Stainless or Not

[Spirit (of Glenans)]

I have read on another forum that replacing the OEM cast iron exhaust elbow, (on a VP engine), with a cast stainless replacement may not be advisable, I am guessing for dissimilar metals issues. I shall probably be replacing mine sooner rather than later and am wondering does anyone on here have any advice on this?

 

[Norman_E]

It can be a problem on turbocharged engines where it may result in corrosion in the turbo. That is what the yanmar agents said to me when they rebuilt my turbo and replaced the stainless elbow with a cast iron one.

 

[vyv_cox]

I had not come across the issue raised by Norman E previously but if a turbo is fitted it seems sensible not to go to stainless steel.

Otherwise, stainless steel is widely used in this application, by Yanmar and others. The failure mode in this case is frequently thermal fatigue, not corrosion.

More recently, aluminium is used by some manufacturers, presumably with no issues.

 

[rotrax]

I would be interested to know why. As a sixty year pro mechanic with lots of hands on experience plus working alongside senior engineers and designers in the vehicle industry I fail to see how the material the elbow is made from will affect the turbo.

It might if it fails and allows water to back up into the adjacent turbine blades and bearings, but the OE Yanmar can do that as well.

I have just sourced a S/S exhaust elbow for our Yanmar 4JH4-HTE Turbo engine. It is Swedish made and has good reviews. It cost under £400. The OE Yanmar would be - IIRC - £1700 in bronze.

 

[V1701]

On a normally aspirated engine the stainless one I had got pinholes in it with only 600 or so hours on the engine, maybe it was a poorly made one. I replaced that with an alu one that seems to be holding up well. Off the shelf from Lancing Marine...

 

[Baddox]

I too struggle to see any risk to the turbo, but there may be a risk of corrosion where the two dissimilar metals join on the wet elbow.

 

[rotrax]

The large top quality bronze Yanmar one which costs £1700.00 had a single pinhole at 1,000 hours. I drilled, tapped and threaded the hole and screwed a 6mm screw through, riveting the end to stop it falling out.

The S/S one is cast and very heavy. If that lasts 600 hours at my age I will be finished with sailing the current boat.

I was involved in Motorsport. Many very high outport Motorsport engines use S/S exhaust headers. From direct experience, a 350 BHP petrol engine doing its lot has the exhaust glowing for about 2.5 metres from the manifold when on a dyno.

Injecting sea water into a hot hard working exhaust might compromise a S/S steel exhaust elbow.

But the one on our Bukh DV 20 in NZ has been there for eight seasons and is still OK. That is custom made from automotive S/S exhaust tubing by a local welder. I had it in bits and cleaned it with caustic soda and inspected it VERY closely. No turbo. low stress engine, perhaps that is the diference.

 

[QBhoy]

It can be a problem on turbocharged engines where it may result in corrosion in the turbo. That is what the yanmar agents said to me when they rebuilt my turbo and replaced the stainless elbow with a cast iron one.

Surely not in the turbo ? Perhaps you meant on the turbo exhaust flange rather ?

 

[Birdseye]

I would be interested to know why. As a sixty year pro mechanic with lots of hands on experience plus working alongside senior engineers and designers in the vehicle industry I fail to see how the material the elbow is made from will affect the turbo.

It might if it fails and allows water to back up into the adjacent turbine blades and bearings, but the OE Yanmar can do that as well.

I have just sourced a S/S exhaust elbow for our Yanmar 4JH4-HTE Turbo engine. It is Swedish made and has good reviews. It cost under £400. The OE Yanmar would be - IIRC - £1700 in bronze.

After a working life in the stainless steel industry, I wouldnt be that keen on using stainless in such a situation. Its not that some stainless steels wont be suitable- we used to make them for everything from nukes to jet engines. Its more the case that a cheep off the shelf fabrication is unlikely to be made of the ideal grade of stainless and more likely to be made of something like 304/A2. Hot salt water mixed with exhaust fumes is quite corrosive.

Cant see why it would affect a turbo but that might be a design issue.

 

[rotrax]

Just about the most agressive situation on an ICE engine is the exhaust valves. During WW1 Bentley developed steels very reliable in use for these vital components used in Aero engines.
It was noted that swarf and duff ones did not rust when on the scrap pile. The steels were early types of stainless....................................

 

[rotrax]

After a working life in the stainless steel industry, I wouldnt be that keen on using stainless in such a situation. Its not that some stainless steels wont be suitable- we used to make them for everything from nukes to jet engines. Its more the case that a cheep off the shelf fabrication is unlikely to be made of the ideal grade of stainless and more likely to be made of something like 304/A2. Hot salt water mixed with exhaust fumes is quite corrosive.

Cant see why it would affect a turbo but that might be a design issue.

I take your concerns onboard.

Before blueing my dosh I researched and looked up customer satisfaction. Nothing negative.

Also, having owned many Saab cars I am happy with Swedish design and engineering.

For what is is, it certainly was not cheap!

 

[vyv_cox]

Just about the most agressive situation on an ICE engine is the exhaust valves. During WW1 Bentley developed steels very reliable in use for these vital components used in Aero engines.
It was noted that swarf and duff ones did not rust when on the scrap pile. The steels were early types of stainless....................................

For interest, this is the introduction to the stainless steels chapter in my book, Metals in Boats: "2013 marked the centenary of the accidental discovery in 1913 that the corrosion resistance of steel is increased dramatically by the addition of chromium."

Harry Brearley of Sheffield stumbled upon this discovery while trying to solve the problem of erosion (not corrosion) of the internal surfaces of gun barrels for the British army during the onset of the First World War. He added 12.8% of chromium to molten iron with 0.24% carbon. The Discovery of Stainless Steel – British Stainless Steel Association

 

[rotrax]

I suspect WO Bentley realised gun barrel erosion and failing ICE exhaust valves had something in common.

Perhaps I should have written 'developed valves' not 'developed steels'. The hollow tulip shape and the new chromium enhanced material did make high output ICE engines far more reliable.

Which was important at 5,000 feet in a Sopwith Camel

 

[vyv_cox]

I suspect WO Bentley realised gun barrel erosion and failing ICE exhaust valves had something in common.

Perhaps I should have written 'developed valves' not 'developed steels'. The hollow tulip shape and the new chromium enhanced material did make high output ICE engines far more reliable.

Which was important at 5,000 feet in a Sopwith Camel

Yes, I feel certain the pilots felt more comfortable knowing their exhaust valves were less likely to plunge into the cylinder head!

I don't know what material exhaust valves are generally made from. I do know that marine inlet valves suffer from condensate corrosion, sulfuric acid from high sulfur fuels, so they are probably not stainless.

 

[rotrax]

When I was racing I tried to find exhaust valves in Nimonic 80 IIRC - it was a long time ago, early 70's.
Austinetic steels were common for exhaust valves on performance engines, often top material for the head with a stem of lower quality slush welded to it. Valve guide wear was better with a two piece valve.

I had our Yanmar head off two years ago to deal with damage inflicted by poor servicing before we had the boat. It is a sixteen valve engine but a simple modification of the eight valve engine with a new head and a turbo. One exhaust valve was bent - someone tried to slacken a nut that is factory set and twisted the opening mechanism for the exhaust valves on No. 4 cylinder. This caused a slight bend in the stem.

When it was in bits I looked closely at the inlet valves in that cylinder and found them perfect. The engine had covered 1180 hours at that time. One Yanmar exhaust valve was 60 quid, the head gasket 360!

I can get four racing quality valves for an 80BHP Speedway engine for 75 quid!

Thanks Vyv, I always find your technical stuff on the money.