I think I'm confusing myself here so maybe someone can help point out where I am thinking wrong: When going from a stock exhaust to a low restriction exhaust, does cylinder scavenging increase or decrease (I know low RPM range suffers from the loss of backpressure and I'm trying to figure out the physics of it).

For example: Starting with a well tuned LS2 in a TBSS with factory single exhaust, muffler, resonator, etc...: When the exhaust is changed from the Y-pipe back, to a larger pipe size and a free flowing straight through muffler, no resonator, the low RPM power drops and low RPM fuel trims go positive. Also, high rpm trims go negative and high RPM power increases. The AFR differences can also be seen on acceleration: In Openloop PE mode the AFR richens at low RPM and leans at high RPM. I would think this is a decrease in low RPM scavenging and an increase in high RPM scavenging. Any thoughts on this?