"There are two types of compressors. Single stage and two stage. The two stage compressor pumps air on both the down stroke and the up stroke and will keep the air pressure at a more constant level. In theory this will produce twice the amount of air as the single stage unit." Not really. Single stage compressors put the air from each piston into the tank, and are generally limited to 110psi. Even if they have two pistons, each one pumps into the tank individualy. Two stage pumps compress the air in the first piston to ~80psi, then cool it, and compress it again in the second stage piston to a higher pressure, usually 175psi max or so. They may be 2, 3, 4 or more cylinders, working together in several different configurations and flow paths. The only compressors that I'm aware of that pump on both strokes are massive horizontal industrial units built prior to 1970. They require a crosshead and a piston rod sealed with packing to obtain the second stroke.