An "SCFM" (standard cubic foot per minute) is a CFM produced with input air at 68 deg F, 36 percent RH, and 14.7 psia pressure (the mere letters "SCFM" refer to no official standard, and while various temperature and RH values are in use, these are the most commonly accepted values). "Displacment CFM" is the rate of volume displaced by a reciprocating piston compressor, which is compared to the delivered CFM to evaluate volumetric efficiency. "Peak horsepower" typically means the electrical power drawn by the motor at the instant of starting; this figure is a meaningless specification because it says next to nothing about the sustainable horsepower delivered by the system. "Peak horsepower" most definitely does not mean anything like "what you get if you run this unit full throttle", "what the motor can deliver for short periods of time", or "what the motor can do if heavily loaded". Also, rated CFM at "90 psi" can really mean the inflated value measured from the CFM input during a pump-up from 0 to 90 psi. Such trickery is what you get in the absence of well-defined engineering testing standards and methods, which is to say, "consumer" mentality. This applies to larger systems like the 5 HP 80-gallon units common in auto repair shops, just as well as the homeowner models.
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