Piston Tops
 

Would polishing the piston tops help reduce heat transfer to the rest
of the piston body?

I have seen in other groups that this is recommended, but not sure that
is has been discussed here. I did a google group search and found only
a couple hits for this group, but many hits for other groups.

TerryB wrote:
> Would polishing the piston tops help reduce heat transfer to the rest
> of the piston body?
>
> I have seen in other groups that this is recommended, but not sure that
> is has been discussed here. I did a google group search and found only
> a couple hits for this group, but many hits for other groups.
>


Carbon build-up is better, if you cant do a proper coating that is..

J.

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

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.

http://www.geocities.com/janbugger/pistonspolished2.jpg


:)

Jan (Coke addict, what can I say)