Exhaust Manifold Materials

[ewidder]

Does anyone have a summary of exhaust manifold materials used on various engines & lines - high volume, foreign makes, etc.? The historic trend has been from cast materials towards welded fabrications. The materials would progress from cast iron & steel(?), to doped castings (Silicon stabilized, Ceramets, etc.), to both welded fabrications & advanced casting techniques.

I came across an interesting thread among these pages regarding running engines on water (dissociated H2 & O2) and the fundamental technical flaws to this concept. One of these involved the excessive cylinder temperatures that would challenge conventional block & head materials. These are temperatures typically found in gas turbine hot-ends, requiring the use of exotic temperature-resistant alloys.

With conventional combustion engines, exhaust gas temperatures downstream of the exhaust valve are in the following ranges (pre-EGR values?):

Spark Ignited: 400 - 1600 F (200 - 850 C)
Compression Ignited: 212 - 1400 F (100 - 750 C)

There was another post that asked why exhaust manifolds weren't made of aluminum. With a melting temperature of ~1200 F (~650 C), aluminum would have strength issues with high-end gas temperatures. Many ferrous castings & alloys are currently used for both strength and thermal distortion resistance. Yet another post (or reply) mentioned titanium exhaust manifolds. Not sure if this is optimal.

Many alloys, both cast & fabricated, are used in present engine designs. I was hoping to see some patterns in material usage. For example, in what years and markets did cast iron (incl. nodular, ductile) yield to advanced casting materials & welded fabrications. (NA-EU-AS / low-med-high perf.).

Any comments, suggestions, data references would be appreciated. This Engineering/Technical forum should be the right place for this discussion. It's amazing how many common vehicle repair posts apprear in this section. I suppose everything is relative - "It's technical to me". But those repair questions are probably more effectively handled from within the specific model forums. (Make-Model-System-Prob/Ques).

Thanks - Ed

[Moppie]

Very relevant to this forum, and also an interesting topic.

I'm a huge fan of old English cars, and noticed the use of cast iron manifolds continued right up until the last gasp of British leyland in the early 80s.
Then when the Japanese moved in (mostly Honda) Rover started using welded steel manifolds.
From what Iv seen of Japanese cars the trend moved from cast to fabricated manifolds also around the early to mid 80s.
Although it Nissan went back to cast manifolds on some of the lower spec cars when they started to run into money troubles in the early 90s.

[KiwiBacon]

While aluminium alloys may have a melting point that may seem high enough, there are two main problems.
1. Being an alloy they start to lose strength at what is known as the "eutectic" temperature, that is the temp where individual parts of the alloys begin to soften.
2. Creep. Materials which slowly deform when subject to stress. The general guideline is half the absolute melting temperature. You need a material with an absolute melting temp of approx 2000K (1700 deg C) to be sure.

My 98 Nissan has cast exhaust manifolds, as do the 95 accord in the family. It's hard to get a good look at more modern cars. Too many heat shields and plastic covers.

[UncleBob]

I would agree the trend is going more towards welded manifolds, but there's still a large percentage of cast manifolds.

As for aluminum....I thought I remember a Cobra?? That used aluminum for the rear half of the exhaust.

[MagicRat]

I find this subject interesting for the reasons each material is chosen, for a particular application.

As I recall, cast iron manifolds have been traditionally used because, compared to welded tube manifolds, they are generally cheaper to manufacture, quieter, and more resistant to heat problems (cast iron has a much higher melting point than steel).

Welded tube manifolds are obviously more expensive to manufacture simply because more procedures are required to mandrel - bend the tubes, assemble and weld, and coat to achieve acceptable rust resistance and long life. However, they are lighter, often are designed for better flow, both of which have the potential to reduce fuel consumption and increase performance.

Also, I suspect welded manifolds lend themselves better for low volume production.

[ewidder]

Thank You all for the replies & observations of trends. I'll be doing some more research into this topic & will post some additional findings. The key parameters are temperature resistance (melting point, ET creep, & ET corrosion resistance), something that characterizes dimensional stability (thermal conductivity & coef. of thermal expansion), and cost factors. There's an SAE standard that covers exhaust manifold materials. My older handbook versions didn't include that one - issued 1999. I'll be back with some more details. - Ed

[curtis73]

There is a drag race class that races snowmobiles. They make their expansion exhausts from concentric rings of Inconel TIG welded together. Not sure of its hardness and fatique ratings, but its the next best steel thing. Check it out.

[KiwiBacon]

Originally Posted by MagicRat

Welded tube manifolds are obviously more expensive to manufacture simply because more procedures are required to mandrel - bend the tubes, assemble and weld, and coat to achieve acceptable rust resistance and long life. However, they are lighter, often are designed for better flow, both of which have the potential to reduce fuel consumption and increase performance.

Like automatic transmissions, there are good cast manifolds and bad cast manifolds.
I see no reason why a good cast manifold can't flow better than a welded manifold, as there are infinite form possibilities with a casting.

[2.2 Straight six]

Originally Posted by curtis73

There is a drag race class that races snowmobiles. They make their expansion exhausts from concentric rings of Inconel TIG welded together. Not sure of its hardness and fatique ratings, but its the next best steel thing. Check it out.

I've seen the icelandics race them, running on alcohol some were dipping into the 7.5xxs in the quarter. i think a couple of the ones i saw were running GSX-R 1000 engines with turbos and methanol fuel. whatever was powering them, they sure were fast.

[SaabJohan]

Originally Posted by ewidder

Does anyone have a summary of exhaust manifold materials used on various engines & lines - high volume, foreign makes, etc.? The historic trend has been from cast materials towards welded fabrications. The materials would progress from cast iron & steel(?), to doped castings (Silicon stabilized, Ceramets, etc.), to both welded fabrications & advanced casting techniques.

I came across an interesting thread among these pages regarding running engines on water (dissociated H2 & O2) and the fundamental technical flaws to this concept. One of these involved the excessive cylinder temperatures that would challenge conventional block & head materials. These are temperatures typically found in gas turbine hot-ends, requiring the use of exotic temperature-resistant alloys.

With conventional combustion engines, exhaust gas temperatures downstream of the exhaust valve are in the following ranges (pre-EGR values?):

Spark Ignited: 400 - 1600 F (200 - 850 C)
Compression Ignited: 212 - 1400 F (100 - 750 C)

There was another post that asked why exhaust manifolds weren't made of aluminum. With a melting temperature of ~1200 F (~650 C), aluminum would have strength issues with high-end gas temperatures. Many ferrous castings & alloys are currently used for both strength and thermal distortion resistance. Yet another post (or reply) mentioned titanium exhaust manifolds. Not sure if this is optimal.

Many alloys, both cast & fabricated, are used in present engine designs. I was hoping to see some patterns in material usage. For example, in what years and markets did cast iron (incl. nodular, ductile) yield to advanced casting materials & welded fabrications. (NA-EU-AS / low-med-high perf.).

Any comments, suggestions, data references would be appreciated. This Engineering/Technical forum should be the right place for this discussion. It's amazing how many common vehicle repair posts apprear in this section. I suppose everything is relative - "It's technical to me". But those repair questions are probably more effectively handled from within the specific model forums. (Make-Model-System-Prob/Ques).

Thanks - Ed

These days exhaust temperatures seem to get higher and higher, particulary for spark ignited engines as these are run leaner and leaner on high loads in order to save fuel. For example, the latest turbocharged engines have turbine inlet temperatures up to 1050 degC. Downsized engines also means that the engines spend more and more time at high loads where the temperatures are higher.

In the past exhaust manifolds was commonly made of grey cast iron, these are ok to about 700 degC. But as exhaust temperatures increased better materials was needed. That meant cast iron with a high nickel content (such as Ni-Resist) or Si-Mo alloyed nodular iron. These alloys are ok to about 900 degC. At temperatures higher than that there are high temperature resistant cast steels that can be used, and of course stainless steel. Stainless is availible both as sheet, tube and cast material. Stainless is typically cast using precision casting, this is a rather expensive method but the finished part can be made very light compared to conventional sand cast iron, and this also requires less of the expensive stainless steel.

Recently exhaust manifolds of hydroformed stainless steel has gotten popular. Hydroforming means that complex shapes can be manufactured rather easily from steel tubes and since the exhaust manifold is thin walled its weight is low. A low weight has gotten more and more important as this decrease catalyst light up time.

Note that the temperatures mentioned are related to conventional engine usage and life expectancy. If an engine spend more time at high loads the demands go up, and if the engine spend its life at a constant load demands go down. The latter is due to thermal fatigue which is only an issue when the temperature of the exhaust manifold go up and down in cycles. Thermal fatigue is what makes you exhaust manifold crack.

Even a good aluminium alloy start to soften at about 200 degC, so aluminium alloys are completely unsuitable for exhaust systems. The exception is water cooled systems as sometimes found in boats.

Titanium have been used in a few cases, but titanium oxidize in the presence of oxygen at temperatures higher than about 600 degC, which generally makes titanium unsuitable. Titanium usually works well for two strokes though.

In some cases high temperature superalloys are used. They are mainly used in racing exhaust manifolds such as F1, but also in some production vehicles. Their use in production vehicles are however limited to certain components as these alloys are expensive. Superalloys are sold under brand names such as Inconel, Nimonic and many others.

[MagicRat]

Originally Posted by KiwiBacon

Like automatic transmissions, there are good cast manifolds and bad cast manifolds.
I see no reason why a good cast manifold can't flow better than a welded manifold, as there are infinite form possibilities with a casting.

I agree, but how many have actually been made?
Back in the early '60's Ford and Chrysler offered cast iron manifolds in very small numbers on high performance V8's. They flowed as well as tube headers of the day because they had very long runners. However, they were enormously heavy and therefore inferior for performance.

[KiwiBacon]

Originally Posted by MagicRat

I agree, but how many have actually been made?

Besides almost every turbo manifold; the 4-2-1 manifold on a 95 honda accord springs immediately to mind.
I have yet to look under the heat-sheild on my 98 nissan to see what's there, but the visible part around the o2 sensor is definitely cast.