Minimum Air Compressor

I've got an Eaton 2 stage pump and a good baldor motor (I forget the frame number but it's a great motor.) that flows 21cfm @125psi coupled to a 80 gallon tank. If I use my good DA or long board airfile I can run it quite a while before the compressor kicks on and fills the tank back up. BUT if I use the Harbor freigh da or my old Mudhog I'll kick on the compressor a lot sooner and it'll keep running longer after I let off of the on the handle. man those things just let the air flow right out. sand blasting is a good air hog too! (i added another 60 gallon tank to help support the sand blasting process. ) and take a break every once in a while. it's 220 30amp setup and I wish it were bigger