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If you plumb it from the rear to the front using a thermostat spacer, you may need to restrict the rear lines with (approx) a 5/8" restrictor. Usually an old thermostat minus the guts is about right.
If there is too little a restriction from water flowing from the rear outlets, the water will not flow within the the engine to the front in the volume that it needs to.. The water will travel the path of least resistance.
It's quite possible the plumbing i.d. will do this naturally but who knows what's actually happening in there?
 

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No need for any 'restrictor'. By the time the water gets to the upper rear portion of the intake, it has run its course as a coolant and needs to go directly to the radiator.
 

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No need for any 'restrictor'. By the time the water gets to the upper rear portion of the intake, it has run its course as a coolant and needs to go directly to the radiator.
I disagree. Most/all of that water has only been inside the cylinder block at that point.
So why doesn't all the water leave the engine there then?
Why? Because what you take from the rear doesn't get to pass through the heads and have a chance to cool them.
Essentually the rear ports can work as a cylinder head bypass and that's not good.
A restrictor may be required to stop too much passing out of the rear and starving the heads of coolant.

Anyway, I'm yet to see any real information on the advantage or even the reuqirement of rear ports. ..at least on the street cars we drive.

Can someone give me one good reason?
 

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Steve, I use 3/8 alum tube, that is about 5/16 ID, that is a lot less than 5/8, you recomend, that is the size of the heater hose, I just use them to blow steam, should I increase to 5/8. ????
 

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I disagree. Most/all of that water has only been inside the cylinder block at that point.
So why doesn't all the water leave the engine there then?
Why? Because what you take from the rear doesn't get to pass through the heads and have a chance to cool them.
Essentually the rear ports can work as a cylinder head bypass and that's not good.
A restrictor may be required to stop too much passing out of the rear and starving the heads of coolant.

Anyway, I'm yet to see any real information on the advantage or even the reuqirement of rear ports. ..at least on the street cars we drive.

Can someone give me one good reason?




The water that appears at the port on the rear of the manifold has already been through the cylinder head, unless you have bolted your intake manifold directly to your block. My intake manifold bolts to my cylinder heads, not the block, and the water that exits the head at the front of the intake has already been through the cylinder head, and it's the same with the rear port. This is the reason you can swap cylinder heads from side to side on a SBC, because the heads have symmetrical water passages.
From a flow perspective, the water at the rear of the cylinder head must be at a higher pressure than that at the front of the engine in order to flow out the front of the manifold and into the radiator. This means that less water goes to the rear of the block than the front, since the front the water goes almost directly from the block inlet to the top of the front cylinder/head. The water that goes to the rear of the block travels past a few hot cylinders then gets to the rear of the block, arriving a little hotter than the water at the front of the block. Less flow of water plus preheated water means the rear cylinders should be hotter than the front ones. Increasing the flow to the rear cylinders is in no way creating a 'cylinder head bypass', instead it is providing more uniform temperatures in the engine.
 

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Thanks John, I aklso assume the smaller i.d. would help proportion the water flow rates, which is why I said may need a restrictor.

onovakind67, I do understand that the water at the rear water port has already been through the cylinder head in order to get to the water port, but that would that water have only passed though only a very small portion of the cylinder head?
Are we to assume/hope that the water leaving the rear port is water that has cooled the two rear cylinders on each bank? ..or perhaps, much more or much less?
The suggested restrictor is simply to ensure the flow rates are what we hope they are.

The restrictor size that I suggeted (5/8" dia) was a detirmined size from a guy down here (John Bennet) that did a lot of R&D on cooling systems. He has a water pump dyno that he flow and pressure balances water pumps with, and he had his own brass impellers made for him that he'd install with a tight clearance long before Edekbrock were doing that with their Victors, and probably before Stewart were as well. In fact, I still have my cast iron BB pump that he modified for me. Worked very well in my application.
In the 80's. he also made a set up where he fitted the thermostat in the lower radiator hose to control the water temp before it entered the block -so cold water from the radiator wouldn't "chill" the front two cylinders. Now all the OEs seem to have the thermostat on the inlet side of the cooling system.
Last I sopke to him, he was manufacturing and suppling parts directly to Ford Australia. He's a smart cookie!

I used to run the 4 corner cooling too. Admittedly, I never had any cooling issues, but that's not to say that I would have with the regular OE set up. I used the restrictor size that I suggested.

I also agree with much of what you have said about the coolant flow. I am not disagreeing with that, I just hope it does what we assume (?) it does.. :)
 
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