Back to original topic, I was quite fascinated to read through the exchange about exhaust wrapping and coatings. My background includes a master’s chemistry and physics, with a chemist father who was fascinated by metallurgy. I’ve also been fussing with vehicles for 40 years, having a lot of fun some fine machinery, with a quite modest budget approach. If anyone is still following this thread, I might have some useful experiential data, along with some further theoretical questions.
When we moved to higher altitude, at first Laramie, Wyoming at 7,200 feet, nearly 20 years ago, my wife’s beloved RX-7 became pretty lethargic, with its complex pollution plumbing excessively stifling the rotary’s ability to breathe that thinner air. Since the rotary inherently runs clean except at idle or very low throttle openings, and we avoid those special conditions, we installed a set of headers, resulting in about a 20% power increase and 10% better mileage, for a considerable practical and environmental improvement. The replacement problems became icing stalling at temperatures around freezing, since the carburetor pre-heater was part of the original manifold, along with excessive under-hood temperatures in midsummer. I began killing two birds with one stone by noticing the gap between the header pipes was similar to the pre-heater connection to the air cleaner. I found a section of old vacuum cleaner pipe of the same diameter, laid it between the header pipes, wrapped all three with drag racer header fiber tape from Summit Racing, and crudely connected the tube (picture 1)
to the air cleaner’s thermostatically controlled intake via metallic flexible hose. A good many years and thousands of miles later, it still works reasonably well, with the headers warming input air coming through the extension tube when needed, while the tape wrap keeps under-hood temperatures perhaps surprisingly lower. Inside the tape, the header pipes are currently in better condition than the rest of the exhaust system, exposed as it is to road salt, etc. Not wrapping each single pipe tightly may well have reduced stress on them.
When I got my ’74 Alfa spider 7 years ago, I was more than annoyed by the hot-foot syndrome arising from the muffler directly underneath the throttle pedal, and, again, high under-hood temperatures. When it came time to replace the exhaust system not too much later, figuring that black body radiation could be notably reduced by the closest coating to white one could get (and having tried white on my Guilia and found it did stay that way for long) I painted it (using a brush, after hours of careful hand sanding) with high-temp aluminum paint from the local auto paint specialist. The positive difference for the feet was dramatic. With that experience, it seemed worth trying the manifold, too (picture 2).
Again, the difference was quite satisfying. With at least 2 years and 6,000 miles, it still looks and acts well, no cracks or other problems, other than I did learn that sanding between coats is requisite to reduce a tendency of the outer coat to flake from inadvertent contact.
Part of the issue for others becomes how the machine is going to be used. Hard racing, long distances, or even regular heavy traffic and its temperature raising delays are going to be quite different in their impact on header stresses than what older vehicles like ours, each of which gets about 3,000 miles a year of use, or others which get modest use have to endure, so results and need for thoroughness of treatment will vary accordingly.
Headers have got to deal with temperature differentials in a lot of dimensions in the best of cases. My solutions cost less than $50, total, to treat two cars. The Mazda work isn’t very pretty, but even that could be improved, like the Alfa’s might be with a more sophisticated form of paint application, but even as is the later looks much better, as well as working better, than the rusty state the headers arrived in just a few years after a quite careful overhaul by the PO.
The question I’ve been scratching my head about is about an internal coating and the practical physics of what happens with a reflective layer that is going to be rapidly covered by fluffy black carbon, at least in older vehicles that cannot be kept in perfect tune or from consuming some oil. The logical part of my mind expects that the now-buried reflective layer might still be helpful, but I’d love to see some experimental results as the carbon layer builds in depth, and whether buildup is reduced, as it could be argued to with higher internal temperatures.
The next step for exhaust scavenging becomes the observation from fluid dynamics that smooth pipes (or streambanks) create much greater reductions in flow rates than do rougher or sinuous ones, from vortex buildup, counter-intuitively if one is coming from traditional streamlining. The details of these arguments are beyond me, but there have been tantalizing hints about possibilities from aircraft and such like engineers.