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I had a Magnaflow custom welded to my WRX for awhile. It was the 5x8 (quieter than the 4x9). I didn't know there was a 5x11, but I don't know if that will fit; the TXS magnaflow muf is the 4x9 and the 9" width just fits in the stock muffler location. The biggest thing is to make sure you get the shorter muffler so the curved axel pipe can have enough room to bend - I had the 5x8x14 and it was perfect: the 5 dimension (top to bottom) could have been a lot larger but the 8 (width) and the 14 (lenght) were just right. Anything larger than a 14 wouldn't have fit.

As far as size goes, the stock piping is 2.25", but you don't need to match that when putting on your own custom muffler. Mine was the 3" in dual 2.5"s out. That was unneccessarily big for just an axleback, my suggestion would be to get a 2.5" muf and have 2.5" piping go back to the axleback flange, then have a flange made for it so it will all bolt together. This way if you want to go to a TXS stealthback later you can bolt your custom axleback right up an not have the muffler be a restriction in the exhaust (the stealthback tapers down to 2.5" pipe just before the axleback flange, so they would matchup and flow perfectly). Just my opinion after going through the same thing.
 
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