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Hi, in new to the jet boating would and just got my dad an old 67 Tahiti jet boat with a 327 Chevy. I am trying to understand the cooling system how it routes correctly, as i do not believe it is . it has log style exhaust and bypass thermostat. from what i find online (whitch its hard to find a good diagram) the inlet water from pump should go to a relief valve to a tee and go to the logs to preheat the water and then to a tee to the block and upper bypass ports the bottom ports go over board or to in to the exhaust. I think i understand the system but if i understand it right why do people run headers as the engine would never get to a good running temp and there would be premature ware on the engine let alone loss of power from running cold. Are running headers just to look cool? I thought i had a better picture of the cooling system sorry. thanks for the help!


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Here are 2 versions one without bypass one with.
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The second diagram shows the correct routing for a buypass system.
Most route the water from the pressurized jet through the exhaust logs for max cooling and engine preheat.
then into the front of the block. out through the top thermostat housing to the snales or turn downs. The bypass system adds a extra source of water to cool the turndowns when the thermostat is closed.
 

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On a jet boat one of the more important pieces of your cooling system is the flow control valve between the pump and the system. Since we don't run thermostats in the boats and the jet pump can provide such a huge volume of water the flow control valve is critical to help maintain engine heat. If I run my 21 foot mini-day with the valve wide open engine temp will barely get off the cold peg. by starting with it open and then gradually reducing it down, I run my engine at 160 wide open for long periods of time. Running the water through the logs first is also to help you get heat in the engine.

Caution: Pay very close attention to your engine temperatures until you know you have everything set right. Boat engines with open cooling systems heat very differently than a boats with a closed cooling system.

There are performance gains to be had with the open headers. The water going through them is to help reduce sound not to keep them cool to the touch. Logs give up some performance but are much safer as they don't get hot like headers. Not to mention, logs are Coast Guard approved for closed engine compartments.
 

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Just an observation, I see you are running turn downs on a jet boat, The engine sits low in most jets and the exhaust tips are in , or under water. Make sure the exhaust flappers are in good shape or the water will wit the rocking of the boat may let water into the engine. Snales will help but you will loose a little more horsepower
 

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Just an observation, I see you are running turn downs on a jet boat, The engine sits low in most jets and the exhaust tips are in , or under water. Make sure the exhaust flappers are in good shape or the water will wit the rocking of the boat may let water into the engine. Snales will help but you will loose a little more horsepower
i ran a 17' jet with an exhaust like this for 20 years, with no flappers, and never had a problem.

if you're running it on a hose, make sure the boat is "bow high". never run it on a hose with the bow down.
 
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