Russell Kent wrote:

Jeff Spitzer wrote:
I'd like to see the results.  I'm assuming the pipes are coated inside and
out.  Really the only problem with mild steel is the extreme corrosion at high
temps.  If the coating remains intact then it will probably work.  A few cracks
in the coating and you're probably on your way to pipe cracking. I hope no-one
is trying to put enough coating on to keep the pipes "cool". That would take
some serious thickness.

I'm not certain I agree with this assertion.  Ceramics have the useful property
of having a low thermal conductivity, even in thin layers (witness the relatively
thin tiles on the Space Shuttle).  If one were to coat the inside of the exhaust
manifold only, then the ceramic layer would slow the conduction of heat to the
underlying metal.  Since mild steel has a substantially higher thermal
conductivity than ceramics, it should wick the heat away and try to dump it in
the air around the outside.  Assuming you have a good flow of air around the
outside, this should allow the exhaust manifold to maintain a lower operating
temperature by causing the exaust gases to retain more of their heat.

Coating *both* the inside and outside of the exhaust header will definitely raise
the operating temperature of the exhaust manifold, since the outer layer will
slow the manifold's ability to dump heat into the cowl air (which might be useful
for cowl temperature problems).  The inner layer would still slow the conduction
of heat into the manifold though.

Coating only the exterior of the manifold will make the manifold's operating
temperature be at least as high as coating both inside and out; it would probably
be higher.

Can any thermal engineers / product enginners out there add some facts to this?

Russell Kent

There is some experimental evidense in the home built community such 
that 
wrapping the exhaust pipes with asbestos and silcon rubber slows the 
oxidation process on the outside of the pipes. (Corrosion). That's 
1400 F. I don't know if that will work at 1700 F.

I always assumed that most of the heat transfer took place by radiation.

Paul
 
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