Thermal coatings

[Craig Dotson]

> Second, exhaust headers. Disregarding turbocharged engines, I see little
> gain here. Less heat transfer would result in higher gas velocity but
> also higher BP, meaning you simply have installed a smaller diameter
> pipe?? Some webvertisements say less friction at the pipe walls hence
> better laminar flow. This must be a joke right? The exhaust flow is a
> fully developed turbulent flow isn't it?

The ceramic coatings I've seen on exhaust headers go on the OUTSIDE of the
header pipe.  The inner diameter is not reduced.  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.  Speed of sound within the exhaust gas is also higher,
slightly raising the "tuned" RPM if it is a tuned manifold.  Coated headers
don't heat underhood air as much, allowing more of the heat to be passed
through the exhaust system further downstream.

> Third, the intake. Now here I can see some gains. If one coated the
> inside of an intake there would be less heat transfer to the intake
> charge, wich is obviously a good thing. However, I haven't seen websites
> advertising thermal coatings on an intake, though I haven't specifically
> searched for them. Isn't this used?

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.  This is regularly done on "mountain
motors," where the block is well in excess of typical "big-block" size and
such adapters are needed to use production bigblock intake manifolds.

Craig Dotson
crdotson at vt.edu
2002 VT FormulaSAE

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