atdjr123

I have dual exhaust on audi 8 and one tailpipe is black while the other is clean, what causes this???

Woodsie0520

It's carbon build up. Turbo engines are known to produce more carbon build up in the engines and exhaust system than naturally aspirated engines. It was really bad on my B8.5 A4, but seems to be much better on my B9 A4. I think that is because the B9 has multiport injection so it cleans up the carbon a bit better.

atdjr123

Thank you for the reply..

Nostromo

I thought I read the EU B9 had multiport, but US only had direct injection?

IMYM

Not sure for Audi, but lot of other cars have a flap/valve that closes one side of the exhaust under normal condition. It only opens when the engine rev is high. This is done to regulate the back pressure.

So if there is a big difference between the two sides, it is a sign that you are too gentle on the car :-P

oktt4000

Cars that use gasoline direct injection (GDI) combined with turbocharging produce black hard carbon particles known as gasoline particulate. This is similar to how diesel engines used to produce black smoke. Diesels now use diesel particulate filters (DPF). In Europe VW/Audi is introducing gasoline particulate filters (GPF) to help with emissions as well as black tail pipes.

The second slide in this link shows pictures of diesel particulate compared to gasoline particlate
https://www.aecc.eu/wp-content/uploa...-GPF-final.pdf

The first three slides show what the manufacturers are doing to help stop the issue
https://www.energy.gov/sites/prod/fi...12_bischof.pdf

HyperS4

Not sure where you are located but the US cars do not have a flap on one side. I have already checked because thats typically the first thing I remove or block off.