Why does one manifold glow red hot?

A manifold glows red hot due to the high temperatures produced during the combustion process in an engine. When exhaust gases pass through the manifold, they can reach temperatures exceeding 1,000 degrees Fahrenheit, causing the metal to emit visible light as it becomes incandescent. This red glow indicates that the manifold is operating at or near its thermal limits, often due to issues like a lean fuel mixture or restricted exhaust flow, which can lead to overheating.