I know it's to do with the amount of carbon etc.
When I had my old 2.0 in the car the exhaust stayed clean for weeks and weeks. But now with the V6 in it, it gets black after a day or so. Not totally, but goes hazey.
Now why would this happen? Both engines have run with catalytic converters. The V6 engine actually has lower milage than the 2.0 I had, just 50,000.
I feel the engine is down in power too.