Re: Hydralic engineer question: pump dynamics johnson/evinrude impellers
Somewhat a combination of a lot of wanted effects.
You may increase number of wings to gain a higher pressure at a lower rpm.
Not quite sure of which engines you are talking about, but the 4 hp might have a higher exhaust back pressure compared to the 3 horse engine. This may call for a higher pump pressure to ensure same volume. The same for engines with pressure regulated cooling systems (spring loaded thermostat/poppet valves etc) which requires a certain pressure at a certain rpm. On inboard engines I often installed 12 wing impellers on engines with an exhaust back-pressure problems to avoid the impeller wings getting 'blown' off. Another way to go is to make a higher impeller with less diameter like Mercruiser did on their drives long time ago.
