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Why Do Some Exhausts Turn Blue? (Explained)

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Last Updated on: 7th September 2023, 12:42 am

If you own a motorbike you may have noticed that the exhaust can turn blue over time, this also happens in some cars, but why does this happen?

Heat passing through your exhaust can cause the exhaust pipe to become blue. The exhaust pipes are built to tolerate heat, but when temperatures rise, the metal of the exhaust begins to interact with the surrounding gas and change to a blue colour, purple, or yellow colour.

In this article, I’ll explain exactly why an exhaust can turn blue.

Causes of an Exhaust Turning Blue

When exhaust pipes turn blue, it’s a regular occurrence, so don’t feel terrible if you’ve experienced it.

The intense heat that passes through exhaust pipes causes them to become blue. There is already significant heat travelling through them, but if there is a higher amount, the outcome is often blue pipes.

  • Revving or Driving Hard: When you increase throttle the engine burns more air and fuel which means more heat is being released, causing the exhaust to become hotter.
  • Lean Air/Fuel Ratio: An engine that is running a lean air/fuel ratio is another potential cause of the exhaust pipes becoming blue.
blue exhaust pipe tip

A lean running engine causes the engine’s combustion chamber to heat up significantly, causing the exhaust travelling through the exhaust pipes to be considerably hotter than usual.

Depending on the kind of metal used, certain motorbike exhaust pipes may be more prone to discolouration.

The heat causes oxidation, which changes the colour of the metal. The metal may interact with the gas molecules surrounding it when heated to a high enough temperature.

The final hue reflects how much the metal interacted with these components.

The base of the header pipes will usually have the greatest colour on them since that’s where the heat is concentrated. The hue will most likely fade as you go down the pipe.

  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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