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.
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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.
:)
Jan (Coke addict, what can I say)
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