>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).
_________________________________________
Greg, I tend to disagree with the weight thing, reasoning being:
1)The exaust is properly supported and bracketed and there is little
if any load on the header (maybe 5 lbs). I disconnected the header and the
pipe stayed in position.
2)The header is a six to one and the crystalization took place on
the inside of the final bend. The headers were wrapped all six in one wrap.
Two of the inside pipes had split the way frozen water pipes split, (like a
banana with too much in it) one even split on both sides like someone shoved
a fruit knife through it and ripped two inches. other crystalized so that a
tap with a screwdriver would poke a hole. Meanwhile the outside of the pipes
(those facing the wrap) were solid and showed only heat discoloration.
I seriously question the coating as the heat will still be there if it's
wrapped. Wouldn't the heat burn or melt the coating?
Alas, I put in a new header without wrap and will heat shield the cab. The
new header is not of the quality of the one I took out, this one is a couple
pounds lighter and not chrome plated.
I seriously appreciate the feedback all have shared. It is an interesting
issue and I'd like to have more definitive information before I start on my
AH BJ8. I've been told they are even hotter than Nigel.
Thanks and aloha, Paul
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