what the difference between the Z's stock exhaust and CAT back?
 

I am not asking for HP numbers, asking the physical concrete difference and theory on why cat back is faster?

cat back just refers to an exhaust system that is complete from the cats back. Instead of just a muffler, or a piece of pipe, etc.. its the full system.. from the cat convertor back.

Why are aftermarket better then stock? More airflow, less resistance, less weight, etc.

The muffler is the big thing in the picture, unless you are going with a titanium exhaust, then the piping will also be super light. I don't know the weight difference on the piping between the stock exhaust and most stainless steel after markets, but I do know that most mufflers will weight about half as much as stock, and will be less resrictive.

True, however, just as big a thing is the larger diameter, less restrictive mandrel bent piping.

I saw on here that was a Ypipe for sale that would add 5rwhp w/ just the y-pipe, what are your guys thoughts on that?

5hp is nothing to get excited over.... I have seen cars vary 5 hp on a dyno only 5 minutes apart.... Catbacks are traditionally the first mods people make; looks, sound, power.......

Ben

I wouldn't just get a y-pipe, because although your freeing up the flow through the y-pipe you are going into the restrictive straight pipe, I got the Fujitsubo systm which increases the diameter to 3" of the straight pipe and also got the y-pipe that way both are increased in diameter.

isnt the stock system mandrel bent?