That's a zeroth order.. eventually, the high side pressure gets so high that some reversion and leakage equals the ability to pump air, but it's at so high a pressure and so low an efficiency that it's more of an air heater than a pump. And like all gizmos, there is a very slight bit of leakage (that's why some roots blowers have an abradable coating, they just wear to tight tolerance) and this becomes a larger percentage of air moved as rotor speeds get very low. But this isn't much of an issue if you look at the efficiency map of the 1320, it gets it's best TEs at 7000-10000 RPM (I think, I'm not looking at the map now). Matt