Tuesday, November 21, 2006

VW - Engine Paint

Engine Paint

In the several years since it was posted my article on painting VW engines has probably produced more mail than any of the other two hundred or so other articles.

That tells me I didn’t do a very good job.

The basic reason for painting your engine to begin with is to protect it from rust and corrosion. But since all paints serve as insulators to some degree, you want to pick a paint that, ideally, will help your engine run as cool as possible.

Within the range of temperatures we’re concerned with, which is basically the maximum range of our oil temperatures . . . say, 400 degrees Fahrenheit as the max . . . a thin coat of flat-black paint will enhance the heat-flow... from surfaces which are in contact with the oil. That means, the valve covers, push-rod tubes, cylinders, generator tower, crankcase (*) and sump-plate.

The physics of this heat-flow enhancement can get a little hairy but they generally fall-in with the reasoning Jazz laid out in his message. Key factors are that the black surface must be thin . . . having to do with the wavelength of the heat-energy being transmitted . . . and must be intimately bonded to the heated surface, the metal to which the paint is applied, and that the paint not contain clay, metallic particles or other substances that act as insulators. In plain language, do not use the so-called ‘high-temperature’ paints, firstly because we aren’t dealing with high temperatures, and finally because such paints act as insulators.

The (*) has to do with aluminum vs magnesium alloy. Paint doesn’t like to stick to aluminum unless the surface has been chemically etched. Since this isn’t practical with the Type IV crankcase, I don’t recommend that it be painted, which is why I specifically mentioned ‘magnesium-alloy’ when talking about painting crankcases. Magnesium is much more chemically reactive than aluminum . . . it is, in effect, ‘self-etching’ (unless passivated) . . . and gains far more benefit from the corrosion-protective qualities of a layer of paint than does aluminum. So paint your early-style crankcase but don’t worry about it if you have a Type IV. (I feel bound to mention that there are such things as self-etching paint intended specifically for aluminum. Most of these are formulated for the aviation industry, are difficult to find except from aviation-oriented suppliers and are expensive. I think such things are beyond the scope of articles directed toward the general population of Volkswagen owners.)

With regard to the aluminum heads, which I also do not recommend be painted, the problem has more to do with the temperatures encountered near the exhaust stacks, which is so high it will destroy all common forms of flat-black paint. There are ways to blacken aluminum and thereby enhance it’s thermal radiation properties . . . you can see examples of this on many motorcycles . . . but the process is beyond the means of the typical Volkswagen owner.

Then we get to the ‘All Black Engine’ confusion.

I trod upon many a toe when I said that folks who chromed their engines hadn’t a clue. That particular thread got its start with regard to the benefits . . .meaning trophies to be won at car-shows . . . of polishing the crankcase.

A polished crankcase, along with chrome valve covers, push-rod tubes, generator tower and sump-plate cause a VW engine to run so hot you wouldn’t believe it . . . the thing literally melts down. Of course, if you live in Lapland, this may be exactly what you want, which is why Volkswagen offered chrome valve covers and push-rod tubes and sump-plates and split bearings . . .all as part of their ‘high-latitude’ package, intended to keep their air-cooled twirler warm and working in a sub-zero climate.

See the problem here? If Volkswagen themselves offered such things . . . and there were part-numbers that would yield-up marvelously well-chromed parts . . .then obviously the things had to be good for the engine, right? Speaking from my perspective in sunny southern Cal, I said ‘No,’ loud and clear. Yet there were those pesky VW part-numbers... Conventional Wisdom wins again.

The truth is, with regard to any part of your engine not in contact with hot oil, you may paint it . . . or chrome it . . . any color you wish. In the case of your shrouding, tin-ware and blower housing, the finish . . . paint, chrome or what-have-you . . . is there only to protect the metal. These metal parts are not a factor in the transfer of heat via radiation. The metal is there to contain the envelope of cooling air. I realize the metal will get hot through both conduction and radiation absorption but the quantity of that heat is minuscule when compared to that being radiated by those parts of the engine in contact with the oil. Indeed, this perception of heat is subjective. When the engine is running and the car is moving, the shrouding and tin-ware is usually only slightly warmer than the ambient air temperature. It is only when the vehicle is brought to a halt and the engine shut off that any significant quantity of heat can be absorbed by the tin-ware. The subjective part is the fact that you can not put your hand on the tin-ware when roaring down the highway at sixty mph... but you can when the vehicle has stopped... by which time the tin-ware feels hot to the touch. And Conventional Wisdom wins again.

Want to polish your crankcase? Chrome your valve covers? Go right ahead. But don’t plan on driving the vehicle.

Finally, those pesky heat-exchangers.... The shrouding of your heat-exchangers . . . the metal canister surrounding the cast aluminum heat-exchanger inside . . . contacts the exhaust system at only one or two points. While the shrouding does get hot through absorption of the heat being radiated by the cast-aluminum heat exchanger, the relatively loose fit of the canister to the exhaust pipe ensures there will always be some amount of air-flow through the heat-exchanger, meaning it seldom gets hot enough to cause the breakdown of regular (as opposed to high-temperature) paint. That means you can paint your heat-exchangers any color you wish. The paint will burn-off in a small area immediately adjacent to the exhaust pipe but the remainder of the metal will be protected . . . and you very definitely need to protect your heat exchangers with a coat of paint, otherwise they will rust out in only a couple of years. The heat exchangers on my ‘67 bug came with the car . . . original equipment. They keep trying to rust, and I keep painting them. I’m sure the rust will eventually win but I think me and the heat exchangers are putting up a hell of a good fight :-)

The heat-exchangers on the Type IV are a different case, one in which I haven’t enough experience to recommend how they should be finished. In my original post on painting your engine I stressed the primary purpose was to protect the metal, to ensure your engine would last as long as possible. The enhanced heat-flow derived from using the proper paint is a freebie but one that should not be scorned through the use of paints or finishes that might reduce the ability of the engine to cool itself.

No comments: