choke flow range c and then decreases until surge occurs at the point indicated on the thin surge line on the map the surge line is a plot of an infinite number of engine speeds at which surge occurs in order to avoid conditions which would result in surge normal engine operation is maintained at a pressure ratio approximately at the upper end of the choke flow range at any given speed a plot of an infinite number of such operating points produces the thin broken operating line for a typical conventional compressor the margin between the surge line and the operating line for steady state operation protects against abnormal conditions which might affect air flow or pressure ratio and also provides for safe and rapid surge free engine acceleration a measure of compressor performance is its operating range a preferred definition of which is equ1 by increasing the operating range of the compressor increased performance is available without the danger of surge the means now to be described increase the operating range and raise the surge range of the typical compressor whose performance has been reflected by the thin lines in fig1 referencing again the preferred embodiment system shown in fig2 5 a slot 68 extends along the height of each channel suction surface 64 at the throat section th the slot 68 extends into the vane 54 to a cavity 70 which opens into a passageway 72 formed in the overlying front wall 56 the passageways 72 in turn open into a manifold 74 which is mounted on the front wall 56 all of the slots 68 are thus placed in fluid communication.