Here is a link to the website of Old Custom Cruisers, a car club in Bristol, Tennessee which owns two Ahrens-Fox pumpers; one from Chicago and one from Maywood, Illinois.
This 750-GPM Ahrens Fox pumper (piston), Model: IM-2 was built for Chicago Fire Department 7 and was stationed in the Stockyard area of Chicago. It was manufactured in 1921 and delivered in 1922.
Hours on Pumper: 750 gal @ 120 lbs for 6 hrs, 375 gal @ 200 lbs for 4 hrs, 250 gal @ 250 lbs for 2 hrs
Engine Details: 6 cylinder engine, 2 plugs per cylinder, duel ignition system, magneto & distributor, 3 speed manual transmission, gas, up draft carburetor, mechanical brakes, spring suspension.
The “Ball”, or the pulsation chamber, reduces the throbbing of the stream as it ejects from the nozzle. The tall chamber on the front retained the vacuum on the intake side of the pump. With minor variation, the style of this pump did not change from 1915 to 1952.
thanks Dennis