1979 Mercruiser 5.7 cooling system questions

colesdadd

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Joined
May 31, 2010
Messages
9
I found out this week that the Lenco heat exchanger I got (used) is a "half system" which is fine. My engine is a 1979 5.7 and I have log style exhaust manifolds. The ends on each manifold have a hose attachment nipple. The risers have one nipple on each one. The manifolds are new, the risers are from my original engine that was raw water cooled. The guys at Lenco told me where to attach each hose. That's where the confusion started. The info I got was to attach the two raw water hoses to the "bottom" of the manifolds.

I'm not sure what they meant by "bottom of the manifold"........ is that the front of the manifold? That could make sense to me if my understanding of how the cooling system is correct, I'll explain my perception of how the cooling systen works.....

The pump in the outdrive sends raw water up through the power steering fluid cooler. It then travels into the heat exchanger cooling the engine antifreeze. It then travels out of the heat exchanger through the exhaust manifolds and back out to the lake/ocean.

Isn't that how it works? If so....... here is question #1, where on each log style manifold do I hook up the hose that will send the raw water back to the lake/ocean? Do I need to use both ends on a manifold or cap one end off? Question #2, what gasket set do I use? Do I need to use the stainless steel part of the gasket set or just the gaskets? And last question is, do I need to get a raw water pump (the kind that works with a V-grove belt from the crank pulley) or is the water pump in the lower unit all that is required?

Can anyone direct me to where I can get the answers I'm looking for? Thanks in advance :)
 

wire2

Lieutenant Commander
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Jun 25, 2007
Messages
1,584
Re: 1979 Mercruiser 5.7 cooling system questions

The log portion should have either a 2nd nipple up high, or have water ports from the log to the riser. If 2 nipples on log, exchanger water goes in the (vertically) lower one, out the upper one & into the riser. The riser will dump the water into the exhaust flow to cool it, and all will go down to the prop.

If there's one nipple and water ports between the log & riser, you just need to send water from the heat exchanger into the nipple. It will flow a similar route as above.
 

colesdadd

Cadet
Joined
May 31, 2010
Messages
9
Re: 1979 Mercruiser 5.7 cooling system questions

Thank you wire2 . . .

Of the two ways you described which would you consider more efficient?

If I were to go with the single nipple setup would I use the gasket set without the stainless steel plate ?

I'm assuming that if I use the front nipple only I would want to use the gasket set that would allow the water to move through the manifold and (unrestricted by the stainless steel plate) flow up through the riser passage ways and then out the back of the risers cooling the exhaust as it exits . . .???

Any thoughts about the water pump in the outdrive? Is it adequate to move enough water or do I need one of those pumps that run off a V-belt? My hope is that the one in the outdrive is all I need......

I used to race drag boats back in the late 70's . . . headers are so much easier lol

Thanks again for your help, much appreciated !
 
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