The 2-stage compressor (the 5hp one), which puts out less volume at 175 PSI, can put out more volume at 90 psi than the other one can, and i'm just talking about the compressor head itself, regardless of the tank size.
I guess i've been out of engineering school too long, because i can't figure out the math very quickly. (well, i'm at work, and i just can't spend too much time on it).
Using the ideal gas laws, but estimating the temperature portion (that's the part i can't seem to calculate correctly) that 175psi compressor will put out around 22-24cfm at 90 psi.
The reason the published CFM is lower, is that much more air has to be sucked into the compressor to fill an identical volume at the higher pressure, therefore, it does it more slowly. When it's allowed to work at the lower pressure you get much more volume.
Ok, that explanation is kind of confusing. How about this: With an empty compressor, when you first turn it on, have you ever noticed how fast the pressure needle moves? It moves much slower when it's nearing the shutoff pressure, right? That's because the pump is working harder, therefore, slower. The single-stage compressor has to be running right near the shutoff pressure all the time to put out 90psi air. The 2-stage on the other hand, is halfway to it's operating pressure, in the "quicker" part of the cycle.
All that just applies to the pump itself. No storage tank. Under normal operating conditions, with the tank fully pumped-up, With the higher pressure one, you also have a huge advantage of the larger tank, pumped up to the higher pressure, is going to be filled with more than twice as many air molecules. so that when you use the air, your short-term draw can be much higher volume, for a longer period of time, before the tank starts to "empty out" and your pressure drops below 90psi.
Bottom line, you'll be much happier with the 2-stage one.
(if anyone out there can correct my math, please do!!)
