This is my first write up, so hopefully It'll help some people on here.
First of all there are many reasons that someone may want a test pipe. Also, some junk yards and scrap yards buy cats, i sold mine for $100. The reasons I did this are...
1. Running low on cash for turbo project(sold cat for $100)
2. Wanted the performance gain from a free flowing exhaust
3. I plan on buying a high flo cat later and I'll just swap them then
There are many disadvantages to having a test pipe as well. It's illegal, some may not like the raspiness of the exhaust, and some need to pass emissions tests. That's just a few, but it's up to you to decide if this is for you or not.
But before i go any further, I must say... THIS IS ILLEGAL IN 50 STATES, I WILL NOT BE HELD LIABLE FOR ANY PENALTY, ANY BODILY INJURY, ANY DAMAGE TO YOUR CAR, OR ANYTHING OF THE SORT.
It is up to you to decide and know safety precautions when working anywhere near a car, or around tools.
Ok, here are some of the tools you may need to do this...
1. basic hand tools(hammer, sockets, wrenches, breaker bar, marker, etc.)
2. air cut off tool(and other air tools may be handy)
3. welder and some basic welding skill(I have some cheap arc welder)
4. jack and jack stands
5. bench grinder with grinding stone and cut off wheel, a plus
6. measuring tape
7. drill and various drill bits and WD-40
Here are the parts you will need...
1. 1 pipe 2.25" diameter(my cat was 16in long, yours may differ so measure it)
2. 4 bolts, 4 nuts, 4 lock washers( i used M8 1.25X40mm bolts, and 8 washers)
3. the flanges that you cut off of the cat
4. exhaust pipe flange gasket material
Ok, now we can start the procedure. The car should be securely on jackstands at this point. If you don't know how to do so, then you shouldn't be going any further than this.
Step 1. First you will need to get the cat off of the car. There will be different methods in doing this because some cars may have seized bolts, and some may not. So, first try to loosen the bolts holding on the cat with a breaker bar and socket. The size of my bolts were 15mm, but they were so rusted I had to cut them off. If you can get a socket on there securely, either loosen them, or just break the bolts. I will discuss how to fix that later. If you cannot get a socket on the bolt heads you will have to cut the bolts off using an air cut off tool. Using the air cut off tool cut the nut side of the bolt as close to the flange as possible. Once all four are all of the way cut try to remove the cat, if difficult give the flanges a few whacks with a hammer until they give. Then remove the cat from the vehicle.
Step 2. If you have any broken bolts, here I will discuss how to remove them. You will need a drill, and various bits from small to big, biggest being around 3/8. Start off with a small bit and try to stay in the center of the bolt. Drill through the bolt being sure to cool and lubricate the bit with WD-40. Repeat process until desired size is reached.
Step 3. Now you've probably noticed that the flanges on the cat are thick and very close to the cat. What we will do here is cut the flange off, preferably using the bench grinder with cut off wheel, if you don't have this you can work at it with the air cut off tool. Cut as close to the flange as you can without cutting the flange up. When you get most of the material cut away you may be able to knock it off with a hammer.
Step 4. Now that you have the flanges off of the cat, now is the time to clean them up a bit to make a good mating surface for the exhaust pipe. Use the grinding stone side of your bench grinder to deburr and smooth the flange. Watch your fingers here.
Step 5. Now that you have reuseable flanges we can move to the pipe. Take your measuring tape and measure the length of the cat without the flanges, take that measurement to the pipe and mark it. You may then want to take this under the car and double check that its close, it should be unless you took too much material off. After you are sure of your measurement, cut the pipe. Now you can use the grinder to deburr the exhaust pipe.
Step 6. Now while the flanges are off of a pipe, now would be a good time to trace an outline of the flange on the exhaust gasket material(if needed). A 10"X10" sheet of this material should be enough for two gaskets. You can cut these out with some strong scissors, drill holes for the bolts to go through, and cut out the 2" hole. Using a drill bit on the edge of the line you made for the hole, slide your scissors in the hole and cut out the center 2" hole.
Ok, now you should have the pipe and flanges ready to be welded together.
Step 7. You can now center the pipe up on the flange, then tack weld it together. Now take a look to make sure it's positioned right, if so, continue welding them fully together. Now wait for the pipe to cool, you can dip it in water to speed things up. The only thing that will do is temper it some, which isn't a bad thing.
Step 8. For this step I recommend getting out your bolts, nuts, marker, and other un-welded flange to ensure proper fitment. The other flange is offset from the other one if you didn't notice on disassembly. First, bolt the un-welded flange to the flange of the catback, next bolt the welded pipe and flange to the downpipe. Lift up the catback and end of testpipe and center the testpipe on the flange as you did the welded one. With a marker handy mark the relation between the pipe and the flange(i marked it in two spots to be sure). At this point you should decide if you are going to need to install a hanger on the test pipe, some may not need it, mine didn't but I did it anyway since I had one laying around. If you don't have one you could use a long bolt(like a head bolt) and angle it off the pipe towards the hanger. Just make sure you test fit and mark the location of what you will be welding on the pipe. Now disassemble, then position the pipe and flange(aligning the marks you drew on the flange and pipe), tack weld the pipe to the flange, and once again inspect for proper alignment. If all is good then continue welding until finished. Now weld on the hanger you will be using in the location you've marked. Then dip it in water to cool.
Step 9. Now that you have a test pipe, it's almost time to install it. What you can do to protect your welds from instanlty rusting is wire brush the welds, prep it with wax and grease remover, and paint it with high temp paint(optional).
Step 10. Install in place of the old cat with the new gaskets (if needed), and don't forget lock washers.
Have fun with the new sound and hp gain, I hope this write up helped you, and I hope you enjoy your testpipe!