Extending generator exhaust pipes

There's been lots of discussion about how to safely wire a genny for emergency domestic use but has anyone successfully extended the exhaust so that it can run safely in the garage?
I'm considering a piece of rigid pipe into the existing exhaust (clamped and sealed somehow), connected to a length of flexible exhaust pipe and then to a length of rigid out of a window or custom-made hole. Presumably all pipework needs to be large bore to minimise back pressure. Sealing is important for safety. Any success stories?
Dave S
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wrote:

I'll be interested in this too. I have a feeling it'll be a no-can-do because if the genny is being run in a confined space (as in a garage situation) then it might cause a fire situation - and trying to find a betting company that will take odds on the insurance companies saying "bog off" to any claims made will be like finding rocking horse pooh for the rose garden.
I'm actually thinking of building a small lean-to on the side of the garage for the genny. That way it would be protected from the direct elements like rain and snow, and also reduce the noise nuisance for the neighbours. And if it catches fire then it'll most likely vapourise the lean-to without causing much other damage.
PoP
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wrote:

My fixed generators have exhausts extending through outbuilding walls. They were already in place when the house was bought. I could certainly take some photos of the pipe work if that would be of any use.

Again, my three gennies are in stone built outbuildings and have been for over ten years as far as I know. The rooms are vented, though in summer they are vented manually---we open the doors!

:-)
Colin
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wrote:

Try asking on uk.rec.engines.stationary
Much depends on the size of the thing, and how much use it's going to get.
But I wouldn't extend downstream of an existing exhaust. The gas has expanded and cooled by this point, and moisture is condensing out. Although you can build it, you'll get problems in service, especially rust.
I'd favour cutting out (or buying as a spare) a new flange to fit the manifold / cylinder head, then welding on pipe from there. Use flexible pipe as necessary for vibration. Then when you're near the end and probably outdoors, fit a silencer box. Arrange suitable drainage for condensate anyway. Parts like U bends and silencer boxes are available from performance car places (Peco used to be convenient), but go to somewhere serving the kit-car market, not the tweaked Nova and baseball cap idiots (retail markup varies enormously). You can also just try browsing the parts catalogue at Edmunds Walker and using light truck parts.
Lag the pipe. This is partly to reduce noise, partly to keep the heat in (improves gas flow, reduces condensation problems). The dimpled stainless steel used for aircraft pipe lagging gives a very neat finish (scrapyard).
If it's a high performance engine, you may need to design a proper system, and that will probably need an expansion box close to the engine. There are any number of books that describe this ("Four-stroke performance tuning in theory and practice" is the one on my shelves).
-- Die Gotterspammerung - Junkmail of the Gods
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Dave wrote:

Not with that specifically, but I have seen loads of staionary engines run via ducting to the outside world.
I am sure if you keep the pipe large, and stick a silencers somewhere in it, it will be fine.

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I run my 7.5 KVa Honda engined (GX390) generator in the shed with a 8 foot length of flexible exhaust pipe attached running out through a hole in the wall. No re-jetting and all has been running fine for about 5 years, although still need to discharge the exhaust into some sort of enclosure as its a bit noisy, especially when the power goes off as everything goes quieter (no tv / radios etc so you are aware of the contant noise of e.g. a generator.
Nick
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