My system uses a ball valve on the 5/8" line with cavitation reducer, prior to the ball valve, between the valve and the passenger side water port on my manifold. (Not my head).
My system uses the EDL-7116 manifold.
It has a special circuit for coolant to flow under the intake, so the air/fuel hits a warm aluminum flat deck, as it flows through the intake. This allows a faster warm up on really cold days here in MT sub-zero winters. I also use a reducer before the ball valve in the 5/8" line, before it hits the firewall.
The reducer shrinks the line down to 1/2" for about 1" , in-line with the 5/8" line, using a metal pipe as a restriction.
The restriction slows the flow under the manifold, and also forces the water velocity towards the heater core to increase, so air will not get stuck in there. The back pressure slows the flow.
So water goes from 5/8" through the manifold, to 1/2" restrict-er, to 5/8" through the core to 3/4" to the radiator, in stages.
Soon as the water gets hot under the manifold coolant circuit, the engine sees warmer-ish fuel in the engine and idles down faster, via the ecm. The coolant temp sensor is involved with TBI, as well.
The 5/8" ball valve shut off works for the summer months, so the circuit doesn't run hot water through the cab during the summer. I can clear the fog on the windows with the fresh air blower, without the heater on.
My water pump feeds the 5/8" line though the 7116 manifold. It comes out the top front manifold via a special port on the driver's side front manifold port, to the driver's side manifold heater port sub-manifold space. (There's a fitting on either side of the square carb section of this manifold).
After the coolant flows under the manifold, it exits out the passenger side port, towards the heater core in the firewall.
The in-line restriction keeps warm coolant under the manifold a bit longer, so the TBI gets a warm manifold in -40 winters. Retaining the heat inside the manifold between short stops helps the engine from running at full idle, or choke, as a carb would see, during warm ups. (The 7116 manifold can run carb or TBI/TPI).
This space is under the intake carb or TBI section and allows coolant to flow under the base plate of the intake dual plane circuit.
The warmer water flows under the intake through the passenger side port, to the restriction and ball valve. It heats up the splash plane of the intake. Like a river of warmer water under the air/fuel intake section of this manifold. This prevents the fuel from freezing on a cold start, before it hits the cylinders, after the coolant warms up the aluminum casting. It also stays hot when you leave the truck to grab a coffee at the store in the morning, without having to re-cycle the entire cold start process. The warm water keeps the aluminum intake warmer-ish for a lot longer than it would if not water warmed.
My 3/4" heater core return line goes to the radiator, NOT the manifold.
Do not want a hot steam pressure zone inside the heater core.
Have seen vapor lock if the return goes back to the engine block and the head gasket blows.
Steam can rupture the heater core if the return does not go back towards the radiator, in theory.
If the coolant is seeking the lowest temps, it will seek to go towards the radiator, where it is cooler, rather than back into the engine, where it most certainly will still be hot.
Radiator thermal flow is a high science.
Some people spend thousands on a hot rod cooling system, so it doesn't over heat or under cool.
If you study the reverse cooling system flows of the Corvette engines, you'll see how much difference reverse flow theory can make or break engine efficiency.
Whatever you do, don't mix up engine parts from a reverse flow engine and a standard small block Chevy 350.
The Merc 454 engines for boats have a coolant re-circulation block under the carburetors for cold weather fuel heating on boat engines. These still sell for $90 or more.
GM spent a lot of money running thermodynamics testing to "coolant-heat the fuel air mixture" on high performance marine engines, using the cooling system to pre-heat the intakes for cold weather reliability.
Most people have no clue how much warming the fuel air flow can assist with cold weather ops.
They spend a fortune with "cold air intakes" in the warmer climates though, don't they?
Ball valve your heater core and learn how to tune your cooling system/fuel intake for your climate.
Designing a custom system for various extreme conditions is worth the effort.
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