also clear hose testing thru out the cooling system can and or will tell you a lot. You do not want air bubbles or water flow to back up

I think the minimum spec, is about 7.5 GPM, from the sea water pump at idle if I recall. Which would make sense. In that CMI video, with a fresh impeller/housing, they are seeing about 10 GPM from the sea pump.

That is at idle though. At 6000rpm screaming across the lake, making big hp, and big HEAT, I am sure the system is capable of much more water flow. I'd want to keep those headers as cool as possible, by giving them as much water as they can take.

At idle speeds, you aren't seeing 1300, 1400, 1500+ degree exhaust temperatures.

I think the big problem with headers, especially at idle speeds, is the distribution of water. Each tube is its own channel so to speak. Its being fed from a single line. There is no guarantee, that each tube, gets an even amount of water flow.

I know for a fact, certain tubes tend to run hotter than others on mine, at idle. one of the things I was looking into last year, was a better drive ratio for the water pump.

In the aftermarket world, say with roots blowers, alot of the crank accessory pulleys, are small. Smaller than mercury used. That slows the pump down. I did some calculations last year, and forgot what they were, but just because you may be idling at 700 engine rpm, depending on the pulley setup, your pump certainly may not be.

My headers have a 1 inch ID connection to the the distribution manifold and the same size ( single ) to the tail pipe, Im a bit concerned going to a -12 5/8 ID connection on the input side of the header.

The gap between the two walls? Almost non existent at the bends which is probably why that's the first place you'll see the discoloration from overheating.

Which tubes are hotter?? maybe look at how it is fed and how it exits... Water takes the path of least resistance. Turbulence can also decrease flow.

Thats what I was looking for, I didnt figure it was much, 1/8 inch at best, thanks for the post

With that much area in the tube,,, I am almost inclined to think that the water at that point it almost working more as an insulator, keeping the heat from transferring to the outer tube rather than removing the heat (cooling)

Mild, with your stellings dumping a 3/4 inch fitting per tube to the tail, I have to think there is a larger volume of water passing between the inner and outer tubes compared to the CMI

that is correct about not much area gap in between the header tubes especially at the bends and very easily for stuff to collect / clog in those narrow areas causing restrictions of water flow thru, hence why sometimes one header portion of the header runs hotter than the one next to it,