Obviously, there has to be a balance between flow and back pressure. If this was no the case, then explain why the power and torque dropped across the board when we installed a very high flowing Hermoff cat on our SVT.
As another example, we have a local SVT owner who decided to install a SHO shop off road y-pipe in place of his stock y-pipe and cat. This resulted in a drop of 6lbsft of peak torque and a loss of HP across the board.
His previous peak numbers before the Y-pipe install were 171hp/157lbsft. His runs after the install were 168.2hp/150.8lbsft. The numbers were lower across the board after the main cat was removed.
We also have a Focus owner that installed a 2 1/2" cat back exhaust on his relatively stock Focus (at the time). His before runs netted an average of 107hp/116lbsft. His runs after netted an average of 103hp/111lbsft.
Does lower backpressure causes lower torque? I would have to say that it is definitely related, and not just peak torque values but values across the entire power band, when the backpressure is reduced too much. I don't claim to be an engineer, but I speak of what I have experienced. I am sure there are others on the board that can give a more scientific explanation.
Bottom line is that in our recent experiences, it appears, on duratec equipped vehicles, it is critical what mods are performed on the exhuast side up to the main cat. After that, the sky's the limit (pun) to what you can do with a cat back exhaust. It would appear that the average gains for any good quality cat back nets 9 to 10hp at the wheels.
