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Old 08-12-2018, 01:30 PM   #1
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Default ONAN exhaust quieting

Hey all,
I just wanted to post this info about quieting the Onan 2500 propane generator. I recently installed a Cummins resonator to quiet the exhaust, and I've been really impressed with how well it worked.
The resonator (which differs from a muffler in that the exhaust flows straight through, and it therefore doesn't affect the exhaust back pressure) was my attempt to cut down the sharp :edge: of the noise. I really didn't expect it to reduce the overall sound that much, but for $35 in parts and $80 at the welding shop, I figured it was worth the experiment.
Now, I wish I had taken before and after reading with w a decibel meter. The noise is much smoother (which was expected from the resonator) and also about 20% quieter ( which is a pleasant surprise). It is by no means silent, but I feel much less self-conscious about running it in the back of a parking lot on a hot day to keep the dogs cool in the van.

So, here is a part number, and the location I installed the resonator. This is on a 2018 Pleasureway Ascent, so you may have a different exhaust pipe setup.

Rick
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Old 08-12-2018, 03:17 PM   #2
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We did the same. Dropped the sound by about 9 dBa and we are considering adding a second one to see if we can actually make the Onan quieter than the van engine at low idle.
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Old 08-12-2018, 03:21 PM   #3
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Originally Posted by Phoebe3 View Post
we are considering adding a second one to see if we can actually make the Onan quieter than the van engine at low idle.
Interesting. I wonder what would happen if the exhaust pipe were tee'd into two paths through separate resonators and exiting on opposite sides of the vehicle.
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Old 08-12-2018, 04:57 PM   #4
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Anybody else notice that Cummins misspelled "resinator" on their box label?

Anyway, I installed one on our 1999 Xplorer last year and was also amazed at the sound "softening". Didn't measure with a db meter but it is much less intrusive and not nearly as annoying. Very worth while upgrade IMO.

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Old 08-12-2018, 05:43 PM   #5
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Found it on Amazon and several other places online. "Resinator" sounds like a 420-type device.

After I double-check to make sure I have room, I think I'll buy one of these.
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Old 08-12-2018, 05:59 PM   #6
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Interesting. I wonder what would happen if the exhaust pipe were tee'd into two paths through separate resonators and exiting on opposite sides of the vehicle.
That's a really interesting question... I don't know that much about exhaust physics... someone elsewhere suggested that a lot of the noise from these generators comes from their mechanical operation, and silencing the case (insulation?) is a good solution as well.
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Old 08-19-2018, 07:40 PM   #7
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Yes, the mechanicals and fan are noisy too, just like your lawnmower, which is in effect what these small generators are.
You can add insulation but you have to make sure it can still get enough air to cool itself(unless it's water cooled).
The newer portable 4 cycle minigenerators from Honda etc are very quiet.
The vehicle engine is water cooled which allows it to be surrounded w much more insulation as only the radiator needs to be exposed to air. The water in the double layers of steel to trap it in the engine also muffles the mechanical noise, so it's hard to make an air cooled one as quiet in a small space that must allow adequate air circulation.
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Old 08-19-2018, 08:14 PM   #8
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Originally Posted by Phoebe3 View Post
We did the same. Dropped the sound by about 9 dBa and we are considering adding a second one to see if we can actually make the Onan quieter than the van engine at low idle.
That I would like to see the results of. Also would like to see the results of splitting the pipe and running one out both sides, or one side and one rear like was suggested above.
Don't know if that would be worth the $$ invested though....??
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Old 08-19-2018, 09:17 PM   #9
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You will get better sound deadening from running mufflers in series and not in parallel. In other words one exhaust pipe with two mufflers in a row.
I tried a parallel system on a riding mower but I did just to look cool so I made dual stacks w two mufflers. It was just as noisy as with one.
However, in series they do multiply the advantage. For instance your catalytic converter on your car acts also as a muffler but it's still noisy(although less than open pipes), but then when another muffler is added behind it it becomes WAAAYYY more quiet.
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Old 08-19-2018, 10:15 PM   #10
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This is likely the same resonator the OP used. It’s still not the quietest but the resonator smooths out the noise to make it much more tolerable. I was able to install it with clamps.

https://youtu.be/gQboSPqKtfE
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Old 08-20-2018, 03:37 AM   #11
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You will get better sound deadening from running mufflers in series and not in parallel. In other words one exhaust pipe with two mufflers in a row.
I tried a parallel system on a riding mower but I did just to look cool so I made dual stacks w two mufflers. It was just as noisy as with one.
However, in series they do multiply the advantage. For instance your catalytic converter on your car acts also as a muffler but it's still noisy(although less than open pipes), but then when another muffler is added behind it it becomes WAAAYYY more quiet.
Would running two of these things in series cause any back pressure that might damage the generator?
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Old 08-20-2018, 04:19 AM   #12
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No running a muffler after a reasonator(or before) won't cause undue back pressure. You have to do something pretty crazy to throttle them that much, and generally the bigger the muffler that you can fit in the same category is going to be the quietest with the least back pressure, also the "straight through" style mufflers have less back pressure generally.
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Old 08-20-2018, 04:20 AM   #13
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Would running two of these things in series cause any back pressure that might damage the generator?

That has been discussed a lot over the years. There is no doubt that single speed engines like the Onan can/should be carefully designed to optimize performance at that rpm as they don't ever run at any other speed. That would include lots of areas of the engine from the carb to the tailpipe and all in between.



For the exhaust system, they can tune to specific rpm by changing tubing diameters pipe lengths, baffles in the muffler, etc so changing any of them can be a performance loss or heat related issue.


But, even though Onan insists that you can only use their straight through resonator, I think it is not fully resolved as to if that is really true or not because we don't know if the entire exhaust system was optimized or not. They do allow quite a bit of pipe length and bends which might indicate it is not all that important. Their resonator is straight through like most are and is likely setup to filter the pulses of a 3600rpm engine, where some others may not be.



Adding another straight through unit would likely not be as much of a potential issue as using a baffled muffler, which could increase back pressure and mess up the exhaust pulse frequency.


All this said, there have been quite a few people who have added all kinds of mufflers to their Onans and were happy with the results. The problem is we don't get the opportunity to follow up on them as they age and see if they issues, or not because of the changes.


We used to hear a lot about the Genturi systems for the exhaust, but I haven't heard them mentioned in a long time.
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Old 08-20-2018, 08:06 PM   #14
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Default Onan resonator

I installed the same resonator on the Onan 2800 in my Xplorer and found that it greatly reduced it's noise, but primarily outside of the vehicle. Still a very nice addition as if you have neighbors present it's just much less annoying. You can read many reviews by folks that have added this that say it made little to no difference, but I'm pretty sure they are talking about the lack of significant noise reduction INSIDE the coach. In my case, after it's installation I realized how much noise was getting inside due to insufficient sound deadening of the generator enclosure, so I lined all of the outside of the enclosure that I could get to (the inside was already lined with a foil backed fiber insulation), including the inside of the access door, with Useal, an aluminum backed butyl rubber product that you can buy at Home depot for a fraction of the cost of Dynamat. As far as I can tell it is exactly the same material, and it greatly reduced the genset engine noise transmission to the interior of the RV.
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Old 08-20-2018, 11:17 PM   #15
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Originally Posted by nicaland View Post
I installed the same resonator on the Onan 2800 in my Xplorer and found that it greatly reduced it's noise, but primarily outside of the vehicle. Still a very nice addition as if you have neighbors present it's just much less annoying. You can read many reviews by folks that have added this that say it made little to no difference, but I'm pretty sure they are talking about the lack of significant noise reduction INSIDE the coach. In my case, after it's installation I realized how much noise was getting inside due to insufficient sound deadening of the generator enclosure, so I lined all of the outside of the enclosure that I could get to (the inside was already lined with a foil backed fiber insulation), including the inside of the access door, with Useal, an aluminum backed butyl rubber product that you can buy at Home depot for a fraction of the cost of Dynamat. As far as I can tell it is exactly the same material, and it greatly reduced the genset engine noise transmission to the interior of the RV.
Thanks for that insight! I often wondered why we had a tremendous reduction and others didn't, even when they welded the resonator in place. It really didn't occur to me that we may have been talking about inside vs outside.

That said, the resonator we installed did, in fact, reduce the inside noise, although I didn't measure it.

We wondered perhaps if Onan quality control was poor and some resonators were made properly and others weren't, thus accounting for the difference in our experiences with the same product installed in the same manner.
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