Last summer, i installed the Flowmaster dual 2 1/2" in 3" out "Y" pipe. After overcoming some other problems, I have concluded that too much back pressure was eliminated. My muffler is a Ravin
Z55 dual 2 1/2" out. It would seem as though the easiest way to restore some back pressure, "move the torque curve back down" would be to reduce the pipe size coming out of the muffler. My question is, how much do I reduce the pipe size?

 

Hmm good question. Try calling Flowmaster and talk to them. I talked to Gibson about there exaust system and they told me that the single pipe system they offered gave the best performance gains for everyday driving due to back pressure.

 

I have a Flowmaster si/do exhaust system on my truck. I used the stock 2-1/2" pipe going into the muffler, and then I used 2-1/4" pipe for the duals coming out the muffler. I was told that this would keep my backpressure up to par. There is no loss in backpressure or low-end torque that I can feel. So stick with 2-1/4" pipe coming out of the muffler, and you should be fine.