If you get a magnifying glass and get a good, close look at the HP rating, you'll see it says "Peak HP". Somewhere, sometime, during the operation of the motor, there is a slight possibility of a chance that 4 HP might be generated in some tiny location within the motor. This, of course, is perfectly useless to anybody, hence the fine print.
I've heard several well-intentioned samaritans suggest going strictly by the amps and volts in determining the actual power output, however, an inefficient motor will draw plenty of amps without doing all that much.
Other helpful folks suggest going strictly by CFM ratings. Personally, I feel it's naive to believe these figures aren't just as inflated as the HP ratings.
My method for buying tools such as air compressors, is to pick the brains of folks who use them to make a living. If they buy junk that falls apart in a hurry, they lose money, so they know what works. This, of course, doesn't apply to slugs who don't finish the job. (Yes, I'm talking about YOU Nick! Finish my damn bathroom!)
Oh yes, the point. If I were in your place, I'd try to match up the specs (volts and amps) of what's being replaced. Not only will this ensure you'll have the power to run the new rig without popping breakers, but you'll get at least as much, probably significantly more, CFM than with the old. Motors have come a long way in 40 years in terms of efficiency.
<DIV>What kind of games are they playing with HP now days?</DIV>
<DIV>My old air compressor had developed a rust hole in the tank and I am
looking for another one.</DIV>
<DIV>The old one (probably 40 years old) is 2 HP and pulls about 20 amp,
230 as best as I can remember without looking. Saw one that was rated at
4 HP and ran on a <STRONG>standard 120 volt cord.</STRONG> In the
vehicular, bull shit. Any <STRONG>real</STRONG> 4 HP motor should pull
at least 30 amp, 230. Pulled out my old W W Grainger cat and didn't
see a 4 HP but the 3 HP I saw pulled 21 amp 230. That's 42 amp 115 and
they are getting 4 HP out of a 20 amp 115?</DIV></BLOCKQUOTE></BODY></HTML>