Raw water pump sizing

Discussion in 'DIY Marinizing' started by wildman, Feb 8, 2009.

  1. wildman
    Joined: Feb 2009
    Posts: 9
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    Location: hobart

    wildman Junior Member

    I've got a marinised Toyota 2H that's been performing beautifully for many years now. However, the raw water pump is showing it's age. It's a gear type pump that leaks no matter what we do and needs replacing. I have acquired a Johnson impeller pump (F7B-8 seems the closest match). Now I need to work out the pulley size needed to get the required water flow.

    Water is pumped through heat exchanger and hot water system then finally the wet exhaust. If I make the assumption that too much flow is not a problem then applying pulley ratios I can come up with a suitable size. But is this assumption correct?

    Can the water flow too fast through the heat exchanger?
    Can the the water flow into the water lift muffler be so great that I create too much back pressure - or worse fill the engine before the exhaust can push it out?
     
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    The Johnson F7B-8 is a good choice, it supplies plenty of raw water for your Toyota when connected with 1/2" rubber hoses. But unlike the old pump, it should not be allowed to run faster than 2500 rpm (some say 3000, I'd prefer to be on the safe side). And it hates running dry for more than a few seconds.
    I installed mine in the lowest part of a loop, so there is no chance of running dry.
     
  3. wildman
    Joined: Feb 2009
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    Location: hobart

    wildman Junior Member

    Thanks CDK. That tells me I'm on the right path. Does your answer imply that driving at no more than 2500 rpm (3000 max) will NOT supply TOO MUCH water?
     

  4. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    That is correct.
     
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