i personally use a thermic 17 lavorwash it is 21 litres a minute at 3000psi, i split this to 2 guns with 03 size nozzles for gum removal so two mewn can work, i like robs idea of using larger pumps with massive water flow but have never gone this route due to keeping the machine fed with water but it is obviously working for him on the type of sites he cleans, as rob says there are so many variables and it is finding one that works best for the kind of work you are carrying out, i know personally i would love more flow for large tarmac areas as 21ltr a minute sometimes is not quite enough