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Wrap around the exhaust pipe?


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Seen discussion on this thread before, but i have a question.   I will have to replace the exhaust port, out of the rear if the manifold,  header???  The bit where the raw water joins the exhaust pipe.   We replaced it with stainless steel one about 8 years ago, so not too unhappy with its age.   I do note from pics that some wrap this with something, obviously not flammable.  And possibly wire around that.  What and Why?   

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1 hour ago, SanFran said:

Seen discussion on this thread before, but i have a question.   I will have to replace the exhaust port, out of the rear if the manifold,  header???  The bit where the raw water joins the exhaust pipe.   We replaced it with stainless steel one about 8 years ago, so not too unhappy with its age.   I do note from pics that some wrap this with something, obviously not flammable.  And possibly wire around that.  What and Why?   

It will be some sort of high temperature insulating blanket eg https://www.bradfordinsulation.co.nz/commercial-and-industrial-insulation/fibertex-board-blanket/fibertex-820 - exhaust temp prior to water cooling is around 600deg C. The wire is to hold it in place. 

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8 minutes ago, SanFran said:

Thanks for the reply.  So it's nothing to do with making it last longer, or make it operate better then...

yup - makes it operate better.  Exhaust wrap is used to retain heat in the exhaust so the engine is more efficient (especially in turbocharged engines, but also elsewhere).

Cooler engine room temps are just a lucky by-product.

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1 hour ago, aardvarkash10 said:

yup - makes it operate better.  Exhaust wrap is used to retain heat in the exhaust so the engine is more efficient (especially in turbocharged engines, but also elsewhere).

Cooler engine room temps are just a lucky by-product.

actually it does not improve operation. In a normally aspirated engine all the action has already happened before the exhaust, in the case of a turbo charged motor whilst heat from the exhaust is good to drive the the turbo, it is bad for the turbo bearings and the oil system(the bearings live a hellish life) and the objective is to keep the charge air as cool as possible, there's a real compromise here to deal with - that's why you will see most turbo marine diesels (certainly the larger ones) with water cooled turbos and air after/intercoolers.

Keeping the exhaust warm does help reduce the possibility of any acid formation in the exhaust piping when the engine is cooling down especially for dry exhaust installations, other than that it is there to stop heat radiation and for protection

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It actually shortens the life of the exhaust pipe. Especially if it is SST,  It's a compromise situation, Wrapping the only seriously hot part of the engine makes a huge reduction to heat generated in the engine compartment. This can improve many things. The engine runs cooler and many components around the engine compartment will be cooler.  The air being sucked in is cooler and thus you get more power and the fuel delivery components are cooler and the cooler fuel is more dense and thus you get more power.
The down side is that lagging the exhaust means more heat in the pipe and lack of oxygen on the surface of the SST which causes it to corrode a little faster.

There are two types of lagging available. Fibre glass or Mineral. Fibre glass requires the use of gloves because it can make your hands itch like crazy. I now use a material that is made by spinning Volcanic Rock into fibers. It is easier to work with in many ways and can stand much hotter temp than the glass one can.
Don't bother with wire. You can tie off the end of the wrapping with a thing like a hose clip that are often sold with the wrap. In some areas where outside wear is possible, I have used exhaust gasket paste to coat the material and that goes hard and holds it all together. After it has dried, I then paint the surface of that with high temp paint to stop it from crumbling.
You will find both wrap materials on Trademe.

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2 hours ago, wheels said:

The down side is that lagging the exhaust means more heat in the pipe and lack of oxygen on the surface of the SST which causes it to corrode a little faster.
 

Agree about retaining heat, but the lagging does not form an airtight seal around the pipe. Corrosion occurs on the interior of an exhaust with the principal issue for SS pipes being acid formation attacking the welds.

Regular grade 300 series SS should not be used in dry exhaust applications, at least not near the exhaust manifold, the exhaust gas temp circa 600deg is a critical temp for these grades as they undergo a phase change around 550 deg C with rapid loss of strength. Carbon steels are better or if the sky is the limit a suitable Inconel grade.

By the way anyone with an old lagged installation should be very careful handling it, asbestos was the go-to material up until the 70's for lagging

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The exhuast pulse work hardens the SST and can cause the stuff to crack and not just on welds.
OK, maybe I don't know why exactly then, but I can assure you that SST will corrode under the wrap, but maybe not due to lack of Oxygen perhaps. I just assumed. I had lagged the exhaust pipe on my boat down to the point where the water enters the exhaust. Perhaps Salt air or moisture in the engine room caused it.

All the SST now wrap is on my plant and it is at about 400DegC. However, I have never unwrapped one to see if corrosion is an issue.
Under test, I was taking the main heating part of the auger/tube to 800DegC. I did not have cracking issues, which did surprise me. I certainly expected it. Kind of freaky to look into a dark box and see all the SST glowing Red.
Normal operating temp is around 500degC. So far no cracking and I do regularly inspect it all to be sure. 6M of 316 Tube/auger sitting at 500deg.

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on my yanmar 16hp the exhaust was wrapped in a cloth/plaster mis and underneath was hiding a problem,heating up/cooling peocess over the yrs allowed the steel to sweat and rust,gave way at the most inconvenient time.Amzing how much water can be pumped in a short period.Replaced and left exposed.I believe new owner still has no issues of rust.Did make a heat shield to deflect heat from engine box back towards engine block.

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