Poor Man's "Generac 4000 XL" Noise Reduction

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Poor Man's "Generac 4000 XL" Noise Reduction

We all want a quiet genset, right? Here are the details of what I did to quiet down my Generac 4000 XL. I am unable to provide pictures, so I'll describe things as best I can.

There are two distinct noises that an engine produces; exhaust noise and engine & mechanical noise. Both must be dealt with in order to reduce the overall noise. I'll first show what I did to reduce the exhaust noise:


The Generac's come with a decent stock muffler. I looked at several alternatives to reducing the muffler noise further. I finally decided to implement a hybrid approach. Rather than replace the stock muffer, I connect its output to a standard $25 automobile muffler. This approach reduces the exhaust noise so well I cannot imagine needing to reduce it any further. Here's how:

I removed the stock muffler and brought it to a welding shop to have a 1 1/4" diameter female elbow welded to it, where the exhaust exits the muffler. This elbow is a standard item at any Home Depo.

Now that I had the means to connect to the muffler, I attached a 1 1/4" male to male coupler then 28 inches of 1 1/2" flexible exhaust hose, then the automobile muffler.

This arrangement ends up with the exhaust directed out the front of the genset, the side containing the electrical plugs.

Note: These flexible exhaust tubes are getting hard to find due to the EPA, but you can still get them at NAPA.

When running, I position the automobile muffler horizontally about 4" off the ground in a cheaply fabricated "cradle". Nothing fancy, just to keep it off the ground. The exhaust travels through the stock muffler, down the elbow, through the 28" flexible exhaust tube, and through the automobile muffler. Super quiet muffler!

Moving on to reducing the engine & mechanical noise:

I thought of building a shroud for my genset, like I did for my backup genset. A Generac, or any standard genset presents several problems to a shroud-based approach. There are two competing factors at work here:

(1) Surrounding the unit with noise dampening material, and (2) Providing adequate airflow for cool operation and intake.

Just as with many other things, a compromise is in order. Knowing that I won't be able to reduce the noise entirely, I set upon a simple solution. Use 2 sections of carpet, one section on the ground under the genset and one section draped over the top.

I obtained, at no charge, a section of new carpeting. Rather than build a shroud and line it with fiberglass, I made a simple 1" by 1" wooden frame that I tie-wrap to the top of the genset itself to act as a shroud "frame", and simply drape the carpet over it. The wooden frame just keeps the carpet from draping too close to the genset's sides. Read on before concluding that this idea is crazy...

Obviously, I cannot cover the entire genset with carpet by merely throwing it over and walking away.

I cut the carpet into a one section measuring 6'(feet) by 4'. I lay this section of carpet on the ground in the desired area to run the genset, carpet side up. Its length is running from side to side, from my point of view. I then position my genset over the carpet, so it rests about 2' to the right of the carpet's left edge. My Generac's electrical outlets are staring me in the face, its stock muffler pointing to the right. A stupid picture of this scene is as follows; (hope it comes out)

[rear] (--- 8' ---)


IIII IIII-I <--genset & muffler (I) (2') IIII I ==========I================= I

:-) <--me [front]

After connecting, starting and warming up my genset:

From its right side, I bring the upper section of carpet up and over it top to its left side. Remember the (2') section of the lower carpet's left edge that we left earlier? Our from-the-right-side-up-over-the-top-and-down-the-left side now joins this 2' section, and is held there by a couple of bricks or cinder blocks.

Now that the carpet is loosely "draped" over the genset, we must tidy things up. I lay several bricks over both edges to keep the carpet angled far enough away from the genset to allow enough air for cooling.


I have run my genset for over 20 hours during the summer under load and have noticed it seems just a bit warmer than without this "shroud". I don't have the means to provide before/after temperature measurements. "Just a bit warmer" is the best way I can explain the difference. The amount is quite acceptable to me. Running it during the winter (Y2K) will be no poblem at all. You can easily make a prototype "shroud" and try it for yourself.

The noise reduction, however is significant!

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 two modifications bring the noise levels down to 79 dB at 3 1/2 ft 68 dB at 23 1/2 ft

The end result is for a small investment and a few hours of labor I ended up with a much quieter genset.

-- Buffalo Bob (buffalo_bob42@hotmail.com), December 09, 1999


FWIW, Here are a few extra suggestions based on what I did.

I placed a folded over piece of 3 1/2" fiberglass batt insulation under the car muffler to eliminate any vibrations. It is fireproof.

I used an auto muffler hanger to support the exhaust pipe where it leaves the generator shack. I also wrapped the exhaust pipe at the exit point with fibreglass batt insulation to eliminate any possible vibration from transferring to the shack. When the generator is not in use, I use a soup can to cover the end of the exhaust pipe to keep mice, etc. out.

I used some concrete blocks set at a 45 degree angle about 2 feet in front of the exhaust outside to deflect the noise away from the direction of the street and off into the bush at the side of our house. There is also a fence behind the blocks.

-- John (jh@NotReal.ca), December 09, 1999.

A quick question, BB - since installation of the muffler, have you experienced a drop in capacity rating of the gen set? Also, your heat increase may well be as a result of the engine working harder because of backpressure from the exhaust system, rather than the enclosure.

You might want to check the rating to make sure the unit is still capable of putting out 4000W. Also, I'm concerned that you're not terribly concerned about the apparent heat load increase. I think you need to quantify this increased heat production, as heat will cause a small genny engine to prematurely fail quicker than any other factor. Don't rely on cooler ambient temperatures at this time of year to result in significantly cooler operating temperatures of your gen set. You also might want to consider going to a slightly lower viscosity oil if the operating temperature is much higher than without the muffler and enclosure.

Oh, lastly, here's a suggestion for checking engine temp differential - run the engine bare bones (no muffler extension or enclosure) under full load for 10 minutes, then shut the engine down. Stick a candy thermometer down the oil fill connection, and measure the temp. Then, do the same thing with the muffler installed and the enclosure in place.

-- Nom (nom@de.plume), December 09, 1999.

Thanks for the feedback guys.

John: Great idea of using bat fiberglass as the cradle!

Nom: Use a candy thermometer? Excellent point. I do think the little bit warmer is caused by the enclosure and not the muffler. I ought to be able to prove this with your measurement method, no?

Be well, both of you!

-- Buffalo Bob (buffalo_bob42@hotmail.com), December 09, 1999.

I've used an automotive muffler with my Generac 5 kW set myself. HUGE drop in noise. Re. air flow and shroud, the manual that came with my generator said you could make a partial shroud if you have to use it in bad weather. The trick is to keep it open on the intake/exhaust side, front and back. My unit puts out a lot of heat so I'd be reluctant to block the air too much.

I have a 20" box fan that could blow a lot of air at the unit if I put it in fron of the gen. The generator seems to throw its hot air out to the back (probably internal fans in the engine and possibly generator). Therefore, put a fresh air fan in front of the unit, set to blow cold air onto the set. This should make a huge difference in cooling.

I think that colder ambients will improve cooling. Heat transfer is dependent on the difference between temperatures, and VERY dependent (square) on air flow over surfaces. Double the air temp. difference, double the heat flow. I'm not sure what ambient Generac uses, but I'd hazard a guess of ~ 70 F. Running at ~ 10 to 30 F. would make a big difference in cooling. Adding the fan would REALLY make a difference. And cooler engines last longer -- can be a lot longer.

After all this rambling, I could picture building a two sided doghouse, with a roof. The sides should be no closer than 12" from the sides of the engine/generator (assuming exhaust is piped away from this area, like mine and bob's). The roof wouldn't matter, that's where the gas tank is anyway. Put a large fan at least 3' in front of the housed generator and set it to blow thru the tunnel.

BTW: "Front" on mine is the engine end, back is the generator end...

-- Gary S. (garys_2k@yahoo.com), December 09, 1999.

Here is another one for you, heard it today. Take that muffler and drop a long hose on it with a hose clamp. Dig a hole in the ground a with a post hole digger 2 or 3 feet deep and put a lid on the hole made of plywood. Make two holes in the plywood, one for the hose, one for to let the air out. Run the hose thru the hole to the bottom of the hole... You get one hell of a lot of noise reduction.

-- (...@.......), December 10, 1999.

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