DIY 2.5'' cat-back
 

I was bored today and figured I would see if I had enough exhaust bits laying around to manufacture a 2.5'' cat-back :)

What I used were some universal bits. A rear axle bend, short 30'' bend, and a bit of straight pipe. The muffler I used is a Simons Turbo, not sure if they are sold over there because it's a Swedish brand. It's a straight through turbo muffler, ie just a perforated pipe and packing inside.
Also cut the flange off the stock silencer (yeah I know you can probably buy them, but this was a bit of an impulse project).

After a couple of hours of measuring, marking, cutting, measuring again, marking again, cutting some more, it was ready for welding
Attachment 28117

None of the parts fit as they came, had to cut and twist or bend all of them. I did not weld the pipes to the muffler, because I want to be able to replace it easily if I want a different sound.

While doing the final assembly on the car, I found out that with everything nice and tight the tailpipe was just hitting the rear bumper, so I cut and bent it down a bit right behind the hanger.

Here's the result! Everything is completely stock other than the cat-back.

http://www.youtube.com/watch?v=_uP4qA1xo-A

It definitely feels like it gained a some HP and torque, accelerating is smoother now, and the "dip" right after it shifts into 2nd is way less noticeable.
It's a nice deep sound without being too obnoxious. So much better than stock! I like the pops when letting go of the throttle :) From the drivers seat you can hear the exhaust if you're listening for it but definitely not too loud.

Total cost around 30 bucks for the universal bits, and the muffler was free.
So far very satisfied!

Attachment 28118

Attachment 28119

PS the tailpipe has a coat of black paint on it, but my test ride was on a muddy road ;)

Did some 0-60 testing today, unfortunately we don't have any flat roads here so a slight uphill towards the end.
My testing method was an OBD reader and the Torque Pro android app which seems to use the speedo info to determine speed, it gave very consistent results.

Stock is about 11.8 seconds according to what I'm finding. Unfortunately didn't measure it myself.
With the new exhaust and everything else bone stock it did 10.5 (it did 10.5 twice, 10.6 once)

After that I did the throttle plate mod while I was at it.
Now it did 10.4 consistently so there was definitely a small gain.

After that I decided to open up my air box a little. Stock paper filter with a bunch of miles on it already. I only tested once before it started pouring and testing was over, but it did a high 10.2 which got rounded to 10.3.

A high flow cat is on order so it's gonna be cool to see if that gives anything. The stock one is rattling so needs to be replaced anyway.
A K&N filter and e-fan conversion are also on the to do list.
It's not exactly a race car but a nice improvement over stock already.

Hard to say how much the uphill bit affects the result but I'll find a stretch of flat road to compare :)