MSR Technical Centre
Research & Development: Exhaust System - Diff-back pipe
 
The diff-back pipe leading into the rear muffler in the exhaust system has a huge bearing on the torque band. Using an "S" bend leading into the rear muffler via 2x 90° mandrel bent pipes back to back produces the most torque in the midrange. Reducing the pipe angles from 90° to 45° moves peak torque to a higher RPM with a slight loss in midrange torque. An almost straight pipe with an angled muffler produces a very linear powerband with a smooth delivery of power due no "kick" as a result of the noticable loss of torque in the midrange. As a result, the "S" bend has a huge bearing on midrange torque. After consulting with Prodrive, they had also confirmed this to be the case.

Changing the diameter of the diff-back pipe that leads into the muffler from 3" to 2.5" also helps improve midrange torque. Pressure at the turbo is not affected by this decrease in diameter. As a matter of fact, it actually helps to keep exhaust gases flowing for better power.

Have a look at the images below (click to enlarge). You can see the differences between diff-back pipe setups and the effect that each one has on the torque band. As the pipe straightened out, I noticed that the torque band shifted up in the RPM range. A straight pipe may offer more top-end power, but the loss of low-end power outweigh the power gains.