SpeedAire compressor

All I have been able to find out about it is that it puts out 6 cfm at 125 psi. (according to a listing on Grainger, now discontinued)

On many spec sheets I see 2 cfm ratings, one at 40 psi and one at 90 psi.

Can one figure what the cfm at 90 and 40 psi would be starting out with the info I have? Just don't want to buy any tools I can't use.

Anyone know of a resource for manuals or other info on this compressor?

Thanks for your help.

Brian S, NE

Rating the smaller compressors at two different discharge pressure points (typically 40 and 90 psig) seems pretty common. Personally, I don't see why they bother. The 40 psig rating is pretty much useless for anything except some low pressure spray guns or for pumping up tires. The difference between the two ratings is caused by the volumetric efficiency characteristic of the particular compressor. That is a function of the piston clearance volume at TDC, how smooth and unobstructed the intake passages are, valve design, etc. There is no decent way to calculate one from the other without knowing specific design and manufacturing (or test) details that are not published. In general, it can be said that the capacity will increase with decreasing discharge pressure. Not much, but some. For instance, your 6 cfm at 125 psig might be 6.5 cfm at 90 psig and 8 cfm at 40 psig. Back in the days when manufacturer's were not playing games with hp ratings, a true 2 hp compressor would have a typical rating of about 6.3 cfm at 90 and 7.8 cfm at 40. It would be less than the 6.3 at 125. I think you are around 6.3 to 6.5 cfm at 90 psig. That's very common for a true 2 hp compressor.

Rod

Charles