Proper hose configuration for exhaust manifolds

After 20 years I am repowering my boat. Engines are new GM 350's (5.7L) which are direct replacements for the old engines. Engines have closed system fresh water cooling but exhaust is raw water cooled (saltwater).

Exhaust manifolds have been Marine Power log style, and yes they lasted all this time! However they are out of production and I have been unable to find replacements.

I have obtained Mercrusier type center riser manifolds for replacements (Osco - 3" exhaust). I want to get the raw water flow thru the manifolds correct.

Would the correct flow be water in the bottom of the manifold, which would continue thru the water jackets thru the riser then out the exhaust?

Or should the flow split, one hose into the bottom of the manifold, one into the rear of the riser?

Also is a "balance hose" necessary between the two risers on each engine?

Thanks in advance for any information

If the heat exchanger has enough capacity, I'd put the manifolds in the coolant side and put the elbows on the raw water side. The flow should always enter the low port and exit out the high port. That will minimize air pockets which don't cool like water. if you go this route, don't forget the blockoff plates between the manifolds and the elbows.

no balance hose is required.

I agree that it would be better to cool the manifolds with coolant. So if I'm thinking right, the flow would be from the HE to the water pump, off either the side or top port of the water pump, to the bottom of the exhaust manifold, out one of the top ports of the manifold (either in front or behind the riser) and back to the intake manifold.

I also heat water in the hot water tank with engine coolant, would I want the coolant to leave the exhaust manifold thru the hot water tank back to the intake or the other way around (hot water tank first, then exhaust manifold)?

On the the raw water side, from the HE to the lower port on the back of the riser.

I hope I'm making sense describing flows! I know what I mean, but to make it clear in a post is sometimes confusing!

The only "link" I'm not certain about is the second one.

The coolant usually leaves the thermostat housing, thru a T or Y fitting. Each hose connects the t-stat housing fitting to the bottom of the exhaust manifold. The upper fitting from the exhaust manifold then passes the coolant back to the HX.

If you want a can post a mercruiser 5.7 cooling routing

justintime said:

If you want a can post a mercruiser 5.7 cooling routing

Click to expand...

I would sure like to see that if you would post it. Thanks.

new board restriction limits file size of images to be very small. Send IM with email address and the diagram can be send via email

Moderator bumped up the attachment size limit, for PDFs.