Thermal coatings

[Arnaud Westenberg]

Craig Dotson wrote:

> The ceramic coatings I've seen on exhaust headers go on the OUTSIDE of 
> the header pipe.  The inner diameter is not reduced.

He he, I'm sorry, I stated that rather poorly :-) I didn't meant to say 
that the coating makes the pipe smaller but it changes it's behaviour 
into something similar to a smaller pipe (higher velocity, more volume 
through the same diameter wich equals (kinda) the original volume 
through a smaller diameter).

> If there is less heat transfer away from the exhuast gases, the gas is 
> hotter and therefore lower density.  Lower density does yield a lower 
> Reynold's number with all other things being equal.

But other things are not equal! Lower density but higher velocity. I'm 
not sure if these cancel each other completely, resulting in the same 
Reynolds number, I would have to do some calculations for that.

> Speed of sound within the exhaust gas is also higher,
> slightly raising the "tuned" RPM if it is a tuned manifold.

Agreed, but that could have been reached through different manifold 
design as well.

> Coated headers don't heat underhood air as much, allowing more of the 
> heat to be passed through the exhaust system further downstream.

In cars I can see the benefits of this, however these advertisements 
speak about motorcycles as well !?

> It probably isn't done primarily because the intake is heated by 
> conduction through the block and heads.  The best way to thermally 
> insulate the intake manifold is to use some kind of thermoplastic or 
> (egads!) wood adapter between the block, heads, and intake.

Hmmm... maybe a combination of both would be interesting.

I'm still not convinced about the benefits of exhaust coatings (besides 
under hood temp)

Thanks for your thoughts.

___

Arnaud

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