On Wed, 26 Jul 1995 Silikal@aol.com wrote:
> Paul Swengler writes:
> > Because it's hot in Hawaii and TR's tend to bring
> >lot'sa heat in the cab I wrapped the exhaust. It cut down the heat
> >substantially!
> >
> >This last week I noticed an exhaust leak sound. Went to look and found that
> >two (that I could see) pipes had split open and others were crystalized.
>
>This brings up a tech tidbit I saw recently. The header wraps (Thermo-Tec
>tape and similar) are almost _too_ good. You should only 100% wrap your
>header if it is high-temp ceramic coated. In all other cases, you
>should leave about a 1/2" to 1" gap between wraps of the tape to release
>some of the heat.
I don't believe that this is true. Solid metal is crystalline to begin
with. It does not crystallize before it breaks. Header wrap, if it is
to be effective, should be applied liberally. Note that from a thermal
perspective, header wrap and ceramic coatings accomplish the same thing;
they insulate the header, raising its temperature.
I've only seen exhaust headers fail for two reasons: 1) they are
forced to hold up the front pipe and/or intermediate muffler, 2) they
rust out. Here's why:
Due to their thin wall construction, exhaust headers are not able to
hold up much more than their own weight, especially at high
temperatures. To expect the header to support the front pipe is usually too
much to ask, and it will begin to crack or bend over time. Since iron
(steel) is a BCC metal and its strength decreases as its temperature
increases, wrapping the header with tape or coating it with a ceramic (an
insulator) can certainly cause it to fail sooner if it's also holding up
other exhaust components.
A wrapped or coated header is either a good thing or a very
bad thing. The difference is determined by the weight loads which are
then applied to the header.
Now, wrapping or coating a header is good for performance, and it sure
helps out with radiant (engine compartment) and convective (passenger
compartment) heat problems. However, a wrapped or coated header experiences
higher temperatures, so it's especially important in these instances to
ensure that the header is well isolated from engine vibration and also that
it is not asked to hold up the front pipe or intermediate muffler(s).
I hope this helps more than it confuses,
Greg
Greg Meboe meboe@wsunix.wsu.edu
Dept. of Mechanical and Materials Engineering
Washington State University, Pullman, Wa.
'85 XJ-12 H.E. (daily) '67 Spit-6 '74 TR-6
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