They were rebuilt at the begining of the season.

toolbox:

What is your speed at WOT to produce 60 psi of water pressure? Where did you install the bypass? In the Merc info we have it says over 30 psi is too much and under 20 psi isn't enough.

I do not know the correct water pressure, but I run about 15 lbs and have never had a problem.
As for the blue impeller, you can run it dry for a minute or 2 without destroying it. You can not do this with the black one.

Water pressure isn't all about the pump speed or the impeller... how much does your pick-up hang down into the water?... If your pick-up is big and hangs low... you are going to get water pressure even if you don't have an impeller.

The bypass is installed per the merc directions. I am running 88. Last year everything was fine then I put on the new lower units and it shot way up there. They have a huge opening on the front of the nose cone. That is where all of the pressure is coming from.

Does it have the pickups on the side also ? I have heard of people pluging the ones in front ....

Big .. you running Circulators ?

I would think that 15 psi is adequate depending upon flow. Low flow would increase pressure and higher pressure may aid the flow if the exhaust is restricted. I don't think proper cooling is dependent solely on pressure.

No, it seems that is low, but I have never had a heating problem. That is why I have never worried about it. I like the idea of the blue impellers living out of water for a few seconds.