Can someone explain relationship of Backpressure to Torque.

I really do not have a clue in this one. If I was to put an X pipe on my GT I am told that the torque will decrease, And I also heard that smaller exhaust diameters are better for torque. What gives? :usa:

Well, the simplest response would be that there isn't much of a relationship.

You have two competing effects at low RPM. One is that the less back pressure, the less work the engine needs to do to force out the spent gases, making for more torque. The other one is that during the camshaft overlap when both valves are open, low backpressure can mean that the incoming air escapes out the exhaust valve, which means it can't burn gas and make more power. At high RPM, the lower pumping losses virtually always win out. At low RPM, it's harder to say.

Realistically, the only way you can tell if a better exhaust will decrease torque at low RPM is to get a dyno graph from an engine with the same specs as yours testing out various exhausts. It's not something that's easy to predict in advance.

Its an arguement that will never end.
On the 4 cylinder side of it, the 2.5 inch exhaust has proven to be the best for overall power and gains and seems ironic that most v8 engines only use 2 inch for duals.
Ive always heard the crossover pipe can play a role in improving the midrange power of the engine and if placed properly in the exhaust can yield more overall torque than without.
Placement is critical.
Madscientist, the cam issue seems like it would be for the performance cams that do have more overlap but for most stock engine setups, I dont think it would really have an effect on the overall performance.
Lastly, the smaller exhaust diameters may increase backpressure for better torque, but when your drag racing, you want top end and larger piping would seem appropriate.
Misnblu

Misnblu, that depends on what the stock cam specs are! There are quite a few engines out there running high overlap profiles these days. Motors tend to be higher winders now. Still, the only way to know for sure what's going to happen is to dyno test it. Sometimes highly cammed motors don't loose torque at low RPM with less back pressure, and sometimes they do while running the stock cam.

And I'd run 2.5" dual exhausts on a V8 anyway, unless it was a very, very mild application.

Back Pressure IS NOT HELPFUL!!!!!! Basically you want to balance the exhaust side with the intake side on a motor. High volumetric efficiency % , swirl/burn efficiency, and good cylinder scavenging all play a role in making torque.