raw water hose size for water injection of riser tailpiece

v8 Engine is 392 cubic inches
Right now I have mine setup to use 5/8 inch hoses, the ends of the risers, that tailpiece has 3" hoses for the exhaust
The riser chamber itself is fresh water cooled using 3/4 inch hoses for engine coolant..
Simply the injection hose is a 3/8 " NPT threaded SS tube cut off and a hose clamped on that to inject water in to the end of the riser
Excess raw water flows to the mixer using a 1.25 inch hose.
I have been thinking that may be more raw water than needed is introduced at the tailpiece, so to limit riser reversion, backflow of raw water back to the log manifold, how about restricting the input by installing a restriction in the ss inlet pipe to 0.375 inches, or maybe 5'16" ? Measuring the ID of the inlet pipe is 0.465".

My system is my own that I setup 20 years ago and has been ok, but I noticed there was some water on manifold becuase the Riser anti reversion lip on one riser rusted away. So was thinking maybe too much raw water is being injected into the tailpiece which attaches to the end of the riser.

How much water really needs to be injected into a tailpiece to cool the exhaust gases?
EXAMPLE here, see how small is that nylon barb for a 3 inch exhaust hose? Mine is way too big I think.
https://www.deepblueyachtsupply.com/...exhaust-elbows

picture


https://www.deepblueyachtsupply.com/...exhaust-elbows

 

So the raw water comes into the heat exchanger, ( the freshwater side of the heat exchanger recircs the engine, manifolds and the risers) and then this raw water then goes where?
Certainly into the tail piece to get the water overboard. You mention a "mixer" Is this another outlet for the raw water?
On our jet installations, freshwater cooled, the raw water inlet into the heat exchanger was 3/4 inch from the pump bowl, into the heat exchanger and ALL the raw water went into the tailpiece(s) after the riser.

Without seeing the entire plumbing circuit but noticing the large fitting on the tailpiece above, it makes sens that the manufacturer expects to run more water through the tailpiece than the small plastic fitting would allow.

 

The raw water exits the intercooler (heat-exchanger), goes to the engine oil and trans oil cooler. That has a split outflow, one side goes to the large bronze exhaust mixer (joins v8 engine banks 1 and 2 exhaust hoses), other outflow exit goes to a Y splitting the flow to each tailpiece attached to the end of the riser.
Yes was surprised seeing the small plastic barb fitting on that 3" water cooled elbow, made me think maybe I am allowing too much water to flow into the exhaust tailpiece that attaches to the end of the riser.
Someone said if you restrict too small the fitting, it might get more easily blocked. I was thinking of making the diameter down to .375 inch from .465 inch. A way to do that, is put a SS bolt to sit in the pipe nipple or grind a SS nut to fit inside the SS pipe connection, and whack it in the end so its a tight fit.

 

I would leave the sizing as you have it if the tail piece is warm to the touch now.

The static water pressure in the tailpiece and the exhaust mixer hoses down stream of the water pump is determined by the flow resistance in the hoses, internal viscous resistance, the number of constrictions in the cooling jackets and the sizing of the hoses after the pump.

(I am assuming that the exhaust vents to atmosphere as compared to an underwater outlet) The downstream side of the water pump is basically venting to atmosphere. 0 psig
So if you have say a 1 1/2 inch discharge downstream of the pump and maybe a small amount of pressure due to the above parameters, there may be very little water going through the "small diameter" fitting that you are considering installing

As a short interval test, I would remove one side of the hose from a tailpiece, and install the size of fitting that you propose and fire up the engine to see how much flow that you will have entering the tailpiece.
As another even easier test is to install the small fitting, run the engine under load, at the rpm and loading that you may occasionally see, and just check the temperature of the tailpiece just by hand.

 

I recall years ago when testing water flow thru that small hose to the tailpiece, it had a continuous steady flow with no air spurting.
Part of how it can do that, the mixer is a few inches lower that the injection point on the tailpiece, not a big drop in height.
I did read some other forum, people put in a cam with more overlap, higher lift and they started to get water reversion into the exhaust manifold, and one suggestion was to cut down the flow into the riser and see if the temperature remained ok.

 

Temperature of exhaust gasses downstream of the mixing elbow will determine how much cooling water is needed.
Exhaust noise is also somewhat dampened by the quantity of water injected.
Rubber exhaust hose is commonly used, and will not tolerate temps higher than what are specified and commonly printed right on the hose.
It is feasible to place a tee fitting ahead of the mixer with a regulating valve to discharge excess water ahead of elbow.
Use caution though, too little water will not fill the discharge elbow resulting in hot spot at the top of the fitting.
Maybe plug up every other discharge hole?

 

I took some pics of my setup. Fresh water coolant runs thru exhaust manifold and the bar riser. The raw water is injected in the bronze tailpiece.
20 years ago I modified 4 new risers to be engine coolant cooled, not raw water cooled.
The 4 exhaust manifolds are 1970 so 50 years old now, any water getting in there, the chamber is well below the exhaust ports, so unlikely to get into the engine cylinder.
The bronze tailpiece is brazed onto the end of the riser. The water exit is brazed over and sealed. I brazed a 3/4 elbow for coolant flow into the riser. coolant comes from the heat exchanger to the engine coolant pump and splits to the riser on each side of engine, moves on to the exhaust manifold, then enters the front part of the cylinder head.

The risers have held up marvelously, except for one where the internal anti-reversion lip has rusted away. That lip keeps water from resonance reverting back up the riser into the exhaust manifold. So when I took this apart, a tiny amount of salt water was sitting in the exhaust manifold.

I recreated an anti- reversion lip using half sand and half PL Premium polyurethane construction adhesive. I have found that adhesive to be very good at resisting heat. So I hope this will keep water from moving backwards. The sand can not burn and is a good filler for the PL, and it is going to be wet from raw water splash.
The bronze pipe brazed on, and the SS pipe silver brazed to the bronze pipe has shown zero degradation. This is exposed to salt water from the lower Chesapeake going on 20 years now. I could buy a new riser and rebraze it together with my mods, but maybe I wont have to do that.


The sand - PL new lip, when the riser is angled into position will be flat to the water line. There are 2 attaching clamps, and these kind of risers can swivel around and point any direction.

I have been wondering if I am injecting too much raw salt water, which is causing the inside surface of the riser to rust faster than it should.

 

I’ve learned (The hard way) to never install water injection where there is any chance of backflow.
Most motors get hotter after shutdown, so expansion in the raw water system can cause more damage than you’d expect!
For safety’s sake, the water injection “tailpiece” Must be located in a “downhill” orientation, so that gravity will pull residual water Away and downstream from the motor.
There will always be some residual raw water drips after the motor is shut down, which needs to go out the exhaust tubing, not down the exhaust passage.
I’d recommend a dry elbow between your riser and the water injection tailpiece to orient that tailpiece into a downwards facing position.
I’m not fond of water jacketed risers, your using engine coolant instead of raw water in them is much safer, but they are still compromised by salt water backflow.
The “dam” in the exhaust elbow is a stopgap, not a solution, that puddle of salt water will eventually bite you.

 

That dam is hopefully going to work ok, the other 3 risers still have their dams (anti - reversion lips) further back up inside the riser right where it begins the turn down is where it is located.
Riser end does angle down hill, the motor sits at an angle. From what I have read, the resonance reversion occurs due to valve overlap of the camshaft. So it creates a partial suction during the engine cycle.
It is another reason why I have resisted putting in a higher performance camshaft. Other is do I really want to do that as it is a lot of work.
Water reversion - Offshoreonly.com https://www.offshoreonly.com/forums/general-q/31038-water-reversion.html
Water goes everywhere and a little is also sucked back into the engine. You can not stop all of the reversion, but you lessen it.
What is Exhaust Reversion? https://www.cpperformance.com/t-what_is_exhaust_reversion.aspx