I have 2.5 inch flowmaster on my chevelle and i want to go to 3 inch, is this going to increase my horsepower by any, and if anyone out there has done this, how much will this cost me about dual exaust and different series of flowmasters, thanks

It will be louder.

The poop I got from Torque Tech was that 500 HP was sort of the pivot point where 3" made a difference. Further, for torquey 450+ HP cars they recommended 3" head pipes and 2.5" tails.

69Chevl, if this is a street car with a mild motor save your money for something else. You will not feel the differance on the street and like gumball said it will be louder.