Quote:
Originally Posted by blackonorange
Huh? You've seen it happen ? How ? I don't understand how they could get in the engine
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The short explanation is that there is some overlap between intake & exhaust valves opening. There is vacuum in the cylinder, and backwards airflow (backflow) is pretty common, generally sucking some small amount of exhaust gas back into the cylinder. In cases where a cat (again, generally a pre-cat because of their proximity to the exhaust ports) has broken down, you have
A) loose pieces of honeycomb floating around
B) significant increases in backflow
and it becomes
technically possible for cat material to flow back into the engine.