Paint your 'wagen

Another popular aircooled question, whith a lot of opinions, a lot of anectdotal information and bugger all science around.

It is well know in physics that a black item will radiate heat better than a silver one, but I think the first question is whether that is what we are looking for.  The majority of heat energy is conducted away by air flowing across the fins, very little is radiated. Any paint or colouring is unlikely to provide a meaningful increase in heat dissipation in any case.

Bob Hoover pointed out the advantage of preventing rust, which he points out quite correctly is an effective insulator.  I personally have not seen cylinders rusted to this degree, including my Kombi which spent most of it's life by the coast (and didn't have much metal left in the chassis, to be honest, but I was a poor student at the time.) Of course it looks good, so if you are of the "Cal-Look" crowd nothing else might matter and you need read no further.

Some guys here in Aus came up with a test rig which they claimed proved paint will make the cylinders run hotter, but there are a number of problems with their test method:

1. The temperature sensors were mounted on the outside of the cylinders. The outside of the cylinders and the fins are where we want to transmit the heat from - a higher surface temperature could indicate more efficient cooling! (Refer Newton's Laws of Cooling.)
2. The method involved filling the cylinder with water, then cooling it with a fan, which suggests questionable accuracy and repeatability.
3. The sensors, I believe, were thermistors which are not the most accurate device (using accurate in the scientific sense.)
4. The tests do not appear to have been performed a number of times.

 The main problem here is what we are measuring - we are trying to measure the efficiency of removing the heat energy out the outside surface of the cylinder (preferably with an airflow of similar characteristics to those produced in a running engine,) while this energy is being constantly supplied to the inside surface.  While using boiling water to supply heat to the inside seems appropriate, we want to know how efficiently that heat is being removed from the same region, not the temperature of the outside surface.

I would like to make some tests of my own at some point in the future, probably along these lines:

1. Drill a hole into the cylinder wall and insert a thermocouple, probably at more than one point and preferably out of the area exposed to the airflow, or inside.
2. Heat the cylinder to a set temperature, probably in a similar way, as the boiling point of water is pretty constant, so the cylinder can be filled for a set period, drained and capped, and the temperature should be pretty constant.
3. Simulate the air pressure, flow and speed of the VW fan.
4. Monitor the rate of temperature drop of the actual cylinder wall under these conditions.

This still may not be repeatable to give reliable results.  I recall when I was young there was similar debate about efficiency in heatsinks (used to cool electronic components.) 

When a heatsink used in a non forced air situation was treated with anodising to make it black, there was little doubt there was an improvement and other tests showed advantages in painting (usually a very light coat from a common spray can of flat black.)

As soon as fan forced air came into the equation (and sometimes convection flow) the advantages would disappear in fact the efficiency of a painted heatsink was usually lower!  The main reasons cited for this were usually insulating properties of the paint, or the reduction in turbulence from the smoother painted surface.  Those cases where efficiency appeared higher with a painted surface could not be discounted as being from the surface preparation for the paint rather than the paint itself.

As an aside, producing a rough finish, a la Bob Hoover's blasting, does tend to improve the efficiency of either radiating or conducting surfaces, but you are unlikely to be able to measure the difference.



So, my opinion?

As anodising is not practical on cast iron, you are not likely to gain any improvement in cooling by painting your cylinders.

A thin enough coat should not cause any significant loss in cooling however, but such a coating may not be enough to prevent rust either.  I'd probably try a combination of phosphoric acid treatment covered by a spray of highly-thinned flat black paint, or a straight (maybe tinted) zinc primer if I was going to try.

PS - I have to arguments with Bob Hoover's comments about chrome. It will significantly reduce radiant heat dissipation on valve covers and pushrod tubes and personally, I'd like to keep all the cooling I can get.

PPS - if anyone can afford gold plating, I'd love to know the results - or you can do it to my engine and I'll let you know.