Compressor and CFM question

[Eiji Fuller]

I have a compressor rated at 5.1 CFM @ 100psi. Does that mean I have more CFM at a lower pressure and less at a higher pressure?

[Glen Butler]

Basically Yes. The CFM is the volume of air the pump can supply at said pressure. Meaning when the motor has a load applied (that is the pressure in the tank) it will slow down thus slowing the pumping action. Manufacturers use 100 PSI as a baseline for giving stats. Some companies use 90 PSI, to make their products sound better, but according to my air tool guy, you lose about 33% on 90psi ratings. That is to say a 5 CFM @ 100 PSI is as good as 7.5 CFM @ 90 PSI

[John Callahan]

Some companies use 90 PSI, to make their products sound better.........
True enough. The other thing that's often overlooked is duty cycle, especially with a portable compressor. Some are only 50%- not an issue most of the time perhaps but if you start running air tools such as ratchets, impacts, or spray guns it could turn into a problem.

[Gary Curtis]

The CFM rating becomes critical when using air tools such as Random Orbital Sanders, Air Drills and Air Ratchets. You can help matters by only using a 5/8 inch hose. A 3/16 inch looks about the same, but reduces air flow considerably.

Because I was restoring old cars and used such tools, I bought a 4hp, 220v compressor from Sears. When painting a car, that machine got quite hot and if I were to do it again, I'd buy a more substantial compressor. By substantial, I mean not a Sears Craftsman. They lie about hp ratings of electric motors. 220 isn't really "better", except that the motor runs cooler when heavy demands are placed on it. And wall circuitry with a lower amp rating will do the job, whereas 110 would pop a breaker if the wire is too thin.

I would get one with a larger air tank so the motor doesn't run so often. The little Hot Dog style compressors are adequate in a woodworking shop because you'll mostly be blowing sawdust off things and using a pin nailer. Not much power require for that.

I am buying an HVLP spray gun called a Conversion Gun. It runs off a compressor instead of a Turbine, as most HVLP guns do. And it requires even more CFM. So I am dumping my old compressor and getting one about 1/3 larger in size and capacity.

Gary Curtis
Northern California

[David Epperson]

I have a compressor rated at 5.1 CFM @ 100psi. Does that mean I have more CFM at a lower pressure and less at a higher pressure?
Also, while it is rarely stated, those ratings are in SCFM - Standard Cubic Feet per Minute. Or rather it's how much air the compressor will suck IN. If you have a 5.1 cubic foot tank(32 gal) - it will not get to 100 psi in one minute.

Standard pressure is about 15psi. So your 5.1 CF tank would take a minute to get to 15psi, another to get to 30, still another to get to 45 - or almost 5 minute to get to 100 psi. It'll really be a bit quicker than that - but as a rule of thumb that'd be close.