Colour change on exhaust

[Gary G]

My 4-1 exhaust is highly polished and has been a coppery colour (outside the engine bay) since I bought the car in May. Just cleaned it today with Autosol (as I've done many times before) and it is now more of a silver colour. Is there any reason for this?

 

C7 GAR

 

[Jason Plato]

Maybe this time you simply rubbed it a bit harder ??

[sieds]

If you wrun it for a couple of hours the coppery color returns caused by the heat.

 

Keep on polishing

[Rob Walker]

Gary,

 

The brown film is just the Stainless Steel oxidizing due to the high temp of the exhaust gases passing through and heating the tube, a good thrashing on a track on a hot day will produce a deeper brown colour. This is the same effect as when you temper steel, as the steel is heated to about 350C a straw coloured oxide forms on cooling if you heat a bit more 450C the oxide turns blue. Therefore if you see your manifold turn blue outside the engine bay this is a good indicator that either you have a weak mixture or your timing is out.

 

Rob

[Gary G]

Thanks for your replies. The heat thing makes sense, because I've driven the car very little over the last 2 months and the exhaust has therefore not been very hot.

 

C7 GAR