If you have air tool's then the compressor is the heart. As was mentioned above pump RPM is something to look at, a lot of manufacturers make up CFM by spinning the guts out of the pump and that will cut down on longivity. Look for a true cast iron compressor, two stage with at least a true 5hp motor with a rpm of 1750, pump speed is 546 rpm. Mine is 80 gal, 5hp, 1740rpm (Leeson motor), 18.4cfm @ 175psi,( it is a horse). In 1996 it cost $1050.00, got another one for my other shop in 2005 $1150.00, same rig, the owner of North Central Air out of Downs Ks. said that it was his best selling unit for small shops and farm use. To be truthful with you all I went to the bank and borrowed $1500. in 96 and got the compressor and a 100lb pressure sand blaster, for around $80 bucks a month for a couple of years. Was the best investment I ever made in shop equipt.My advice is buy the best compressor to do the job even if you have to finance it for a while, it will outlive the inexpensive one time and time over.
