I was watching a car show and they were talking about how they took apart the engine in some classic car and found that all the exposed surfaces on the inside of the motor had been polished. Push rods, crankshaft, oil galleries, the inside of the block, all of it was polished perfectly smooth. Highly unusual, to say the least.
However, what stuck in my mind was that I seem to remember something about polished metal surfaces don’t transfer heat as well as dull surfaces… but I could be hallucinating that.
It seems that a rough surface would have more surface area and be better at transferring heat, but there could be other considerations I’m not aware of as well as wondering if the difference would be enough to really matter.
So would polishing the inside of the engine affect the cooling in any significant way or have any other measurable effect on engine performance?
With an engine spinning at 1000+ rpms, smooth surfaces on the moving parts might make more power due to better aerodynamics. However a rough surface may have more surface area allowing for better heat transfer.
Back in the day porting and polishing the intake and exhaust system was used to produce more power. On the other hand, golf balls purposely have dimple to help the fly further.
Strictly speaking, a dull surface will transfer more heat than a smooth surface. The reason is because a dull surface will create a more turbulent boundary layer (the layer of fluid that is immediately closest to the surface), and more turbulence translates to a higher heat transfer coefficient.
In reality, the theoretical improvement of heat transfer using a dull surface usually does not offset the practical disadvantages. Dull surfaces create more friction/wear/heat. In chemical processing applications, they’re more prone to fouling, formation of scale and encrustations, and are harder to clean. Furthermore, there are methods of “harnessing” turbulence without the use of dull plates: Google “plate heat exchangers” and you can see that the use of “ribbed” smooth plates. These create more turbulence than a dull surface could, while maintaining ease of sanitation. I believe some large chemical companies have also tried using finned inserts in their shell and tube heat exchangers, but with minimal success.
There’s also an effect on radiative heat transfer. The efficiency with which a surface will emit radiation is the same as the efficiency with which it absorbs it. A polished shiny surface will absorb very little radiation, because it’s reflecting it instead, and so it will also emit very little radiation. Convective transfer is probably more relevant than radiative in an engine compartment, but it would still be a factor.
We would have to distinguish between polishing the surfaces which “touch” (or rather, glide near each other with lube inbetween),
and polishing surfaces which are not so close in contact…
Where the surfaces touch, I think they polish them right up now, whereas they used to hone it with herring bone pattern, so that the surfaces run in … That is, they make it all a little tight and then it wears out the herring bone where required, and that means its fitting at the designed tolerance real quick… These days they must make it polished and a just right fit… Hence the thinner oil… real fine polish, and a close fit…
I suggest it only makes sense that they were looking at a blue printed motor , and saw that there was no herring bone, it was a motor that was prepared to be a just right fit, no run in required…
The non-contact surfaces, why would it matter ? who’d care. Oil splashes there, oil falls or runs off, so what ? Although it is possible someone polished up non-contact surfaces just for the hell of it.
It sounds like the polishing was done to get rid of stress risers on moving parts, where cracks could begin and the part could ultimately be pulled apart. Remember that the only moving part in the combustion chamber is the piston. The piston heads are polished to minimize heat transfer as the fuel and air burn (The best thing to do with all that heat is send it out the exhaust after it has done its work.)
The piston skirts are also polished sometimes (usually by the piston manufacturer, I think), but this is to keep the skirts from breaking. The pistons are sized to run at almost an interference fit to the cylinder walls, and heat transfer isn’t a problem.
The combustion cylinder areas of the heads are also sometimes polished, usually to limit heat transfer out of the combustion charge and sometimes because the builder thinks that they can improve airflow in and out of the cylinder. The intake and exhaust ports can also be ported and polished, again to improve airflow.