4) Powder coat or thick smooth paint is bad as it smooths out the rough surface and reduces convection cooling. Multiple non-authoritative sources mostly related to Harley's
This is correct.
Out of curiosity, I made a simplifed calculation of the loss by having a green sand iron casting surface (e.g. , a cooling rib) coated. Assuming an average roughness of 300 um (which is about as fine as it gets) and a sinusoidal roughness distribution in an x-y planar frame with a vawe length of 3mm (which is a coarse approximation), the area difference to a completely level surface is 19.3 %. Reducing the average roughness to 200 um reduces the area difference to 8.7 % .
How much do peaks and dales contribute to convection? The air passing by is assumed to be turbulent, unfortunately the turbulent flow field close to a rough surfaces isn't fully understood by the scientific community.
Roughly speaking, the horizontal turbulent air flow induces further turbulence in the dales. It's well known that turbulent flow enhances heat transfer due to the high circumferential speed of swirls. This is captured by the heat transfer coefficient (h), which is part of Newton's law of cooling. Isaac Newton took (h) to be a constant but it's not. Even for smooth surfaces, the coefficient (h) shows a dependancy of the air speed. For rough surfaces, the dependency (h) of v (the horizontal air speed) is very strong. An example: A fin with moderate roughness (Ra=22 um maximum roughness) was exposed to vinds ranging from 3.4 and 15 m/s (ok, modest vind speeds for a motorcycle). The computed heat transfer coefficient rose from 72 to 230 W/m2/K.
In Newton's law of cooling, the area (A) is the projected area.
Downside of painting cast cooling ribs isn't the diminished area, it's the reduced secondary turbulence. Hence, it's advisable to apply a thin paint layer.
In summary, to obtain the highest possible convective heat transfer of an air cooled engine, you should
* maximize the finned area A
* use separate barrels which are better for cooling than joint castings
* keep or enhance roughness of cooling ribs
* keep the barrels unpainted (which is possible when using certain aluminum alloys)
* duct the engine, ensuring even rearward ribs receive fresh air
* use enforced cooling by fan (as VW, Porsche did).
* use a casting material with a high thermal conductivity coefficient k which evens out the difference between well cooled and less well cooled areas
* operate the engine at low ambient temperatures (moist seaside air cools better than hot desert air).
- Knut