i have the "quincy 350-375" , and i'v had it for almost 40 yrs. !! i have always ran it on a 5 hp single phase @ 1750 rpm ! (about 25 cfm @ 100)comp mtr is at 900 rpm
it had rings,and some valve work done about 10 yrs. ago,and i'm on my 3rd elec. motor-- over the yrs. the 5 hp. motors have gone from 200 to about 325.00 (1750 ind. type)
the thing about the quincy "350-375" is you can run any motor from 3 hp to 40 hp.depd. on how much air you need .
**note** it makes no difference how big your storage tank is(60,80,or120) if you take 10 lbs. of air out of it --it's going to turn on--this type of compressor motor also can run on 100 % duty cycle--in other words it can run 24/7 !! any compressor that runs on 3600 rpm only has a 50 % duty cycle,and if it runs all day trying to stay up ----it will burn up in short time !
**i don't work for "quincy" or have any affiliation with them**
this is only my opinion......steven