wrote:
> TerryB wrote:
> > Would polishing the piston tops help reduce heat transfer to the rest
> > of the piston body?
> >
> -------------------------------------------------------
>
> Yes. Also for the chamber. After about 20 hours of run-time the
> polish will be gone but the main benefit will remain. Here's why:
>
> The key point is not to just make the surfaces shiny but to make them
> perfectly uniform. The piston-top and the as-cast surface of the
> chamber are irregular. The tool marks on the piston-top serve to
> INCREASE its surface area; the larger the surface, the more heat it
> picks up. Polishing, which is always done before balancing (or cc'ing)
> should remove ALL of the tool marks and nomenclature from the top of
> the piston and leave a mirror-bright finish.
>
> A mirror-bright surface absorbs less radiated heat. Once the entire
> fuel-air charge has been ignited most of the heat absorption is via
> conduction. The conduction phase is significantly longer than the
> reflection phase which is why having a uniform surface is the primary
> goal of polishing.
>
> Thermal barrier coatings do a better job of heat rejection. Rather
> than polish the surface, it is ABRADED, typically with #120 media at
> low pressure. This creates an 'infinite' surface; under 30x
> magnification you will see that the surface is now a field of jagged
> edges. The ceramic-metallic coating fills the pits and bonds to those
> edges at the molecular level when the piston is baked. The resulting
> surface is then polished. It won't come out mirror smooth but it will
> be uniform and the resulting cermet alloy is a very poor conductor of
> heat.
>
> The less heat absorbed by the chamber & piston, the higher the
> effective pressure in the cylinder. The net result is to see more
> torque at the crank for the same amount of fuel. Not a lot more,
> because VW's don't have very large pistons, but the improvement is
> built-in and doesn't wear out, assuming the coating has been properly
> applied.
>
> -Bob Hoover

Thank you Sir, this is the answer I was hoping for.