Need ideas on cooling system design

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I’m in the process of mocking up all of the cooling system components on my hot rod bread truck build and I’m at a point where I’m not sure if my plan will work and I’d like to ask for ideas.
In these stepvans the heater core is much higher than the top of the radiator and there is no place to mount a surge or recovery tank above the heater core. There is a place above the brake booster to mount a blind expansion tank that will be above the heater core . My thoughts are that to fill it I could pull a vacuum on the system at the remote recovery tank and slow fill the system from the radiator drain valve till I see coolant entering the upper expansion tank.
Canton racing claims that we should allow 6% for an expansion space. My thoughts are to build the upper expansion tank large enough so that at temp it would be nearly full and not to the point of pushing coolant to the recovery tank. This design is dumb because if I have to top it up at some point I would have to add to the recovery tank while the system is warm and then let the shrinkage from the cool down cycle pull it up into the expansion tank.
I’m open for any ideas on this as I’m not convinced that what I have drawn will work as planned. TIA
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Why don't you do what GM did on the Northstar, and I'm sure other makers do too. Run a small hose from the highest point of the system directly to the main expansion tank. This tank is always open to the system, so there should be a minimal constant flow from that high point. This way, any small air pockets in the system will continuously be directed to the expansion tank, then sent back into the system. Since everything is under the same constant pressure, it won't cause a leak or overflow.
I'd say a tee in the upper heater core hose, even if you have to route it a bit higher than you want, should do the trick. On the Northstar it was the uppermost hollow bolt used to secrure the throttle body to the intake. It provided flow continuously when the engine was running, if it didn't, such as getting plugged, air would accumulate in the block and cause overheating.
 
Johnny G I cant tap into the heater hose below the heater. The heater core inlet and outlet is an -10 on the bottom of the core and no practical way to tap that circuit up at the heater. Even if I could tap up there I would still be at the bottom of the heater core. I’m convinced that I must create an expansion space at a higher level than the heater core which would naturally collect any air.
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More pics of build required.(y)
here’s one of the 2 pics that hit Instagram..... not good. Shortly after they posted them my messenger blew up.
This thing has been in tin shop jail for 2 years and I just got it back from them in July. I retired last January and I have been working towards a full mock up of everything 7 days a week since July.
I am in que to get it into the body shop in April but I still have a bunch to go before paint.
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That looks like a cool project. Please keep us updated as it progresses.

So there's not room to mount a factory-style pressurized surge tank with the coolant level above the heater core?

Not sure if it helps, but I found this diagram while trying to understand coolant flow on my Silverado. Green highlighting is for low pressure. The heater out to surge tank connection is a bit misleading as shown. On the trucks, there is a "Y" that joins the heater out with the larger-diameter surge tank hose, but it is close to the water pump. The single end of the Y then connects to the water pump. There's also a lot of info about how the factory system works in post #2 of the link below.

lsx_coolant_4116771c801fb5cb32923e1c7c743ae86f392a8f.jpg





https://www.silveradosierra.com/thr...tion-on-5-3-when-does-thermostat-open.454794/

Edit: I just noticed a difference between this diagram and the one posted at the link. The link shows the steam tubes going from the heads to the radiator, and then a separate vent hose from radiator to surge tank. That's how the factory system on my truck is set up.
 
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That looks like a cool project. Please keep us updated as it progresses.

So there's not room to mount a factory-style pressurized surge tank with the coolant level above the heater core?

Not sure if it helps, but I found this diagram while trying to understand coolant flow on my Silverado. Green highlighting is for low pressure. The heater out to surge tank connection is a bit misleading as shown. On the trucks, there is a "Y" that joins the heater out with the larger-diameter surge tank hose, but it is close to the water pump. The single end of the Y then connects to the water pump. There's also a lot of info about how the factory system works in post #2 of the link below.

lsx_coolant_4116771c801fb5cb32923e1c7c743ae86f392a8f.jpg





https://www.silveradosierra.com/thr...tion-on-5-3-when-does-thermostat-open.454794/
Yes I have room for a tank above heatercore elevation. If I draw a horizontal line from the highest point of the heater core there is a 3” void above the brake booster to put a tank but it can’t have a cap as that is where the top of the dash is. If it had a cap I would have to fill it from inside the van. This is going to sound stupid but a could just make a tank with a lid that mounts up under that ledge above the booster with the 18lb cap on it but I would have to unbolt it and pull it out of there to fill......... a little bit gangsta but it would work because it’s the highest point in the system by 3”.
 
Is there any way to make an access panel you could unbolt or flip open to fill the tank? Or some sort of extension neck that would make the cap accessible? I have only seen the system on full-sized trucks, but maybe some model out there has a filler design that would work. With all the LS swaps, somebody might make custom tanks. Oh--what about vans?
 
Here's the tank for a 2004 Corvette. It's lower profile than the truck tank, and has a T fitting attached vs. the Y hose assembly.

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A 2004 Chevy van doesn't seem to have a pressurized tank. Instead, it has a radiator cap on a raised neck.


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Thanks for the ideas and wisdom. I can make what ever tank I need once I have some back up that what I’m doing will even work.
If I go by what canton racing says about 6% expansion area for 3 gallons total capacity I would only need 23 ounces (plus the fudge factor) of expansion area at the highest point in the system . Let’s say I make that upper blind tank hold 40 oz and the bottom recovery hold hold 40 is there such a thing as having too much expansion volume.
The other part of this is..... does the heater circuit have sufficient flow to wash out any air that would ever collect in it?
With slow vacuum filling I would have to assume that there should be little to no air anywhere in the system.
As far as the access hole or door.... yes I sure could put a nice flush billet fuel door access up there and eliminate the lower remote tank all together.
Thanks for the wisdom!
 
Buses tend to have a surge tank lower than the HVAC unit where the heater core sits in the “attic” above the engine or in a roof-top HVAC system(unless it’s a touring coach or said bus uses baseboard mounted heaters). They use one of these:

Another approach is to build a bleed “port” near the heater core, much like a Toyota or Honda has at the highest point to bleed the cooling system.
 

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