I'm looking at the specs for an IR 213C impact wrench here:
http://www.irtools.com/IS/modelComp_...61,862,311,219
It shows that the "Av. Air Consump. cfm" to be 4.2, and the "Min. @ load cfm" to be 22. What's the 22cfm? It seems like it would take a lot of compressor to sustain that flow.