Thinking some more about that, I would be inclined to say the opposite is true: If an engine runs crap without its intended muffler, and well WITH, that does not indicate "something wrong with the engine", on the contrary, it indicates that both the engine as well as the muffler are extremely well designed as two parts that together form a "system". Similar to for example how a bike frame or car chassis and its suspension can be complementing each other,