SCFM with out an adequate reserve in the tank is no better than a large tank with a lesser pump. at least with the larger tank, you have enough air to get some duration of use from the tool, although the wait will be longer.
even a 20 SCFM @ 90psi pump(7.5hp range) wont run a die grinder if its on a 4 gallon pancake tank for more than a few seconds at a time. your wait time will be drastically shorter, but it wont keep up with the tool running continuously.
if you need to run any sort of tool for any long duration, you need tank size to back it up plain and simple. you also need a pump that will fill it at a reasonable rate, but unless you get into the true real 10hp models($$$) there is no continuous running of air hog tools possible.
its a trade off. best bang for your buck is a decent tank, then the most SCFM pump you can afford on top of it.
Oh geeze... Give me a break.
This week I was running duty cycle tests on our air system. We have 160 gallons of tank capacity. With a 12.5 SCFM draw, the tanks went from 150 PSI to 110 in less than five minutes. I can get the exact numbers on Monday, but thats pretty close.
How long is an 80 gallon tank going to last at 21 SCFM? I'm going with not very.
What usually happens is you run the pressure down to a point where the air tool draws less... And that's what the compressor puts out. A bigger tank is good for a couple of minutes, but your never going to do any kind of sustained running like you would running a DA or a cut off wheel.
BTW
A good 7.5 HP system should get you about 24 SCFM at 175 PSI (That's what each of the two IR's I use put out).... That said, its expected that the designer of the compressor does a reasonable job of matching components.
Reasonable is that both factors are important. My point is that don't look at only tank size, You need SCFM for any kind of continuous usage. The tank is a good short term buffer and helps to cool the air but that's about it.
