I tried it at six it might lift more head but did not test...
I use to install chemical filtration equipment to meet the Canadian Environmental standards. This
pump moves about an once per stroke at about one stroke just under a second. I have a couple of double bellows... I also have 2 pumps that are like the kidney dialysis pumps. They are capable of incredible pressure that you cant stop by holding it... I want to put them in to an atomizing grow bed...
Those pumps are very expensive and built to very demanding tolerances.