JLW wrote:
>
> Roger et al.
>
> Unless I hear some compelling reasons for not doing it, I am planning to
> get my exhaust system ceramic coated. Such as 'Jet Hot' coated.
>
> Roger Mills wrote:
>
> > Not unless you've got a stainless steel exhaust - insulating it has been
> > shown to increase the rate of corrosion of mild steel systems and Europa
> > have made a strong recommendation that all lagging should be removed.
> > Regards
I've pasted below a treatise on this topic. Veracity: can't say, but
he cites his sources. Suggests one needs to coat the parts internally
also, but no coating on the 914 for the health of the turbo.
Regards,
Fred F., A063
-----------------
The following is information our company (The Fighter Works) has
researched (most of the following is taken from our newsletter). You
should find it intriguing:
There are a number of ways to effectively beat the heat when it comes
to exhaust components. Early methods included high temperature paint,
chrome plating, and porcelain coatings. Later, header wraps became
available (which, as described by others, can have a disastrous effect
upon the life expectancy of the exhaust system component -- in the
world of motorsport, it's not uncommon to find headers crumbling and
held together with the wrap due to heat stress). Each header heat
"system" offered a different level of effectiveness (and each had it's
own drawbacks), but those early concepts were nothing compared to
what's available today. Contemporary header coatings have the
capability of extending the service life of components by resisting
corrosion and ultimately reducing thermal fatigue. According to HPC
(High Performance Coatings), thermal fatigue/oxidation, which is often
mistaken for rust on an exhaust system component, is a bigger problem
than good old fashioned rust. Thermal fatigue/oxidation is rooted in
cyclical temperature changes and the intrusion (into the material) of
hot gas. AirBorn Coatings offers more insight:
"The corrosion problem with low carbon steel headers is that the big
EGT's are exceeding the base metal's normal operational range and
degrading it. This degradation is called oxidation and results in a
type of corrosion that is classified as "exfoliation" (when the metal
"falls apart in sheets"). Think about when you take an old exhaust
system out of a car, lay it on the ground and all that stuff comes
falling out. What do you think that does to the laminar flow of the
exhaust gases? It simply compromises the flow of the exhaust."
"Thermal fatigue is another problem causing base metal deterioration
and it's caused by cyclic temperature changes. This problem is more
prevalent in cast manifolds exhibiting cracks. With tubular headers,
thermal fatigue exists, but oxidation occurs more rapidly which
effectively masks the phenomenon."
Today's high quality coatings have the capability to retard oxidation
and excessive heat buildup, even when exhaust gas temperatures
intermittently exceed 1800F. Header coatings are actually bonded
onto steel, which in turn, extends the actual life expectancy of the
component by as much as four times.
There are some major advantages to coatings other than ultimate
component life: For example, a set of coated headers can reduce under
hood temperatures by 30F or more in a race car (depending upon the
application, and of course, the airflow surrounding the engine). This
is done by effectively insulating the header, which in turn, keeps the
heat inside the header tube. For some applications, this translates
into a cooler intake air temperature charge. Needless to say, if
under hood temperatures in a race car can be reduced, so can
firewall-forward temmperatures in an aircraft.
Obviously, exhaust gas flow is also a consideration. Given the
internal difference between a oxided, scaly exhaust tube and a smooth,
coated version, it's easy to see that there are other performance
improvements available. Laminar exhaust flow is improved with an
exhaust tube that is coated inside and out. Factor in this superior
(faster) exhaust flow and better cylinder scavenging and you can see
the opportunity exists to produce more horsepower.
There is a catch to this internal coating process. Not all companies
have the capability of coating a specific component internally. Some
companies coat the external surfaces of the exhaust first (usually by
way of a spray process), then coat the internal surfaces via "flow
coating". There are probably more ways than one to flow coat. Some
companies keep this information proprietary, but AirBorn Coatings
simply uses a special applicator that goes inside the respective tubes
to set up the coating. Not coating exhaust tubes internally will have
a definitive effect upon the performance of the coating and the
exhaust system. HPC feels the primary benefits of coating internally
are heat reduction (the heat originates inside the tube, and that's
where you want to keep it) and of course, improved laminar flow. HPC
states that an exhaust system which is coated on the external surfaces
will only show minor temperature reduction, but of course, will still
look visually superior.
Can power be increased by using these race car header coatings on your
aircraft engine? According to the manufacturers surveyed, you'll
typically see a power gain in the range of 1-3% with a
metallic-ceramic coated header. A relatively mild engine might only
see minimal gains, but a high horsepower combination (large
displacement radial, water cooled V12) could see as much as a 12-15
horsepower gain with a coated exhaust system. Another important point
to consider is the capability of working on the engine. A coated
header will definitely dissipate heat in a much more efficient
manner. This translates into quicker, easier and less painful
maintenance.
There's more too: A ceramic-metallic coating will not "blue" like
chrome. And it's resistant to staining as well. These coatings are
also "weldable". This means if you have to modify a coated section of
exhaust it is possible. It should also be pointed out that
ceramic-metallic coatings are very resistant to abrasion. So
resistant, that in most cases, you can hit them with a hammer. The
tube will bend, but the coating won't chip or peel. A little know
fact about these coatings is that they don't last indefinitely. The
folks at HPC point out that a ceramic-metallic coating is
self-sacrificing (in order to protect the base material). This means
that eventually (perhaps ten or twelve years down the road), the
coating may sacrifice itself to the point where it will eventually be
totally consumed. At that stage, the parts will have to be re-coated.
Keep in mind that headers and related exhaust system components aren't
the only items that can be coated. Some racers have found that disc
brake calipers are more effective when coated (temperature reductions
of 200 F or more aren't out of the question, but remember, the
ceramic coating keeps the heat inside, and reduces the external
temperature). Springs can be coated too (don't confuse
ceramic-metallic coatings with the old porcelain coatings --
ceramic-metallic coatings are flexible enough to work on springs,
porcelain isn't). Unlike plating processes, ceramic-metallic coatings
cannot foster hydrogen embrittlement.
When a coating is applied to the outside face of a given component,
the surface looks like it's polished, but at the same time, it is not
affected by petroleum products such as oil, gasoline and coolant. The
coatings won't oxidize or tarnish like polished aluminum does either.
Another factor is cost. Even though the coatings aren't inexpensive (a
typical set of full length V8 race car headers can cost $225.00 -
$275.00 to coat inside an out, depending upon the header configuration
and the coating company), they are cost effective. How? Simple. A
ceramic-metallic coating is so competent, it can extend the life of
the header dramatically. In the world of motorsports, racers have
even been known to use the process to "save" a set of high dollar
headers that otherwise, would have been tossed into the nearest
dumpster. There is a hitch though: Part of this salvage process
depends upon the coating company's capability of eliminating all
corrosion on and in the header(s).
In order to save a given exhaust system component, the coating company
must insure that all corrosion and scale is removed. Typically,
components prepped for coating are media blasted (most often with a
fine aluminum oxide spray). A ccording to AirBorn Coatings, glass
beading parts is not recommended. Glass bead is physically round, and
while it is a good cleaner, it actually closes the pores of the metal
being cleaned. Aluminum oxide or fine sand etches the metal and
cleans (opens) the pores, which allows the coatings to bond to the
metal. Unlike other blasting mediums, aluminum oxide doesn't
sacrifice the base material. From an environmental perspective,
aluminum oxide may be superior in that it can be reclaimed to be used
again. The use of a blasting technique works well on the outer
surfaces, but in order to remove the scale and corrosion on the ID of
something like a header, there only two solutions: The header can be
chemically stripped or it can be mechanically cleaned. The method of
internal cleaning selected depends upon the header configuration.
Another issue is polishing. Professional coating companies often use
a "vibratory" polishing system. This method is based upon the parts
being placed in a special polishing machine. Inside are a series of
spheres which are tumbled around the respective component. The
multiple "balls" (and there are plenty of them) serve to polish the
part by way of a tumbling action. This polishing is gentle but
effective, with the final result appearing almost chrome-like.
Coatings aren't cheap. But they are effective. At least at this
point in time, our research has shown that coatings are one of the
most capable means of reducing firewall-forward temperatures and
improving the performance and life expectancy of exhaust system
components.
Two sources we believe are reliable include:
AirBorn Coatings
52 Village Street
East Hartford, CT 06108
PH# 860-528-8281
HPC (High Performance Coatings)
550 W 3615 S
Salt Lake City, UT 84115-4250
PH# 800-456-4721 (orders)
PH# 801-262-6807 (tech)
******************************
I hope this sheds some light on the coating/exhaust system wrap
issue. By the way, many well funded Indycar/CART teams use coatings
---From the above companies.
Wayne Scraba
The Fighter Works
Yak 3 Airframes, Yak Parts, IAR Jet Fighters
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