Looking up upgrade after Dyno Don's y pipe. Would it make much sense to increase from my 3 inch catback to a 3.5 or even a 4 inch. Will the 2.5 inch y pipe even flow enough to fully use a 3.5 or 4 inch mufflex or should I keep my 3 inch on right now?

 

Some things to ponder. Two 2.5" pipes equal 9.8 square inches. One 3.5" pipe equals 9.6 square inches. Pretty close. One 3" pipe equals 7 square inches.

 

So you really would not gain much going to a 4 over a 3.5 with a 2.5 inch y

 

The other point to ponder....
You might need 9.8 sq. ins. at the collector,(when ex. is hot) but by the time the exhaust condenses down the line, you probably won't need more than 7.5 sq. ins.(because it cools and therefor reduces in volume).

In other words, there are plenty of 11 sec. & low 12 sec. cars using 3"

 

And having a 3 inch cutout right after the y pipe probably helps a bit also