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Onan stock muffler 155-1258 or cheap Car muffler(s)

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Old 01-21-2010, 04:41:57 PM
HondaBob HondaBob is offline
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Default Onan stock muffler 155-1258 or cheap Car muffler(s)

I want a good quiet muffler for a recently acquired Onan 6.5 KW generator set, Model 6.5 NH-3CR 16004P, Serial # F833680756. It will be used for home standby use and occasional use at Trail Ride events.

Although my unit is a Spec P, it has a Spec K exhaust manifold 154-1864, no flange where it connects to the vertical exhaust tube.

A few questions please:

1. I have never heard the unit run with the original Onan stock muffler 155-1258, and therefore don't know how quiet it will be. Someone added a non-stock muffler that it uses now, and that one needs replacement. I can buy a $70 Onan 155-1258 or a cheap $20 Automotive muffler, but I don't know which will be quieter. I have read here about using two cheap automotive muffler in series. Any opinions on this please?

2. I have read many posts here that speak about the need for using a flexible exhaust coupler when adding a (non stock) muffler to an Onan engine. This is usually advised to to protect the engine heads from cracking, and that makes sense to me.

When I look at the stock exhaust layout for an Onan 6.5 KW generator set, Model 6.5 NH-3CR 16004P, the exhaust manifold connects to a vertical exhaust tube. This exhaust tube connects directy to one end of the stock Onan muffler, 155-1258. The other end of the muffler is held to the vehicle via a muffler hanger.

This stock arrangement uses no flexible exhaust method. It is obvious that the exhaust manifold directly bears the weight of one end of the stock muffler. I would think this weight, plus the added strain of an RV in motion going down a bumpy road, would place a great deal of stress on the Onan exhaust manifold and cylinder heads. Being that this is a stock arrangement, and that it apparantly works well (I have read no complaints) - I wonder if I really need to use a exhaust coupler if use decide to use an automotive muffler?

I appreciate your time and any comments you may have.
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Old 01-21-2010, 05:18:14 PM
bobbyz72 bobbyz72 is offline
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Default Re: Onan stock muffler 155-1258 or cheap Car muffler(s)

When i got mine, i used a car muffler, about $35 and rigged it up nicly to my generator, no flex, all welded pipe, and i had all kinds of vibration, no matter how i strapped it. Get the onan muffler, its longer, but worth it. With the car muffler, the sound had a very low tone to it. It actully made my ears hurt. With the onan muffler... silence! I used flex this time, and run it under the wagon its mounted on. Im also now working on another onan, and i cut the exhause pipes, and welded pipe to the exhause and put in a flex section. Comming off the T that you see, will be a 90, flex, then the onan muffler.
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Old 01-21-2010, 08:03:48 PM
David C David C is offline
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Default Re: Onan stock muffler 155-1258 or cheap Car muffler(s)

Click on my name and I have a couple of pictures of 5.0CCKs using some Ford Focus mufflers from a salvage yard. They are some sort of alloy, so they sould last a long time. They are also nice and quiet.

David C.
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