So exactly what I said....
The exhaust creates the scavenging affect. Which HELPS evacuate the cylinder of burnt gases. Cam overlap, collector length, primary tube size and length. Among so many other things help. But it ONLY HELPS. The majority of the vacuum that pulls in air/gas is created by the piston on the intake stroke. That's Otto engine 101. You really should know that. Scavenging is basically cleaning the cylinder before you draw fresh air in it. Ideally scavenging would pull out alllll the exhaust, and just start to pull a little of fresh air/fuel so that the cylinder is only compressing all new fresh air and fuel. That's scavenging.
