Thought I'd post a how too in its own thread as the other one is kinda misleading a bit. The original thread is right here though:
Borla vs backfire ... backfire wins and borla shreaded up and to my surprise FG!
In a nutshell I since i blew up the other one I used the perfortrated pipe of the borla and did the rest my self. But you can buy the Perfortrated pipe on ebay or you can have it made locally at most sheet metal places (they basically grab a perfortraded sheet and roll it to make a tube. I don't know the exact spacing and hole size needed so maybe buying premade stuff is easier). Other things that you'll beed will be Coarse stainless steel wool (Number 3), and medium Stainless steel wool (Number One), Ceramic furnace insulation (rated to about 2500*f), and a welder.
One thing to do if you're just buying or have a straight muffler is to add a few angles in there to reduce the sound abit more. Mine since it was an offset mufffler already had about 7 degree angle on the pipe so i stuck with that. Here's what mine looked like when all taken out of the casing and everything unwrapped of it:
Next thing to do is start wrapping the coarse wool, on the Pipe. The reason that I wrapped it with the coarse stuff first is because it has more material and the heat can't detroyit as easily. In my case I used up 10 pads of the coarse to get 2 layers on it. Then from there you want to grab some tape (I used the paper type tape so its not as overly tight wrapped as apperently it could make thing matters worse). This is how things look like then:
You do the same thing now with the Medium wool and wrap it up. It should also look like the above. Basically you're adding SS wool to fill up the size of your muffler tubing that you'll be using. I used 4" SS with about 1/8" side wall and used 1/16" Side plates.
Now when you're doing the medium grade leave about 1/2"-3/4" from inside walls of the tubing to fit the insulation wool. So once its all filler up you want to cut a size that is going wrap around the tubing properly and still be somewhat easy to fit in. I made sure mine was nice and tight when stuck inside the tube. That stuff will shrink a bit and once the tape is burnt up the SS wool should expand a bit.
So now grab your tube, slide it over it and it should look something like this now:
Mine was 19.5" Long in total for that center section. Have some plates welded on there and the ends should look something like this:
You'll notice that the tube isn't sitting right in the centre. I figures since my diameter is already small enough for the ouside tubing, where ever there's a bend in the pipe there's more room behind it for the air to travel through and cut down on sound a but more. Thats my theory. If you have a big enough piece I'm sure it wouldn't matter as much. And there you have it.
I have some videos of it uploaded on youtube. The first one is of ouside the car with the new muffler on, the second one is of back when I just had a magnaflow, and the last one is of inside the car in the passeger side to show you how quiet it is inside. I did have my windows open and I was in a parking lot so you are hearing abit of echo from the muffler in that one.
Also both videos were taken with the same camera. You can really tell the difference in noise below 4k rpm. Alot more muffled down. Sorry i don't have any with the old borla on but it wasn't much quieter than with the magnaflow alone. Inside the car the noise is about 1/3 of what it was with the borla and about 1/2 the noise on low rpms outside the car.