choice of material for turbo exhaust manifold

[kenre]

I Have a customer who purchased an aftermarket turbo exhaust manifold constructed from 321 stainless. It needs a few changes and repairs to bring it up to scratch. Owner isnt too keen on having bits of metal that are falling off the welds inside going through his new $2500 turbo.

Due to the mods needed and the fact that the car will be driven VERY hard I would like to know what would be better, the 321 or build a new one from Steam pipe bends.

From the specs it seems that the 321 would be very good in this application, but i would like some first hand feedback.

Thanks,
Ken

 

[NickE]

321 stainless is excellent in hot exaust gas, at least from my understanding. Retains strength and resists sensitization that can plague the non-stabilized 300series.

What material are you talking about when you say steam pipe bends? if you are talking about the steam handling pipe from a boiler then I definatley would go with the 321, the exaust gas in turbocharged applications can get to temps >1500F and I just dont think that the mild steel used in boilerpipe would have the strength at temp to provide good design. (IE weight/strenght/durability) I do know that 439 adn 409 SS are also used in exaust design. I have a 439ss downpipe (on turbine exaust) on my car, while there is some redrust present it isnt excessive. I dont know what the factory headers are made of, proabably 439 or similar inexpensive ferritic grade.

 

[Warpspeed]

This could be one of the few applications where going "low tech" may have some advantages.

Steam pipe made from thick wall mild steel tubing, with cast steel bends, will have less expansion than stainless. The thicker walls and higher ductility can also make it less prone to cracking.

Stainless is probably better for a long runner light weight racing manifold where large radius sweeping compound curves can allow enough stress relieving flex.

Steel may be better for road cars with a tight compact layout, (and low internal gas volumes). Weight is also a disadvantage, but for an everyday road car manifold weight probably does not matter quite so much.

There is probably no right or wrong answer, it is more of a judgment call really.

 

[kenre]

Thanks for the replys guys.

It is a road car, mainly a weekend blast through the hills type.

4 cyl Nissan i think, and is a very compact manifold. Customers main worry is if it could crack or not.

Cheers,
Ken

 

[Tmoose]

Material is important, but geometry can cause problems with low cycle (heat) and high cycle (vibration) fatigue. Welded on flanges would seem to be at an immediate disadvantage compared to a cast manifold with fully developed radiuses, if the manifold is stubby and restrained.

 

[kenre]

Tmoose, The 321 stainless manifold is a welded unit. Welds are nice looking on the outside, but look like chicken doo-doo inside. Manufacturer didnt use purge gas inside the tubing when welded. even some of the tubing doesnt line up square, which could be a potential failure point. If we decide to use the 321 it will need to be cut to pieces and welded properly.

Either way it will be like starting from scratch, thus the reason for my original question.

Another question, How does 304 ss handle duties as exhaust tubing? not for turbo application.

Ken

 

[NickE]

There are lots of aftermarket exausts manufactured from 304 tubing, I attribute this to the rarity and cost of 321, along with the availability of 304L (in which the carbon content is controlled to a max of 0.03%wt). 304L needs no PWHT and resists sensitization.

 

[sbmar]

Ken,

We have been building both wet, dry and combo exhaust systems for diesels for about 20 years.. All types, sizes and shapes from 5 HP engines to 600++ hp engines used 1000's of hours per year--Maybe 500 systems over the years and just a small handful of re-do's during that time...

Although we do not build the manifold itself (we build the riser/adaptation sections after the turbo/manifold), both 304L and 316L has shown to give very good long term service with internal temps between 300 and 1400F... Obviously design and the quality of construction plays a big role in how well these exhaust components hold up, but I'd give my vote to these common alloys if the rest is right..If you drop me a note, I could send you pics of 100's of exhaust components we have fabricated over the years to help in your decision on what to do..BTW, we do use 321 for our flexible "bellow" sections, and if we have to mix SS with mild steel, we use 309 to weld them together.........................Tony

Tony Athens

http://www.sbmar.com