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For the rest of the people that might be reading for the sake of learning remember that valves open and close so exhausts and cylinders work in pulses not a seamless flow of air as most people incorrectly assume. Therefore there are always areas of high pressure and low pressure just outside of your exhaust valves (just like if you drop a rock in water, the ripples represent high and low pressure). You're always trying to create the lowest pressure possible outside of the exhaust valve and the highest pressure possible outside the intake valve. the physics of this world dictate that a fluid will always move from the area of high pressure to low pressure at a higher velocity the greater the difference in pressure.
Using the rock in water example, it's easy to see that by confining the exhaust you will effectively have a greater high pressure and a lower low pressure than that of a wide open exhaust (the energy will disperse/diffuse in a fan shape). This is why a properly tuned exhaust can take advantage of a large low pressure pulse to scavenge.
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