No engine runs better with more backpressure. A certain cam, cylinder head and carb combination may suffer from reversion that a smaller exhaust can cover the effects of so it seems to run better. Or you can do the research and put together the right parts that work well together and have more power and efficiency when you are done. This is another myth along with the "torque motor" myth and the "engine cools better with less coolant flow" myth that modern science and engineering have debunked.
