3/4" gravity enough to flush/ winterize?

I agree, if it gets diluted in a pocket, better to have the extra protection.

I see what you mean theoretically, but I'm not sure it has any relevance in practice.
Firstly, the raw water circuit of any half decent engines should be designed to allow a straight flow with no stagnating water pockets - and the same goes for wet exhausts.
Secondly, even if we assume that in some areas the antifreeze could not push out the water and mix with it instead, I don't see how a refractometer could help, considering that those areas could be anywhere inside the block.

I've seen several different engine types/brands being winterized, and with all of them, bar none, the raw water output stays crystal clear till it becomes fully coloured, in a matter of a second or two. To my simple mind, this is a good enough indicator that the antifreeze liquid "pushed out" the water without mixing with it in any meaningful/harmful way.

There you go. Don't let the over-thinkers scare you. (30 years of winterizing stuff under my belt....)

What you call overthinking, I call added piece of mind. Which is why I chose to use -100 as opposed to -50. I don't run a boat that needs to be winterized anymore, but if I did I would still choose the added protection. Have always taken the extra measure... even when the boat was in a heated shed for the winter. (Been winterizing stuff since 1990, so I guess I can say I have 30 years under my belt as well.)

I'm staying out of the "how many years" contest, because I never thought there's anything to be particularly proud of, in being old.
'Fiuaskme, getting old sucks, and the only reason why I don't mind is that it's still better than the alternative.

That said, I can't understand your concerns, for the two reasons I tried to explain in my previous post.
And I'm also skeptic about your "peace of mind" reasoning, btw.
My humble suggestion to anyone who's really interested in peace of mind is to avoid running any boat.

Getting old is the ultimate success in life, greatly enhanced if a long time relationship goes with it. It also brings you to a point where you can be totally honest. F---anyone who doesn't like it, because the only thing you care about is health. You also know more than you've ever known before, and knowledge is the most valuable thing you can own and it can never be taken away. You couldn't pay me to go back to being 20 again, especially considering the world they're facing.
Being a newbie is great because you hopefully come in with knowledge of the latest technology, but that's nothing compared to the knowledge that comes from experience.

Agreed... "Piece of mind" is how the marine industry get's you to spend more money than you reasonably need to. -50 Antifreeze begins to slush at -50. It's still got a ways to go before it can freeze solid and expand. Coastal areas from Maine to NJ are not going to see the kind of extended deep freeze that would ever cause this to be a problem. And considering the -100 AF is almost double the price and one might need 20-30 gallons between engines, genny, AC, and water systems, seems like it's a waste of money. I have never heard even and anecdotal story of -50 AF, used properly, ever being an issue.

Well, you're not getting a "piece" of my mind.

Touché. Should have said Piece of my wallet....

That too...... I do my own work.

True, that's indeed an advantage.
Also because it allows you to "smell" not so honest comments much more easily...

Very cool handy pump. Thanks Capt J , think I might get one of these.

45 years under mine, always was able to just remove the intake hose pour the antifreeze in the hose or suck it out of a 5 gallon bucket . I always add an extra gallon over what is really needed.
When you see the antifreeze coming out the exhaust, it takes a gallon or two laying in the exhaust hose/length of hose 6,8 10 + feet just to roll out the end exhaust , you are well past the point of having it all flow through the motor.
Dad's boat had 8v71s with big sea strainers . All you had to do was remove the strainer top, fire one up, pour in the antifreeze ( 4 gallons for the 8v71) then shut her down. Done.
I could do each motor in 15 minutes.