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Steel manifold - what are the issues - Induction/Turbo/Chargecooler/Manifold/Exhaust - The Lotus Forums Jump to content
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Dave Freeman

Steel manifold - what are the issues

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As you know I now have a broken stainless exhaust manifold, the last manifold was the standard cast type. I put a thread up to see if anyone was interested in another group buy for the Alunox header, so far no takers and yes it is expensive, but as Dave (changes) says this Alunox header is good and does provide increased BHP and torque (for his particular engine). This could be the route I will take, however I have been contacted by a company suggesting that mild steel could be used, it's cheaper and has life time guarantee.

Have I missed something or does this look like a viable alternative. Why would you not use mild steel instead of the brittle stainless.

Can anyone shed some light on this.

Best regards all


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What's the exhaust manifold temp when running the engine hard?

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Guessing but it must be around the 1000 degrees, what's your thoughts Michael.

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All metals obviously have a range of temperature that they 'work' in. Some materials can handle extremes of heat better than others, but are more brittle.

Mild steels or (low carbon steels) can handle the temperature of the manifold, i.e. you are not going to reach the plastic deformation point of the material (without the addition of an outside force - think turbo hanging off the manifold as ours do), but you may get an effect called carburising of the material as it heat cycles and is exposed to the exhaust gasses. Generally, carburising involves quenching the metal after heating, but you may still get a lesser carburising effect from air cooling, water spray etc.

The more the material is carburised, the harder it becomes and more brittle. As carbon content increases, and we are not talking about a great increase, the temperature of the ductility point of the metal decreases.

Also as carbon content increases, weldability decreases. So repair later on down the track becomes more difficult.

Tubular expansion should be manageable as stainless, for the same size, actually expands more with temp increase than mild steel.


See the problems?


Whatever it is made of, I believe it needs to start out as a quite low carbon steel. As it is used, it will surface harden (internally) until one day the heat cycle will crack it. How long that takes, is why companies test materials and products for a long time.


Talk to them and do research. The internet is full of actual information.

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I think the fact being mild steel is not a problem. It does have more hot hardness than the stainless steel but I still think the company you are talking abouts design has not got enough bracing for the turbo. The turbo is very heavy and when glowing red hot even the   3mm wall will if you ask me allow the turbo to droop. I know they offer lifetime guarantee but would that cover the £1000.00 labour cost to remove and refit?



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The discussion on even considering a mild steel tubular manifold

on a turbo seems silly too me... If they had any longevity we would all be 

using them end of story... The fact is they were tried years ago and

failed  very quickly..The designers tried options s/s being one. The lesser

quality s/s were better but still eventually failed, 321 s/s seems the optimum 

for cost and reliability,   The cost is why they are not popular along with

some early design feature's which created some issues with stress fracture's.

The Alunox system confronts all these points and addresses them. Their 

design incorporates the expansion joints which minimise possibility of stress

fractures, the Turbo has an external support to take the weight and also allow

the natural flex needed with extreme heat.  It is also made from 321 s/s by a

company that does this as a specialist.

 We are in a situation where the cast ones are failing and cast replacements do

​not meet the old standards... We also expect more from our Esprits so look for

upgrades where applicable. Any exhaust manifold has to take into account that

​it provides a function, and must do it effectively.. It seems the Alunox system ticks

all the box's and has been tested to extremes by me. The results are documented

with video proof... No other manufacturer offers this data.. As for reliability, time is

the key. I doubt any road car will give the system more abuse than mine which will

act as a bench mark. As a designer myself I have confidence in this product and believe

it is the best it can be without going silly money...

Should anyone wish to fit and test others to the same standards we will all be interested

in the results... but always remember the 3 P's   penny pinching = problems. 

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