The basic concept of exhaust "tuning" it based on pulses in the exhaust system. It's kinda complicated, but basically, the flow coming out of the engine isnt at a constant rate. It comes out in pulses, or slugs of air, each time an exhaust valve opens. This causes pulses in the exhaust stream. Now when these are flowing in a tube, some interesting things happen. Essentially, when they hit restrictions (collectors, etc.) or when they reach the end/opening of a tube, there are low-pressure pulses/waves reflected back UP the tube (towards the engine). Now if you can tune the length of the tube and the type of restriction, you can time these refleected pulses to coincide with exhaust valve opening events. Essentially, if you get the length of the tube correct, at a certain RPM range, this low-pressure pulse will make its way back to the exhaust valve right as it (the valve) opens. That means this reflected low-pressure pulse can actually help evacuate more exhaust gas out of the cylinder, by essentiually sucking the hot exhaust gas out of the cylinder, leading to better exhaust flow. Unfortunately, a certain length of tube will only cause this beneficial effect in a relatively narrow RPM range, which is why long-tube headers and shortie headers will move torque and power peaks up and down along the RPM range. This same principle can be used to take advantage of these pulses in different ways. I can only imagine flowmasters work on this same basic principle, by using these pulses to essentially internally cancel out some of the increased backpressure/restriction placed on a system by adding a muffler. Now when you try and apply this concept to a turbo engine, you can see why it has little effect.. any pulses/pressure waves in the exhaust would be so disrupted and "chopped up" by travelling through the turbine housing/wheel that any "tuning" effects after the turbo would be kinda pointless, as the pulses wouldnt exist, or if they do, would be so weak as to be negligible. I'm sure this is an over simplification, and I'm sure there are those on this board that could do a MUCH better job of explaining it all (and I'm sure there are probably some things in this that are flat wrong, keep in mind this is info I'm trying to remember from my aero- and flow-dynamics classes at the AF Academy 4 years ago), but hopefully this will give some idea of the principles involved.
Thank you for your time for righting all that.
