Why Do Some Exhausts Turn Blue? (Explained)

If you own a motorcycle, you may have observed that the exhaust may become blue over time; this also occurs in some cars.

Heat from the exhaust gas can cause the exhaust pipe to become blue. The exhaust pipes are designed to withstand heat, but as temperatures increase, the metal of the exhaust starts to interact with the surrounding air and becomes blue, purple, or yellow.

The colour of the exhaust may increase as the exhaust system gets hotter, especially if the exhaust pops and bangs or backfires.

Why Does an Exhaust Turn Blue?

Exhaust pipes becoming blue is a common occurrence. They go a blue colour due to the intense heat that travels through them.

• Driving hard. When you increase the throttle, the engine burns more air and fuel, which means more heat is emitted which must all pass through the exhaust system, causing the exhaust to grow hotter.

• Lean air/fuel ratio. Another possible reason of blue exhaust pipes is an engine with a lean air/fuel ratio. A lean air/fuel mixture with too little fuel will burn much hotter than a rich air/fuel ratio.

When an engine runs lean, the combustion chamber heats up dramatically, causing the exhaust travelling through the exhaust pipes to be far hotter than typical. Certain exhaust pipes may be more prone to discoloration depending on the kind of metal used.

The heat induces oxidation, which alters the metal's colour. When heated to a high enough temperature, the metal may interact with the gas molecules around it. The final colour indicates how much the metal interacts with these components.

The base of the header pipes (manifold) will often have the most colour since that is where the heat is focused. As you go down the pipe towards the end of the exhaust system, the colour will most likely disappear.

Sometimes the pipe can go blue, purple, or even yellow.