance.

 

I was referring to the section to the right of the crush bent part. Unless that also applies to your statement. Which, btw, is very informative.

 

Here's another picture of the whole section.

 

If he put this in in place of cats there will probably be gains from it.

 

I would guess it's a net gain. I'm not particularly concerned about this as far as engine safety (or something) because I don't think it's damaging anything. I'm just kind of curious as to the effect of it going from a (for example, not sure of the actual numbers) 2.25" pipe to 2.5".

 

if its a daily driver /not used for racing and you dont have smog inspection in your state, dont worry about it

the best he can do is redo the whole exhaust if doesnt feel too happy

 

No smog and it's a weekend fun car, but not track or anything. It's likely going back to having a cat on it because sitting in traffic with the windows down on a straight pipe isn't the best situation... always "fun" having it smell like you're deep-throating a tailpipe.