Another thing to consider is how the water flows through the tails. Mine have "dams" in them so the water is forced to fill the pipes and not just flow along the bottom. These were added in, I'm not sure if that is a common practice or not when they are made.

Dam's = restriction ?? which builds pressure to fill the tube's for cooling,...............all this pressure is giving me a headache.....

https://youtu.be/a01QQZyl-_I <iframe width="560" height="315" src="https://www.youtube.com/embed/a01QQZyl-_I" frameborder="0" allowfullscreen></iframe>

I'd guess that the opening at the top is large enough to allow the water to flow freely once it fills the tube. I don't think it would increase pressure.

I meant four 3/4 inch lines feeding the tubes at the inlet side At the outlet, it is a 1 inch tube , to the tailpipe. The connection at the headers "supply" , coming from tstat , is also 1 inch, like your .

Here one example off the top of my head cause I know this one in my sleep.

Merc 496 and of course closed cooled - Sea water output test.

in the water testing only at 1000 rpms for 15 seconds needs to equal min of 2.5 gallons of water flowed in a empty bucket

Here is some figures I dug up while sourcing parts on diesel builds. I used an online calculator that takes orifice sizes and pressures then gives a maximum GPM flow rate possible.

.50" Dia @ 20psi = 23.31GPM
.50" Dia @ 30psi = 28.55GPM
.50" Dia @ 40psi = 32.97GPM

.75" Dia @ 20psi = 52.45GPM
.75" Dia @ 30psi = 64.24GPM
.75" Dia @ 40psi = 74.18GPM

1" Dia @ 20psi = 93.25GPM
1" Dia @ 30psi = 114.21GPM
1" Dia @ 40psi = 131.88GPM

1.25" Dia @ 20psi = 145.71GPM
1.25" Dia @ 30psi = 178.46GPM
1.25" Dia @ 40psi = 206.07GPM

1.50" Dia @ 20psi = 209.83GPM
1.50" Dia @ 30psi = 256.99GPM
1.50" Dia @ 40psi = 296.75GPM

Also according to Hardin, a single stage merc style impeller pump flows the following based off the pump RPM not engine. Seem to line up with the examples above.

1000 rpm: 11.5 gpm
1400 rpm: 15.5 gpm
2000 rpm:​ 22 gpm

Hope that helps.

10-4....I was thinking that collector must look like medusa LOL CMI are the same

I had thought about running a bypass so I could run a T-stat ?? seems like one thing is good to do but it will possibly lead to other issues. I hate boats

What some run, on the jet boat side especially, is called a "cracker valve".

Basically a spring pressure valve on the headers that only opens above 5-10lbs line pressure. This setup allows the headers to heat up more at idle, so when you jump on the throttle the cold tubes ain't screwing with your fueling mixture. Also claims to help with reversion issues. Did some reading on it a while back, some Merritt to the design. However this mod does require a small bypass tube dumping that 10gpm idle water flow somewhere down stream.