The question of painting the heat shield for the carburetors came up, and
I have something I could share here.
> What do you think of 'Jet-Hot' coating the carb heat shield? <
I know nothing about the stuff except what I've heard here on this list,
and that is sounds like pretty good high temperature paint. However, it
sounds like it would actually tolerate the heat and stay on the heat
shield, which is more than I can say for the stuff I painted it with.
Otherwise, don't count on it making much difference for the heat problem.
A thin layer of paint (or anything for that matter) is a very poor
thermal insulator.
Once upon a time I engaged the services of a thermal engineer in an
effort to "get the heat out" of an electronic engine control computer.
This device was to be in a pocket calculator size aluminum box, so didn't
have much surface area to dissipate the heat. It was 1" thick plus 1/4"
high fins on both sides, totaling 1-1/2" overall, and still was
marginally overheating from within. The decision was "It wants to be
painted", as a painted surface radiates heat a little better than a bare
metal surface. So if painted, the heat generated inside the box would
dissipate a little easier, reducing the inside temperature.
This can help a bit (little bit) on an exhaust manifold, where the heat
comes from within, if it's painted on the outside. But I don't know that
you really want the exhaust manifold radiating more heat under the hood.
I'd like it to radiate less and put the heat out the tail pipe. Also, if
you paint the manifold on the inside as well, it just about cancels out
the radiant effect.
I think the Jet-Hot treatment would best be applied to tubular headers,
where the coating may help resist corrosion and burning through the thin
walls. For this it is important to coat the inside as well. And, it may
keep it looking nice rather than going rusty (steel) or dull (stainless)
or blue (chrome). Also, tubular headers are so thin that the thickness
of the coating may actually have a noticeable effect as thermal
insulation. You may know that headers dissipate a lot of heat under the
hood, much more than a cast iron manifold. It is common to wrap headers
with asbestos tape to retain heat in the exhaust rather than dumping heat
under the hood. (More on asbestos later).
Now back to that heat shield. The primary purpose of the heat shield is
to stop radiant energy from the exhaust manifold from getting directly to
the carburetors. The shield has very little effect as a barrier when it
comes to hot air, because the air is free to circulate under the hood and
can easily make its' way around to the other side of the shield.
The way the heat shield works is to stop the infrared radiant energy from
getting to the carbs by reflecting it, or by absorbing it and
re-radiating it or turning it into heat. Absorbtion raises the
temperature of the heat shield, so it must then dissipate the heat
somewhere. It primarily looses a lot of its' heat to the air around it
by conduction, and the air carries the heat away by convection and by
forced ventilation. The shield also re-radiates away some of the energy
in the form of infrared, and that radiation goes away in all directions,
so some goes back towards the engine, and some towards the carbs. The
net result is that the shield will stop some radiation by reflection, and
almost all of the remaining radiant energy by absorbtion, dumping most of
that into the air, and shedding much of what is left back in the
direction of the source.
Painting the shield actually reduces the reflectivity, which is bad.
But, you may recall that the MGA heat shield has an asbestos pad on the
engine side of it, so reflectivity there is minimal anyway, so go ahead
and paint it. Painting it also increases the infrared dissipation a bit,
not necessarily good because half of that is going in the direction of
the carbs. It's not the same as when the heat is coming from within and
the objective is to get the internal temperature down. Here you're
trying to keep the heat from getting past. On thin sheet metal the paint
probably increases thermal insulation as much as it increases radiation,
so it's about a wash in thermal effect. I'd paint it just to keep it
from going rusty, and Jet-Hot sounds like "pretty good paint".
Now the real trick of the heat shield is the asbestos pad. You may have
noticed that it's pretty thin. It does function somewhat as a thermal
insulator, but that effect is fairly small because as previously stated,
the air is free to circulate. What's really going on here is that the
asbestos is absorbing most of the radiant energy. Asbestos has a low
thermal coefficient (doesn't hold much heat), so its' temperature goes up
pretty fast, and gets hot primarily on the surface on the engine side.
Then at elevated temperature it does act as a thermal barrier, keeping
much of its' own heat away from the sheet metal shield, and dumping a lot
of heat into the air on the engine side of the shield. Even better,
asbestos radiates energy in the infrared like crazy. More than half of
that goes back towards the engine (hotter on the engine side), and the
rest towards the sheet metal. The sheet metal then reflects and/or
re-radiates a lot of that back into the asbestos where it can again be
re-radiated out, about half towards the engine. After a few cycles of
this efffect, the end result is that about 90% of the infrared radiation
from the exhaust manifold is stopped short and dissipated on the engine
side of the head shield.
(Some of you may recognize this as the same function as the ceramic tiles
on the space shuttle).
The point of all this? Paint the heat shield with anything you want, but
be sure the asbestos pad is in place when you're done.
Barney Gaylord
1958 MGA with an attitude
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