The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc.), it is said to run lean. ![]() If an engine burns with less oxygen present (13:1, 12:1, etc.), it is said to run rich. This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). Now, valve burning occurs as a result of a very lean-burning engine. ![]() Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well. The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road.
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