How exactly does the Merc seawater cooling system work
 

Can see the thermostat housing, but not how the passages are inside are setup. Comes from raw water into thermostat housing what it exactly does from there?

Mine goes from sea pump to oil cooler. Oil cooler to thermostat housing. Thermostat housing to water pump. Water pump circulates. When the the thermostat opens it realeases water out of the exhaust. However they all have a bypass of sorts to constantly have water flowing through the exhaust as that's necessary on inboard mounted engines.

Freshwater, not closed cooling I'm assuming ?

You asking of the typical 4 port t-stat ?
1 large in/ 1 large to recirc pump, 1 x 2 outs to exhaust ports ?

I'll see if I can find the pic I drew 1000's of years ago to show exactly this. LOL.

In the meantime. Think of the Tstat housing in 2 halfs.

Water enters the lower and goes to recirculating pump. When the lower half fills it then is able to enter the upper half thru a 'bypass' opening. The upper half is what has the 2 outlets to the exhaust manifolds. This way, the water entering from the sea water pump is prioritized to the recirculating pump. Again, The excess goes up thru the bypass opening into second half which send it to the exhaust.

The t-stat rests/seals against upper 2nd half. Directly over , but not on, the t-stat opening in the intake manifold. When the t-stat is closed a lot of this water get's redirected with incoming water towards the recirculating pump. When the t-stat opens, the hot ater coming from the intake manifold has a straight shot to upper t-stat housing half which goes right out into the exhaust manifolds.

The 4 port Mercruiser t-stat housing is actually, in my mind a stroke of genius. It does not impeded water flow, let's the thermostat do it's ob quite well,and it actually directs water to where it needs to be prioritized. all while letting lots of water thru to the exhaust and out the boat.

Yes open cooling system. Im doing a LS so little different than SBC BBC where those go in the waterpump, pump pushes through the block, out the intake and thermostat blocks it from leaving until it opens up - These have the thermostats on the inlet side so it would be sea pump, oil cooler, into thermostat neck, then through engine and out the pump outlet

http://www.pirate4x4.com/tech/billav...20numbered.jpg

So like I was thinking the sea water pump flow is mainly to pump inlet (its not a straight shot through to the outlet port that leads to pump so wasnt sure) Some gets bled to the exhaust outlet two ports - what size is this orifice?

The thermostat isnt sealing the block per se, its sealing access to the upper therm housing chamber (exh dumps) Just making a loop from water pump inlet back to itself until it opens then most flows to exh dumps. Theres a 3/4 line or so from intake to top of pump too?

Trying to figure out the best way to rig cooling system up

How to make that happen on a LS is the trick

Hardin Marine - LS Engine Parts and Accessories

LS Crossover and Thermostat - Hardin Marine

Cant use that crossover, have to retain LSA waterpump as tensioner, idlers etc all mount to it