he's talking about duals out of the muffler. i have dual 2.5s out of my flowmaster.

 

Let me point out in saying that your PF&E y-pipe has a merged collector with a 2 1/2" exit- so why go into a 3" pipe from that point?

3" is not going to help flow if there is a prior section of pipe (hotter gases also I might add because it is closer to the engine) that designate 2 1/2" dia of flow before it even reaches the 3" section.

Now once it does reach the 3" section, the hotter gases cool and slow down because they are first off further from the engine with every inch, and seciondly you are slowing them up more by increasing the pipe diameter. All of the cooler slown down air now has to be PUSHED all the way out through the muffler and out the tailpipes. It takes pressure to push those gases and this robs flow out of the moptr into the headers, thus hindering flow into the motor.

 

yea, i agree definately that if you have a smaller pipe anywhere in there, there's no reason to go bigger after that.