Originally Posted by laserred96gt
American Thunder catbacks use 2 chamber flows (at least for 96 and up) . As far as power the whole catback is supposed to make more power than the weld in mufflers because they use mandrel bent piping and they flow better. As for the sound I prefer the original 40's sound better than the delta flow mufflers ( which they use in 96-97 catbacks, im not sure about older stangs).
Your right the Force II system is a 3 chamber muffler and the American Thunder is the 2 chamber muffler. I went from the 2 chamber to the 3 chamber and I liked the 3 chamber better because of noise levels. I didn't notice any decline in performance.