Poor Man's Generator Noise Reduction (repost)

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Poor Man's Generator Noise Reduction

We all want a quiet genset, right? Here are the details of what I did to quiet down my small genset.

A small gas engine makes a lot of noise. I have read that the better engines such as Honda are inherently more quiet than cheaper ones. My engine is a cheap one and therefore the results are more significant. I have a 1000 watt generator powered by a used 3 1/2 HP Briggs & Stratton engine which just screams at full load.

First of all there are two distinct noises that an engine produces; exhaust noise and engine mechanical noise. Before I began this project, I thought that exhaust noise would be the greater of the two, now I am not so sure.

To reduce the exhaust noise I went to my local lawnmower repair facility. I highly recommend finding the one in your area and spending a little time getting to know the folks there. They are very helpful and there are always "jewels" to find out there in their graveyard. I talked to them about muffler options and quickly found that there are no "general" stock items for such an application. I did use the yellow pages and called a few generator places, including Honda, and found there are some new spark arrester mufflers, like the type used on Generac 4000 and 5500 XL's. These mufflers "promise" to be quiet but at a hefty price $85 plus. I have not heard anything operating with these type of mufflers, so I don't know how effective they are at noise reduction. If any of you have a genset with one of these then please share your experience.

Out in the lawnmower junkyard, I got a 100 cc moped muffler and mounting bracket for $6. A quick trip to Home Depot, some 1/2 inch galvanized pipe and fittings and, presto - a great super-low tone muffler.

Now that I got the exhaust noise greatly reduced, it was time to deal with the engine's mechanical noise. I got some free scrap 3/8" plywood from a construction site (amasing what they will let you have for free if you just ask). I built a 5 sided box shroud, 4 sides and a top, and lined the inside with fiberglass (also free from scrap).

I know what some of you are thinking, what about ventilation for the heat? This shroud sits over the engine portion of my verticle shaft genset, in such a way that allows plenty of room for the engine's forced air paths to exit. Most of the forced air is directed at the exhaust port and cylinder head. The top of the shroud has a 6" circular opening to draw cool air right into the engine's cooling fan.

I ran my genset for 5 hours under load and noticed it seemed to be no hotter than without the shroud. I am not saying it was not ANY hotter, just that I could tell no difference. The noise reduction, however is terrific! Before making the wooden shroud, I used a large cardboard box as a prototype. The wooden box was amasingly better, due to the fiberglass insulation no doubt.

With the engine's mechanical noise dramatically reduced, I found that the moped muffler I used was emitting more noise that I wanted. You know how a project can get once you get really into it. I went to my local auto parts store and bought the cheapest car muffler they had for $25. I installed the car muffler and WOW, one quiet genset! If there is ANY exhaust noise, I would need NASA scientists to come by with sensing equipment to show me, my ears hear none at all.

For comparative purposes; The Generac 4000XL's noise specs are 86 dB at 3 1/2 ft 73 dB at 23 1/2 ft

My Generator's noise specs are 83 dB at 3 1/2 ft 71 dB at 23 1/2 ft

I think that if I started with a new engine the specs would be a little better. This engine burns 1/2 quart per 15 hours of use, so obviously there is some serious wear that's already taken place during its life.

The end result is for a $ 35-40 investment, both mufflers (who knew) the galvanized fittings and all, and a few hours of labor I ended up with an very quiet genset. I wish now that I had also taken dB readings before starting this project. I can tell you, it was a LOT louder. I think I'll bring it all down and show the guys at the lawnmower repair shop, they'll get a kick out of it.

-- Buffalo Bob (buffalo_bob42@hotmail.com), December 08, 1999


Bob: That's a great post and it sounds like you've beat the sound problem but I'm concerned with the back pressure. I understand the back pressure will interfere with engine operation over the long haul. What do you think??


-- chuck@newriess.com (Charlie@newriess.com), December 08, 1999.

As far as back pressure, keep in mind I am going through a rather short distance of 1/2" pipe (stock muffler pipe size) into a short run of larger pipe of 1 1/4" into an automobile muffler. I am no expert on fluid or gas dynamics but I would bet that my arrangement offers no more significant resistance than the stock muffler did.

I inquired about this to several mechanics and they agreed that, given the type of engine used, so long as the exhaust goes into a larger opening and muffler then things will be just fine.

-- Buffalo Bob (buffalo_bob42@hotmail.com), December 08, 1999.

Bob, here's something I did that caused an unexpected but noticeable reduction in noise.

I have a 6.5kw generator powered by a dual-fuel Honda 13hp engine. At the suggestion of several more-knowledgeable mechanics than I, at the 90hr oil change, I replaced the regular 10W-30 crankcase oil with Mobil 1 synthetic oil. (I don't have any stock in Mobil.)

When I next started the engine, it was immediately obvious that it was running much quieter and seemingly smoother. An hour later, the wife wandered by, and -- no mechanic, she -- without any prompting, she also commented on how much nicer it sounded to her. It seemed that much of the metal-to-metal clanking noise had greatly diminished.

I was sufficiently impressed that I plan to put synthetic oil in all of my small engines that power my chipper/shredder, log splitter, pumps, etc.

Might help to quiet your B&S, Bob.

-- Norm Harrold (nharrold@tymewyse.com), December 08, 1999.

Another suggestion: try surrounding the generator w/sandbags - if you leave the top open, the sound will be directed upwards (not as easy to locate).

-- James (jpeet@u.washington.edu), December 08, 1999.

Great post Bob. One suggestion though. Installing a different muffler may require you to change the carburetion settings.

I'd check the spark plug that's in there now to make sure the generator isn't running dangerously lean (if so, the ceramic will be whitish tan to white - and there may be little specks of melted aluminum on the ceramic and electrodes). Obviously, if this is the case you don't want to run it any more until you put in a (much) larger main jet.

Otherwise, if the plugs looks pretty good (at least tan or darker), I'd put a new plug in, run it for a few hours, then do a reading on the plug and change either the main jet or fuel/air screws accordingly. If you don't know how, talk to the guys at the shop and see if they can get it dialed it in for you.

Doing this will improve engine life, power output, and fuel economy. Good luck.

P.S. Thanks for the post. I think I just might give it a try myself!

-- Clyde (clydeblalock@hotmail.com), December 08, 1999.

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