Relief hole in underwater exhaust pipe?

bikerider

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I have an Evinrude outboard with a Briggs and Stratton power head that has exhaust tubing that runs from the engine to below the water line (when mounted on the boat). I have not water tested the motor with this exhaust system. I have read conflicting information online regarding needing a relief hole drilled in the pipe above the water line. Some say you don't need to drill one and others say if you don't, you can suck water up into the engine. I want to get a straight answer before I water test the motor with it's underwater exhaust. Thanks
 

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wrench 3

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That looks like an interesting project.
They call them idle vents and the ones on the outdrives are just below the water line. So I assumed that they were there to stop the extra pressure needed to push the water out of the exhaust system from messing up the idle.
 

jimmbo

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I doubt you will suck any water up into the cylinder, but You will experience back pressure when trying to start it. The idle relie, no longer applicable, is visible in your photo(about 1/2 way between the bottom of the powerhead, and the top of the gearcase, it is also where you will see cooling water discharge. It is above the water line even when the boat was not moving, as engine speeds increase, addition waterflow will seal the relief shut.
 

bikerider

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I doubt you will suck any water up into the cylinder, but You will experience back pressure when trying to start it. The idle relie, no longer applicable, is visible in your photo(about 1/2 way between the bottom of the powerhead, and the top of the gearcase, it is also where you will see cooling water discharge. It is above the water line even when the boat was not moving, as engine speeds increase, addition waterflow will seal the relief shut.
That's what that is eh? Interesting. Do you think the added back pressure with my setup would be a hindrance or just a noticeable difference?
 

jimmbo

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If the backpressure wasn't a concern, the manufacturers wouldn't have Idle Reliefs on their engines
 

GA_Boater

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Your lawnmower motor is now a marine engine, isn't it? :smile:

Try running the motor in a barrel. You'll soon know if an idle relief is needed.
 

bikerider

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Good point. I suppose it is. I wish I had a barrel to try it out in. I would be taking the same risk of possibly sucking water into the engine though just like going to water test it at the lake. I'm just concerned about ruining the engine after putting a lot of work into the project, you know?
 

bikerider

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That looks like an interesting project.
They call them idle vents and the ones on the outdrives are just below the water line. So I assumed that they were there to stop the extra pressure needed to push the water out of the exhaust system from messing up the idle.

As I understand it, the idle vent is for the sake of balance and not as protection for against water entering the engine via the exhaust?
 

GA_Boater

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Idle relief is so a motor will idle with submerged exhaust.

If you run in a barrel you won't suck water up the exhaust. Just don't do any quick throttle changes, like from high revs to idle. Do it slowly.

Outboards don't have flappers like some I/O's to prevent water ingestion.
 

Chris1956

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Why not plumb that iron pipe exhaust into the original exhaust passage? I would plumb it below the waterline, so the casing doesn't heat up too much. The original exhaust characteristics of the OB midsection will then come into play.
 

bikerider

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Stoopid question: why not run a lawn mower muffler?

I had the factory muffler on the engine during my initial water tests. The exhaust fumes were really getting to me. Especially with a tail wind, so I wanted to try and solve that problem by running the exhaust into the water.
 

bikerider

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Why not plumb that iron pipe exhaust into the original exhaust passage? I would plumb it below the waterline, so the casing doesn't heat up too much. The original exhaust characteristics of the OB midsection will then come into play.

I saw someone else do it that way on a lawn mower engine converted outboard but they had left the original water pump impeller in place and routed the water pump tube to cool the exhaust. That is a bit out of my skill level. I removed my water pump impeller a long time ago. Plus, I am okay with how the pipe looks externally.
 

bikerider

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I went ahead and drilled a small hole in the exhaust tubing above the water line and I'm waiting for a dry day to water test it at the local lake.
 

bikerider

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I'm waiting for a dry day to water test it at the local lake.

Today ended up being that day. I misjudged where to drill the relief hole in the exhaust tube because with me in the boat the water was just covering it. I also need to distribute the weight differently too. The back of the boat sat way too low in the water. The motor felt like it was laboring more than usual when that relief hole was under water so I didn't push it. But I did put around the lake at idle and low throttle and the motor otherwise seemed to do ok. I noticed that when I fist started out (relief hole submerged) the top of the gas cap started to spew gas. (I have a one way breather valve mounted in the top of the cap. I had read of someone else having this problem with their converted outboard. So for now I'm going to drill another hole further up the pipe where I know it will not be under water, re-seal the gas cap one way breather tube, move my battery much further forward and maybe one or two other things and give it another go.
 

bikerider

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I did another test run this evening. I had no issues with water covering the new hole I drilled in the exhaust tube. The weight redistribution helped a lot. The boat feels well setup now. I turned on my gps app and topped out at 6mph with the average being 5mph. When I ran the motor on the boat before with the stock muffler I was topping out at 7mph easily. With the exhaust tube to the water the engine feels quite bogged down (as mentioned earlier in this thread). I'm going to take a wild guess and say that the only way to un-restrict the motor is to eliminate the under water exhaust?
 

jimmbo

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The length of the small exhaust pipe is probably creating Back Pressure, its twisting and turning don't help either, and Cast Iron pipe isn't very smooth. Using a larger pipe would reduce the Back Pressure as would removing a few directional changes.
 

bikerider

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The length of the small exhaust pipe is probably creating Back Pressure, its twisting and turning don't help either, and Cast Iron pipe isn't very smooth. Using a larger pipe would reduce the Back Pressure as would removing a few directional changes.

At this point I'm wondering if these engines can run well at all with underwater exhaust. I could try and bump up the tubing size or shortening it. I have also been considering shortening the pipe to where it is just above the water line and see how bad the exhaust fumes are. (My original reason for the underwater exhaust).
 

jimmbo

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It would not be quiet. That is why exhaust is discharged under water
 
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