The X axis on most compressor maps reads in lbs/minute, although I have seen a few that read in cfm. My question is this: at higher altitudes, would a a turbo still flow it's rated amount in lbs/minute or cfm? I would think cfm, but I'm not sure if I'm missing something.

 

Air flow is the same....the density is much less. That is why cold air is preffered, the same amount of air flows, but the density is much higher. Example 1 cf of air at seal level has 100 molecules of air, while at 4000, 1cf of air has 75 molecules. Though it isn't really the air, but the oxygen, and air only has ~20%oxygen. This is why people use N2O, it is an oxidizer, and that little bit of air that same cfm, has MUCH more power. Also why a turbo charger is used, makes the air more dense by compressing it. A turbo can make a 4k' car perform like a sea level car at lower psi, except that turbo'd car compressed the air making it hotter, so you would need a bit more psi to 'level the playing ground'.



I think CFM and lbs/min and all that is just preference like metrics, vs SAE, they can both be converted.