My manifold is glowing red hot? Why?
Manifolds do glow red hot under high load in many engines - especially when fitted with Turbo Chargers. See the article on Why is my turbo glowing red hot for the answer that covers both the turbo and the manifold.
My manifold temperature is not even? Why?
First, some spots may be cooler than others, this can be cause by a few things:
You think its hotter, because of the colour, but you haven't measured it
Relying on colour isn't a reliable way to tell how hot the manifold is at a certain point. With this light glow, it will appear different colours under different light levels. Therefore always check with an infrared thermometer that has been calibrated. Using cheap un-calibrated tools gives unreliable results.
Here you see the engine from the first angle.
Now another angle, you can see the white and yellow circled portions looks blacker (i.e. cooler) than in the image above. This is due to the lighting and demonstrates why they should be measured.
Uneven thickness of manifold
No two manifolds are identical, some may have thicker spots than others, leading to lower external temperatures. You can see from the colour - again better id you measure! The example below you can see the deliberately thicker part of the manifold - it appears darker, because it is cooler.
Different gas flow
At different points on the manifold, you will have different gas flow rates. As you can see from the image below, on the left the first engine cylinder connects, in the centre the second and then on the tight the third. This shows that as there is less hot gas (and hence less energy in the gas) traveling through the pipe, it is cooler.
It is worth noting also, that the manifold is likely thicker on the bend than in the straight section, which may also contribute to this effect.
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