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  1. #1
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    Out Of Water Testing / Muffs / Backpressure

    I hear people talk about this all the time so I wanted to start a little discussion on it. People always talk about a lack of backpressure when running motors on muffs, potentially causing damage etc. Obvioulsy loading the motor on the water, with a boat loaded the way it would normally be would be the best. But asside from that is there any real reason not to run a motor out of the water? The motors obvioulsy have an idle relief, a lot have above water exhaust, and how much back pressure could there be with water running past the bullet at 60mph+. In my mind, once the exhaust is out of the tuner, no back pressure is needed or maybe even wanted..... am I wrong?


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  2. #2
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    I 100% agree with you. My 0.02.
    The only thing back pressure affects is idle and some midrange. And in those area its hard to "damage" a motor. The proof of this is revealed in adjusting carbureted engines: if back pressure affected how outboards ran then the a/f mixture would be different with and without back pressure and we would not be able to adjust carbs out of the water. I adjust carb motors without back pressure and then water test them and they run exactly the same. Most of the time [1]. This proves that back pressure (from being submerged in water) doesn't do much. Once on plane there is no back pressure aside from what is cause by pressure differentials and harmonics in the exhaust tuner itself. And on plane is where damage to engine...

    Side note [1]:
    Old J/E engines from the 60s, 70s and 80s did not like to idle out of the water. They would run lean. Dump em in the water and the back pressure DID make a big difference. They would run much better with back pressure. That being said I have a hard time believing you could damage an engine idling it out of the water for hours even running a tad lean.

  3. #3
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    I try not to touhc the J/E stuff from pre-2000.

    On the mercs, I usually just adjust the idle 150-200rpm above spec if they are out of the water. Other than that, most is on muffs, dyno if I want to load them. Worst case, water test. But I'm 1.5hr from water.


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  4. #4
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    Being a marine tech. for 35 years. I have always told customers, I can,t guarantee my work until I lake test. or ramp test. Depending on situation. But I was always close to water. At the shop I run every motor, I can, in a barrel. So yes I think it makes a difference. The other advantage of a barrel is it simulates the motor in the water, and it,s quiet compared to a flushette
    Last edited by billybob; 03-22-2023 at 10:52 AM. Reason: change

  5. #5
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    I never heard that idling on muffs would cause any damage, just that for idle adjustments, in the water with a load was the final test. I would also assume no load vs load test would be different with different pitch propellers. My 19p prop will see less load than my motor with a 26p propeller.

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  7. #6
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    Quote Originally Posted by Fishwhisper View Post
    I never heard that idling on muffs would cause any damage, just that for idle adjustments, in the water with a load was the final test. I would also assume no load vs load test would be different with different pitch propellers. My 19p prop will see less load than my motor with a 26p propeller.
    ^^^ for sure. But thats more to make sure your motor doesn't die when you put it in gear; or idle too fast you can't get it out of gear.
    This also doesn't account for load testing to make sure the engine RUNS good throughout the rpm range. But I don't think Blake was asking about that, I think he's talking more about running an engine out of the water loaded like with a propshaft dyno but no back pressure from the water.

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    I see now, still don’t see how it would cause any problems. Only possible way would be extra heat through the exhaust tube and lower unit and possibly making the lower unit warmer due to no cooling water around it. But water should still be coming down through the water jacket and exhaust into the lower unit.

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    Muffs Were Made For Flushin

    Quote Originally Posted by InjectorService View Post
    I hear people talk about this all the time so I wanted to start a little discussion on it. People always talk about a lack of backpressure when running motors on muffs, potentially causing damage etc. Obvioulsy loading the motor on the water, with a boat loaded the way it would normally be would be the best. But asside from that is there any real reason not to run a motor out of the water? The motors obvioulsy have an idle relief, a lot have above water exhaust, and how much back pressure could there be with water running past the bullet at 60mph+. In my mind, once the exhaust is out of the tuner, no back pressure is needed or maybe even wanted..... am I wrong?
    I think muffs were made for flushin but not load test tuning. I have also heard whacky stories about barrel test tank tuning where the exhaust gases would replace air in the agitated water mix? Do engines breathe? (LOL) Aside from powerhead combustion...........Today's new motors are rigged with flush hose connections but are not recommended for running engines..........only salt flushing.

  10. #9
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    Thanks for all the responses. It seems the consensus is generally the same. You can run/test adjust out of water without issue. But confirm diagnosis under load on the water if possible.

    The reson that sparked this, I had a friend that bought a boat motor years back, and the dealer he got it from WOULD NOT even start the motor on muffs. He had to take the boat without hearing it run. Sounds like a sketchy situation to me.


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  11. #10
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    That is sketchy. I will say I had a problem with destroying my impeller while using muffs with my motor, always ran fine on the muffs but the next time I’m the water the pump wouldn’t pump. So I stopped using muffs and bought a 100 gallon livestock tank to run my motor in, haven’t toasted an impeller since. I did always use the cheap muffs you buy at local stores, not the official Mercury OEM muffs that have the rod clamp, probably would work better.

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    Muffs 2 Condoms

    Quote Originally Posted by Fishwhisper View Post
    That is sketchy. I will say I had a problem with destroying my impeller while using muffs with my motor, always ran fine on the muffs but the next time Iím the water the pump wouldnít pump. So I stopped using muffs and bought a 100 gallon livestock tank to run my motor in, havenít toasted an impeller since. I did always use the cheap muffs you buy at local stores, not the official Mercury OEM muffs that have the rod clamp, probably would work better.
    I wonder if the hose over pressured the pump impeller? When you think about it, the inlet holes (added together in cross section) probably do not add up to 1 inch in area? Still water at rest relies on the impeller's ability to "pull" it over the orifice in the pump body. Muffs are going to deliver more water than the pump can "eat" at idle. Speaking of inlets, I made a slip over "condom" for an SSM unit once. Worked OK with a test wheel. Has anybody figured out a "condom" for nose cone gear cases?

  13. #12
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    Quote Originally Posted by WaterZebra View Post
    Has anybody figured out a "condom" for nose cone gear cases?
    Bob's... in nose cone accessories.
    https://bobsmachine.com/product/fresh-water-flush-kit/

    Also. I think the impeller blades are folded forward and dong nothing once you're at speed... I don't see how a garden hose could hurt the pump.
    Last edited by David - WI; 03-23-2023 at 03:26 PM.
    Let's go Brandon!

  14. #13
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    Quote Originally Posted by David - WI View Post
    Bob's... in nose cone accessories.
    https://bobsmachine.com/product/fresh-water-flush-kit/

    Also. I think the impeller blades are folded forward and dong nothing once you're at speed... I don't see how a garden hose could hurt the pump.
    Square muffs don't seal on a lot of motors and burn the pump via sucking air. Round ones that are soft rubber seal good. I run multiple engines a day 5 to 6 days a week on muffs and never hurt impellers. I have customers that fry their impellers using muffs... You just need to be smarter than your flushing device. If when you start the motor up it doesn't suck the water out of the hose itself then its probably sucking air from around the muffs: not good.

    I have, however, had problems with running engines in bags/buckets. Occasionally, not always. People say all the time on here when this comes up that the wp housing must be submerged when you are running in a bucket/bag. Like it would in the lake. The water level has to be higher than the pump, otherwise it won't prime and will melt itself in the air its in before it has a chance to suck water. I have found this to be true most of the time but not always. Good rule of thumb though if you are running in a bucket to get the whole lower unit below the water.

  15. #14
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    The one thing that can happen in a bucket too, is if you do it in gear, you can blow the water away from the intake & suck air as well. I've seen people do this before. Like said above, be smarter than your flushing device or whatever you are using to run your motor.


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  16. #15
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    My 100 gallon livestock tank gets the water about 2-3 inches above the gearcase and I have not put it in gear since the first and only time I put it in gear, it about emptied half the tank in a few seconds. And I know I probably could have gotten the muffs working correctly if I wanted to but my trips to the water are about a 2 hour drive and infrequent enough that I didn’t won’t to risk messing up a trip with muffs. I did use the rectangular ones, maybe I should have use round ones.

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