Sorry guys, but any claims that a 2 stage compressor is "more efficient" than a single stage of similar construction is purely a myth.
2 compressors with equal hp, running at the same speed, similar pump construction but one being single stage and the other two stage, the single stage will put out more CFM every time.
The reason is real simple. In a 2 cyl compressor, for example, you have 2 cylinders pumping air to the tank with a single stage, and one cylinder pumping to the tank in a 2 stage, while the 2nd cylinder is just pumping to the cylinder that's actually supplying the tank.
If you want to get an idea of which compressor is most efficient, check the discharge temperature of the air coming from the single and 2 stage units. The 2 stage will always have the higher discharge temperature. Heat rejection is the most common and most accurate method of measuring wasted work. Higher discharge temp = more wasted work = less efficiency.
Simple reason for this as well. The 2 stage runs the same quantity of air thru 2 compression cycles. Each cycle represents work input to the air. Some of the work is stored as potential energy in the form of compressed air, while the rest is converted to heat, either carried in the air or rejected thru the compressor casting, piping, intercooler piping, etc. Take a specific volume of air and compress it once, and you'll have some discharge temperature. Now run that same chunk of air thru a 2nd compression cycle, and it'll get hotter every time. Each compression stroke represents a combination of useful and wasted work, so when you compress the same air twice you will always be adding the wasted work to that air as well as the useful work. Compress it once and push it to the tank, as in the case of a single stage, and you've added less heat, and less wasted work, to that volume of air.
For most uses, whether its a body and paint shop, or a machine and fab shop like my shop, a single stage compressor will do anything that needs to be done via air, and it'll do it cheaper both on a monthy operating cost and on a long term maintenance cost, because the single stage will typically be operated at a lower temperature and lower level of mechanical load. For some reason though, there's been such an effective job of selling people on the need for 2 stage compressors that its difficult to even find large cast iron single stage pumps today.
We've got 3 compressors at our shop. One is a 5hp 2 stage IR, made before the US discovered manufacturing in China, which we've always run at a cutout pressure of 175-180 psi. Because my dad has been in the heavy equipment business for about half a century, the pressure is set that high so it doesn't take half a day to fill a big tire to 120 psi. Its piped into the shop air system, but valved off from that system unless we happen to need it for something unusual.
The 2nd compressor is a Quincy single stage made in the mid 50's. 2 cyl pump, rated for use with a 5, 7.5, or 10hp motor. Its got a 3hp single phase 1750 motor on it because it was originally bought for use in a state highway maintenance garage in a remote mountain location where the only electrical service into the building was a 100 amp single phase.
The third is a 2 stage 10hp 3 cylinder radial. Its the one that's normally used for general shop air supply. We normally keep it set for 140 cutout and 105 cut in pressures.
The 10 went down one day when I was bead blasting a bunch of parts. I opened the valve and started the 5hp IR. Found it couldn't quite keep up with the blaster. Almost, but not quite. So I started the 3hp Quincy as well, and finished the job.
Then, out of curiosity, I tried the IR, and then the Quincy, separately, just to get some idea of how much air each was putting out. Nothing else using air at the time. Laid a chunk of steel on the blaster's pedal and let each one run until their pressure was no longer dropping. IOW, would one hold a continuous 35psi and the other a continuous 70psi, or whatever, since it would give some reasonable measure of the output of each.
The IR ran a while and eventually settled out at 66psi. Then the Quincy settled at 62psi. Very little difference in output, since both were discharging thru the same orifice and both pressures were measured with the same gauge.
Remember though, the Quincy is only 3hp while the IR is 5hp. Two things going on here to make their performance so nearly the same. One was the inherent better efficiency of a single stage, primarily due to less work being wasted as heat. The other is that the slow running speed of the Quincy made its pumping efficiency even higher than normal via reduced intake pressure losses, and even less than normal heat input into the air due to the reduced speed necessary for it to play nice with the 3 hp motor.
Obviously, if you have a need for high pressure air, the single stage isn't the way to go. OTOH, its real difficult to find any sort of tool or other air operated device that's designed for an operating pressure greater than 90psi. Personally, if I ran across someone who had a quality 10hp single stage in good shape, and they had a need for a 2 stage, I'd swap 'em my 10 horse in a heartbeat.
Regarding the 5hp IR single stage mentioned above, the 18 cfm is a very rational output number. 4 cfm per horsepower isn't unusual at all for a single stage. Dad's got a number of portable compressors from small to pretty big, all with CFM measured at 90psi output. Little 100 cfm jobs, driven by a 25hp diesel, up to a 1600cfm driven by a 400hp diesel. Right across the board, even though the collection is made up of several brands, the CFM number is right around 4x the engine horsepower number.