At 08:45 AM 4/22/98 -0700, Simon Matthews wrote:
>I remember a business trip when I worked for Lucas to Fiat in Turin. We
had installed an electronic ignition timing system on a car which was run
on a rolling road. After running the car for a few minutes, someone pointed
out a bright orange tube in the engine compartment to me, It took me a few
moments to realize that this was the downpipe (= exhaust header pipe in
American?).
>.....
>Bright orange = how many degrees?
Simon,
I hope you don't object to my posting this response to the mgs list. It
seems to bear some interest for many list members, considering recent
correspondence.
About 1700dF. Delete now if that's all you wanted to know.
First some information that you didn't ask for, just for background. From
the heat treating section of Machinery's Handbook:
Deg-F Color of plain carbon steel
430 Very pale yellow
440 Light yellow
450 Pale straw-yellow
460 Straw-yellow
470 Deep straw-yellow
480 Dark yellow
490 Yellow-brown
500 Brown-yellow
510 Spotted brown yellow
520 Brown-purple
530 Light purple
540 Light purple
550 Full purple
560 Full blue
570 Dark blue
640 Light blue
These color changes come from changes in the grain structure of the
material as it changes temperature. In this temperature range, a small
change in temperature causes a significant change in color, so the color
alone can be used to judge the temperature. The differences in grain
structure are used to advantage during heat treating, tempering, and
quenching to achieve the desired strength, toughness, and/or hardness, all
of which are compromises. These color changes come from reflected light,
effectively changes in the surface color of the metal. At these
temperatures the metal does not glow in the dark. You may notice however
that any change in color indicates a temperature in excess of the limit for
a powder coat paint (about 400dF max).
There are paints formulated to withstand temperatures up to 1200dF, and
generally suitable for painting exhaust systems, particularly iron
manifolds. When you paint the surface you can no longer see the color
changes described above, you just see the color of the paint. When the
1200dF limit of the paint is exceeded, the paint will start to discolor,
char, and burn off by the time things starts to glow visibly.
The thickness of a cast iron exhaust manifold acts as a thermal barrier
limiting the heat flow rate, so you can think of it as an insulator.
Between the limited heat flow rate, and the heat dissipation rate from the
surface to the surrounding air, there is a practical limit to the external
surface temperature of the manifold. As a general rule, the external
temperature of an iron manifold will not exceed 1200dF if the engine is
tuned properly, although certain racing applications can generate more
heat, and an improperly tuned engine can generate a lot of excess heat in
the exhaust.
The thin steel header pipe immediately after the iron manifold provides
very little resistance to outward heat flow, so this pipe can heat up very
quickly to higher temperatures. In normal driving conditions, enough of
the exhaust heat is dissipated in the iron manifold so that the exhaust
temperature in the header pipe does not exceed 1200dF, so the paint should
also not burn off there. If, however, the engine is run at full throttle
for extended periods of time, more heat can be generated inside of the
header pipe such that the temperature will rise to the point where the pipe
will start top glow visibly. By that time the paint would be long gone,
and for these higher temperatures you would need a ceramic coating.
Now getting more directly to your original question, from Mark's Standard
Handbook for Mechanical Engineers:
Deg-F Color of iron or steel
990 Dark blood red, black red
1050 Dark red, blood red, low red
1175 Dark cherry red
1250 Medium cherry red
1375 Cherry, full red
1550 Light cherry, light red
1650 Orange, free scaling heat
1725 Light orange
1825 Yellow
1975 Light yellow
2200 White
So it appears that your description of "bright orange" may be somewhere
around 1700dF. Colors from light red to orange indicate forging
temperatures where the iron or steel is already partially plastic and can
be easily deformed by impact. So, metals that glow red or brighter from
heat will likely suffer from thermal distortion, and can suffer shrinkage
and stress cracks while cooling. This is why header pipes will suffer a
short life if wrapped heavily with insulating tape. They will overheat and
then crack when cooling.
If one is inclined to wrap tubular header pipes to reduce the engine
compartment temperature level, it would be wise not to over do the
insulation. It may be appropriate to strive for a heat flow rate similar
the characteristics of the original iron manifold. For this you do not
need a lot of insulation, perhaps two layers of thin insulation tape, or
one 50% overlap spiral wrap. Even at that insulation level, the tubular
header pipes will see operating temperatures closer to the internal surface
temperature of the iron manifold, and will still have fairly high thermal
stress.
And thanks for the question. Now I get to archive this one for later be
addition to my web site.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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