Thru transom dry exhaust.
 

Hello all I am new to here. Was wondering if anyone has attempted this. I need to run water jacketed exhaust all the way out thru the hull. The water and exhaust can’t mix. It’s do to my cam. I am going to dump the water at the very end out the bottom of the pipe with like a half inch fitting/bung. My question is when you are building it. How do you make it so the water surrounds the whole pipe? If you put it on the bottom at the end technically wouldn’t it only run water along the bottom of the pipe? Thank you

the water dumps need to be on the top so the pipe fills

Would it still apply if the exhaust is going downhill? Meaning the discharge side is lower than the intake. It’s only a half inch gap that’s needs to fill up. Would the water pump be able to over come the angle? Thank you

Most have the dumps on the top and add just a small weep hole at the bottom end of the pipe so that water can run out. At least those of us that run in salt water.

JPBrew,

I have a somewhat similar situation with my exhaust. I modified OEM 496 exhaust elbows by extending both the inner and outer tube, and added a second dam at the end, functionally equivalent to the upper dam. extending the dump about 9" from original. The exhaust stays dry until just before the exhaust tips inside the transom. I also made a set of water jacketed risers that added 8° to the tilt, which does leave the jacket at the top of the elbow dry-ish. It's been my observation that, at idle, the apex of the elbow gets warm to the touch, just beyond the point of comfort, but not so hot I think it's an issue. I think the water flowing through it is enough to sink heat away from what would likely be dry sufficiently to keep it cool enough to be a problem. At anything above idle, there's enough flow/splash in the jacket and extension they stay pretty cool. I've actually lifted the hatch up at cruise and put a hand on them.... Raw water cold.

Thanks. Brad.

Top?
I`ve never seen exhaust dumps at the top. Never seen a boat shooting water up at the swim platform.
The bottom is where you want it .
Unless you have a 2" hole the entire pipe is filled, it doesnt drain that quick.
Mine were inside and at the bottom, my tips were just as cold as the rest of the exhaust.
The tips picture is not mine.

https://cimg5.ibsrv.net/gimg/www.off...52e7d16f19.jpg
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Dry exhaust frome what I know is only used for Cam overlap issues. The other thing it is loud. In my new build it has to be Dry for the Cam overlap. There is about a 10 inch drop from the header collectors at a 45 degree angle to the exit out the transon. The O2 sensors are in the header collectors, they are very sensitive to mosture as well. Outside pipe is 4.5 inch OD and inner is 4 inch ID. I am welding up the lower half to the tails at the exit end to retain some water at idle but think the pipes will be full with the pump I am using. A small hole in the bottom is added for drainage after the engine is off. The top half the pipes are open with the standard 1/8-3/16 inch gap betten the innner and outer pipe. This will caise the water to go out over the exhaust and quite it down some as well. I can not see how any water at idle could travel up a 45 degree incline with a 10 inch rise but we will see. I'll have to carry a few O2s just in case it could. An O2 failure would show any water intrusion well before getting to the valves at idle.

My CMI tails are dry all the way through the transom and are just cut off with no separate water dumps. The water dumps out directly from the water jacket.

Needs weep holes...to drain for fresh as well.

Weep holes top and bottom also help cool any added mufflers clamped on etc.

This is what I have for a dry exhaust. First time for me as well and think it will work. Any input would be helpful.
Baffles are for two reasons, for reducing sound at idle and stop reversion. They bolt in at tail end. Old school spiral baffles that do work at idle but no real reduction at anything past half throttle.


https://cimg3.ibsrv.net/gimg/www.off...dd6fac76a8.jpg
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Your tail pipe Angles/style looks exactly what I am doing right now, except mine IS going to Be with 3.5" innner Pipe and 4" outer. Do you Have a picture of 02 sensor bungs location?

Your tail pipe Angles/style looks exactly what I am doing right now, except mine IS going to Be with 3.5" innner Pipe and 4" outer. Do you Have a picture of 02 sensor bungs location?[/QUOTE]

I don't have a photo od O2 bungs. But the headers are CMIs and the bungs are in the collectors and straight out the side, in the standard location for boats. One in each collector.

The ends of my tails are similar, except mine have 3 1" welds between the inner and outer pipes
Even if your tails were normal wet and mixed just inside the transom, I can't see them sucking any water back up with that much downward angle.

I don't have a photo od O2 bungs. But the headers are CMIs and the bungs are in the collectors and straight out the side, in the standard location for boats. One in each collector.[/QUOTE]
You Mean here.
https://cimg1.ibsrv.net/gimg/www.off...0eea084993.jpg

Yes, that is the spot, same in each collector.

Not easy to put a O2 bung there after the fact. The O2 bung will be fine in the front of the tail pipe just after the clamp maybe an inch or two, should be easy to weld in there.

Found a photo of the O2 bung hole in my collectors. Look at the crossover bung on top of the collector. The blue tape under it is the O2 bung hole.

Acually on another boat the O2 bungs are in the tail pipes as they were added with the new EFI system last year.



https://cimg7.ibsrv.net/gimg/www.off...427d9d94b6.jpg

Those are dry to the tips. A buddy of mine had dry to the tip and reverted bad at an idle. I made the mistake and ordered my tails dry to the tip from CMI and ended up welded them up/fully dry. And honestly not really much louder then dry to the tip which was already really loud. I've ran them both ways. exhaust sounds a little more crisp fully dry.
Then installed 3/4 stainless bungs on the top side of tails per CMI recommendations 6" back from the inner transom wall. Plumbed accordingly out the side of boat. 4 yrs later no cracks from welding
the tails up. Basically no signs of water reversion. I'll see if i can post picts later tonight.