80 or 90% air?? what would the other 10-20% be if it weren't air? Typically scavenging is referring to removal of exhaust gas using a chain of exhaust gas pulses that are tuned. Turbos are not going to help scavenging in that manner. They create positive pressure in the manifold. You can't get something for nothing.. it's the positive pressure in the exhaust manifold caused by the actual flow of the exhaust, and the continous expansion of exhaust gas that makes this pressure. I have a 7.3 powerstroke. I know it makes boost because my boost gauge reads around 20PSI when I'm on it. If you check the exhaust backpressure sensor reading, it's always about 50% higher than the boost pressure. I don't know of any overlap in the valves, so the forced evacuation theory only holds true on the old Detroit 2 stroke diesels that had a supercharger for that. I might be wrong.. it's happened b4, but I'm pretty sure of this.
Don.