Tools, Sharpening and Power Machinery - Air Compressor CFM too low?

I just purchased a 33 gallon 1.6 hp sears oil less air compressor. It is rated at 6.3 scfm at 40 psi and 4.9 scfm at 90 psi. I decided to calculate how many cfm of air i was actually getting.
It took 3 minutes and 20 seconds to reach 90 psi. I calculated the tank as 33 gal / 7.48 cf/gal = 4.41 cubic feet of air in the tank.

So I think it should have been able to fill the tank in about 54 seconds....(NOT taking into account that the compressor gives more air at a lower pressure)

Instead taking 3:20 to reach 90 psi means my at best the compressor is giving me 1.3 CFM at 90 PSI.
(4.41 cubic feet / 3.333 minutes)

So here is my question Am I calculating this wrong or should I return the compressor for being defective?

Here is the data I collected:
38 psi 60 seconds
60 psi 120 seconds
82 psi 180 seconds
90 psi 200 seconds
102 psi 240 seconds
118 psi 300 seconds
134 psi 360 seconds
148 psi 420 seconds

Was this done at 68 degrees F and 36% RH? :) Joke.. Anyway, welcome to the weird world of compressor ratings by manufacturers...If you try to take it back based on this, they'll probably tell you that it's the displacement cfm and not the actual cfm that they are advertising :wall: Based on the pump up time, I say you have a good unit there, try to run it at 50/50 duty cycle and it may actually last a while. Also, if you keep it, knock the pressure down if you can, you don't need 150 psi and it obviously is very inefficient at that setting.

Wow, I turn mine on, it pressurizes, I use it and smile. Just don't have time to do all those calculations. But good luck with your problem.

Yeah, Larry, but these oiless jobs that Sears pushes on the consumers are extremely ineffective. I understand the guy's concern, but in the long run you get what you pay for. Because of the one piece connecting rod and piston design, it never really seals well to the cylinder and probably compresses only 40% of the air that it intakes. Hence, the low numbers. Hate to imagine what happens when it starts wearing out.

Oh, I agree. When on a jobsite and a trade has an oilless compressor, I make them put it in the yard and run hoses in. Noise is unbearable, and they seem to cycle alot more than my double hot dog Ridgid.
And the cheaper they can make them, the more money China makes, right?? But I digress politically, sorry.