Your Curtis pump is a old one, maybe 1950's or '60's vintage.
They are a very well-built, high quality pump, with the 'ring oiling' system.
You haven't a chance of anywhere near 20-ish cfm, tho.
One with those bore sizes would be meant to develop 5-ish to 6-ish cfm, with a 2hp motor, running 400 to maybe 500 pump rpm.
Theoretically, you could run one of these at its max rated speed, I think 850 but don't remember exactly, with a 3hp motor, for more cfm, at the price of much lesser life between overhauls.
That size was a very common installation in gas stations, years ago. Run in the 400-500 pump rpm range, and with the oil and the intake air cleaner element changed regularly, they run reliably for many, many years in service.
Added on edit......I don't have Curtis specs easily to hand, but per Quincy specs, the most air you can get with 5hp is 15.6 cfm, with either the Quncy model 270 single-stage, or the model 350 two-stage. Those are much larger pumps, the bores on a 350 being 6" and 3.25".
cheers
Carla
