Air Compressor Volume

The 11.7 scfm, by itself without any qualifying statement, is not particularly meaningfull. The air delivery (throughput) of all reciprocating compressors is determined by the displacement of the machine (rpm x bore area x stroke) and the volumetric efficiency of that particular machine. The volumetric efficiency is determined by the unique design of the particular machine and is also a function of the discharge pressure. It always decreases with increasing discharge pressure for the same machine. You may recall seeing compressor ratings expressed as, for example, 9 scfm@40psig and 6 scfm@90psig. The 11.7 scfm that you mention should have some discharge pressure specifically associated with it if the manufacturer is intellectually honest.

I would NOT recommend using any tool rating as a good way to measure compressor capacity. The competitive nature of the pneumatic tool industry is such that all kinds of games are played with the rated air consumption of tools. For example: 1) some are rated in scfm, both with and without a specified throttle pressure, 2) some are rated in actual cfm at some throttle pressure, (vastly different) and 3) some are rated with an average cfm, either standard or actual, that is the result of an unspecified duty cycle (say 1 minute on and 2 minutes off) that skews the number downward and does NOT indicate the volume that needs to be sustained when the tool is used on any kind of a continuous basis. To top off the confusion manufacturers are not consistent in terminology by some dropping the s in favor of just cfm. Simply using the term cfm with no reference pressure is not only confusing...it is meaningless. Unless you know exactly how that tool is rated (you usually never do) it does not serve well as a measure of compressor capacity other than on an extremely general basis.

I have an old Sears 2hp, single stage machine that is rated at 7.8 scfm@40psig and 6.3 scfm@90psig. Assuming yours is a single stage also, and similar, your 3hp would be about 11.7 scfm@40psig and 9.5 scfm@90 psig. Check to see if your 11.7 scfm is tied to 40 psig pressure (or thereabouts). If it is, your compressor would be quite similar to mine except 50% more capacity and I would guess that your air delivery at 90 psig would fall close to 9.5 scfm.

Rod

11.3 CFM @ 90 psi, 3HP continuous, 7.9 HP intermittent.

I thought I was buying more capacity than I needed, a big compressor as it was the largest one in the store. Turns out I need all of it. I like the compressor as it is very quiet. Just dicovered using an air powered nibbler to trim metal building R-panels. Pretty handy.