I am looking into buying a cat back exhaust however I do not know what size piping would be the best for fuel economy. I am considering the Magnaflow 2.25 in. exhaust. I do not want a can for a muffler either. Would a 2.5 be better or would that be taking away from back pressure?

 

i think 2.5 would be best anything above that on a na you would loose back pressure..

 

2.5". You want unrestricted airflow, but you want that airflow to be laminar, i.e. moving in the same direction and with decent speed. Go too big, and the exhaust stream slows down and gets choppy. At that point the exhaust can't do it's main job as well, mainly getting hot gasses away from the engine. Low back pressure doesn't burn valves, running lean and not being able to remove exhaust gasses does.

If you go custom, get the longest resonator you can find before the muffler.

 

Considering that the oem exhaust starts at 2.25" and tapers down to 1.75" after the "scuba tank", IMO 2.25" would be a big improvement. Although 2.5" or larger may be better for peak dyno numbers. I would try to match the oem "taper" to keep exhaust gas velocity high as the gas temp decreases but increase the size throughout. The Magnaflow or ebay 2.25" exhausts should both work well, if not optimally.

 

The magnaflow cat-back on my 09 xB is a nice exhaust. Its well built but too quiet. To me its just a hair louder than stock. Wish i would have went with a custom exhaust or went with one of the borla cat-backs.