Generator exhaust noise

[Jeff64]

Ever since I bought the Interstate I have thought the generator exhaust seems to be a bit loud, Everyone I have listen seems to agree it's a bit loud, not a rattle just a noisy exhaust, much better sound when standing on the passenger side. I've done a little research and have had several guys suggest a Super Trapp muffler and resonator, from what I can tell it would definitely improve the situation, however it installs at the end of the exhaust pipe so unless I'm missing something you would have to cut the existing pipe back several inches, maybe 12" (the Super Trapp muffler is around 9" long) which would put the muffler and the exhaust exit point just underneath the van rather than exiting out where it currently is, the Super Trapp is too big around to be mounted directly underneath the sidewall and would need to be held back some. My concern and question to other members is, would you deem this a danger? would it be possible for the exhaust fumes to get through the floor and enter the coach? Looking for some input.

Thanks!

Jeff

[Protagonist]

Originally Posted by Jeff64

would it be possible for the exhaust fumes to get through the floor and enter the coach? Looking for some input.

You do have floor penetrations near the generator (low point drains, for one), so your concern is valid. It may be worthwhile to add a short section of exhaust pipe to the end of the Super Trapp to get it back out to where the original exhaust pipe ends.

I never found my generator to be particularly loud. Maybe that's because, never having owned a motorhome with built-in generator before, I didn't have any particular expectations as to noise level. I've certainly heard louder generators at some of the campgrounds I've visited.

[xrvr]

Be careful of warranty voiding by doing exhaust mods. Try a Venturi, spelling may be wrong, tube to direct exhaust fumes and noise up over mh. Jim

[xrvr]

Try genturi. Also onan offers a resonator, my local AS dealer has one. Jim

[Protagonist]

Originally Posted by zigzagguzzi

Be careful of warranty voiding by doing exhaust mods. Try a Venturi, spelling may be wrong, tube to direct exhaust fumes and noise up over mh. Jim

Having looked at the Gen-Turi at Camping World, I can say with reasonable certainty that it will do much more for redirecting fumes than for noise reduction.

You may consider getting an elbow that you can clamp onto the generator exhaust pipe, so that you can turn it downward to point the exhaust at the ground. Any engine exhaust is loudest exactly in line with the end of the exhaust, so if it points at the ground and not toward people, it will seem quieter. Just make sure it's far enough from the ground that you don't increase back-pressure, and don't use the elbow if you park on grass.

[xrvr]

If the sound is directed up above the MH it is much less obtrusive. I have seen them used. Also you do not need to worry about fumes on the ground, just make sure the end of the genturi is not near an open roof vent. Jim

[Protagonist]

Originally Posted by zigzagguzzi

If the sound is directed up above the MH it is much less obtrusive. I have seen them used. Also you do not need to worry about fumes on the ground, just make sure the end of the genturi is not near an open roof vent. Jim

The reason I doubt the efficiency of a Gen-Turi for noise reduction is that you have an elbow the same diameter as the exhaust, feeding into a chimney of a larger diameter, with a gap through which sound can escape. Granted, there is some reduction, because as with the downward-facing elbow I suggested, the upward-facing elbow points the noise away from listeners. But I don't think the reduction in noise will be any better than what I suggested.

The reasons I suggested the downward-facing elbow as an alternative are:
1 - it's cheaper;
2 - it stores in a smaller volume when not in use;
3 - You don't have to install anchor points on the side of your RV to hang the chimney from.
4 - rainwater will not collect in a downward-facing elbow the way it can in an upward-facing one.

If Airstreams were parked cheek-by-jowl at a rally, I'd certainly invest in one to avoid gassing my neighbors with generator exhaust. But for noise reduction alone, I don't think a Gen-Turi is worth the ~$170 price tag.

But that is only my opinion, worth exactly the same 2¢ as anyone else's. I've done my bit by offering an alternative, whether anyone tries it or not. It's all food for thought, and may prompt others to come up with better ideas.

[Jeff64]

Thank you for the comments, I actually have a genturi for my other MH it is a good product but is primarily designed to divert exhaust fumes, it does help reduce noise a bit. I think the Onan resonator seems to be the answer, there is a good You tube video on it, I believe it is a small enough diameter to exit similar to the existing exhaust. The Super trap is probably the superior muffler/resonator but does not exit the exhaust from the opposite end, it disperses it 360 degrees around the muffler and is several inches bigger in diameter and considerably more expensive, and would need to be mounted under the coach. So for this particular application I think I'm going to order the Onan resonator.

Jeff

[russ240]

Originally Posted by Jeff64

I think I'm going to order the Onan resonator.

Jeff

Please do report back on how it works! I've been contemplating some sort of muffler upgrade for over 2 years now but have not pulled the trigger.

[kaylorsan]

I recently added the resonator (Cummins resonator - 155-2449). I think I paid $57 for it on eBay. Install was a breeze. I cut the existing exhaust pipe with a tubing cutter and used muffler bolts to attach it. If I recall correctly, the existing setup had 2 clamps securing the exhaust.



I used one of them, and added a bigger one (2.5"?) to clamp around the body of the resonator. I had to drill two new holes in the sheet metal heat deflector. Took about 30 minutes to complete. Used the existing exhaust hanger.

(shown without the second clamp attached)

I downloaded a free iPhone app to measure decibels. The resonator averaged about a 2 dB reduction in multiple tests. Really not much. However, the sound (pitch? frequency?) changed in a way that produced a less annoying sound.

(with resonator)

(without resonator)

I'm happy with the results, even tho it didn't really quiet it that much. In reality, it does what the website says it does (http://www.rveparts.com/cummins-onan...uiet-155-2449/) in terms of noise reduction.

FWIW, I also added exhaust tips to the gen and engine lines. Those look much better than stock. Both were bolt-on models (Main - Magnaflow 35208,
Generator - 35594) I needed to drill a drain hole in the generator tip

[joemikeb]

Thanks for the report. The resonator is third on my modifications list, right after adding an internal propane switch in series with the existing outside switch. I already checked off the first item, Koni shocks and struts.

[kaylorsan]

Originally Posted by joemikeb

...I already checked off the first item, Koni shocks and struts.

I did the shocks, but have not yet done the struts. I've been considering it, but just haven't found the time to get them done. The shocks made a big difference for me. Are you satisfied with the struts? Did you do the shocks and struts at the same time?

[73shark]

Nice looking install. IIRC, a 3db change represents a doubling or halving. So a 2db drop isn't bad. But that was in power, don't know if it applies to sound levels.

I looked at the 35208 but thought it would be too long so went w/ a shorter, larger diameter one that I had to modify since it was designed to be welded and I didn't want to try welding st-st.

[joemikeb]

Originally Posted by kaylorsan

I did the shocks, but have not yet done the struts. I've been considering it, but just haven't found the time to get them done. The shocks made a big difference for me. Are you satisfied with the struts? Did you do the shocks and struts at the same time?

Yep both at the same time. I have been VERY pleased with the improved handling. The Interstate carves a corner almost as sure footedly as my VW GTI and that is saying a LOT. Overall it feels more like a "sports suspension". Stiffer but still comfortable up front. The one thing they didn't cure is the ride in the lounge area. Even at moderate speeds the seat belts need to be tight to prevent launching passengers into the ceiling.

Not sure what, if anything, to do about that.

[Jeff64]

I can finally report back on my solution for what I consider to be noisy generator exhaust. I ordered the cummins resonator and I also ordered the 5S Super trapp muffler with 6" resonator from Jacks small engine, I tried both and the Super trapp was the winner hands down, there is almost no exhaust noise just the noise of the engine itself. I'm attaching a picture. I intend to add a hanger at the muffler attach point and omit the original factory hanger.

[David M]

Originally Posted by Jeff64

I can finally report back on my solution for what I consider to be noisy generator exhaust. I ordered the cummins resonator and I also ordered the 5S Super trapp muffler with 6" resonator from Jacks small engine, I tried both and the Super trapp was the winner hands down, there is almost no exhaust noise just the noise of the engine itself. I'm attaching a picture. I intend to add a hanger at the muffler attach point and omit the original factory hanger.

Very nice Jeff! Did you get a chance to test the output of the generator yet? Does it still output enough power to run the roof air? TIA
David

[SilvrSausage]

I had my auto muffler guy re-do the system on my SOB Onan generator. By adding some curves and length, we came up with a setup that used a small car muffler, and moved the exhaust point to the rear; all while still allowing the pull-out generator tray to work normally.

The small car muffler did a superior job of quieting the exhaust, with so much more flow capacity that back pressure was a non issue. I discovered that the exhaust is only ONE of the noise factors from the generator. Shrouds, sagged mounts, and the sound of the engine itself make a dissonant symphony, but muffling the exhaust goes a long way at a campsite.

A resonator is basically a canister with some manner of simple baffles or chambers, but no damping material. It reduces the amplitude of the exhaust pulses, but in peaks and valleys across the frequency scale.

OTOH, a muffler includes sound damping material like fiberglass packing. It does a superior job of damping the sound waves across the scale. It also has a more complex flow pattern than a resonator, with perforated pipes carrying the flow forward and back, from chamber to chamber. The sound damping is far better, but this introduces more back pressure into the system.

Finally, a glasspack muffler is a perforated or riffled tube going straight through, with fiberglass between the perforated tube and outer shell. These don't muffle well, and riffle designs introduce some backpressure through turbulence; these are mainly used on performance cars. However, they have a place in motorhome systems; on the engine exhaust, small glasspacks added near the end can help reduce the strange resonances from the looong pipes. Also, a glasspack might be a good supplement or endpiece in a longer generator system, but not as the only muffler. There are many "exhaust tips" on the market that amount to a small glasspack and may be worth looking at.

One thought about using a car muffler; having a 1-1/2" generator exhaust pipe enter a 2"+ muffler inlet is good. You may find that the sound damping is better with the outlet reduced again to a 1-1/2 or 1-3/4" tailpipe instead of 2" or whatever size the muffler outlet is. If size is no object, this will give quieter results than hunting up a generator-size muffler.

My 25c on generator exhaust mods.

[russ240]

Originally Posted by Jeff64

ordered the 5S Super trapp muffler with 6" resonator from Jacks small engine

Thanks for finding a great option for us! I just went to the website but there seem to be lots of variations. Can you provide the part numbers you ordered?

Thanks again!

[xrvr]

Are there not warranty issues with CumminsOnan as far as using a non factory muffler? Used to be. Jim

[Inland RV Center, In]

Originally Posted by russ240

Thanks for finding a great option for us! I just went to the website but there seem to be lots of variations. Can you provide the part numbers you ordered?

Thanks again!

The expansion of the hot exhaust gas, is the noise maker.

Cooling off the exhaust, greatly reduces the noise.

Simply adding a longer tail pipe of what ever length, and you will be amazed at the reduction of decibels.

Andy