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Perhaps we should clarify this a bit further.
Cams have overlap -- the number of degrees at which the exhaust valve doesn't fully close after the intake starts to open -- for two equally important reasons:
1) There is low pressure (less than atmospheric) in the exhaust behind the departing pulse of exhaust gasses. That's a moving column of gas that has mass and velocity, and tends to stay in motion. That low pressure helps get the column of intake charge moving.
2) The mass of the intake column, once set in motion, helps expel the last dregs of the exhaust gasses from the combustion chamber. It's not possible to pump it all out on the exhaust stroke.
So, efficiency (economy, power) comes from the low pressure in the exhaust being matched to the amount of overlap, and to a lesser extent the length of the intake column. If the low pressure builds too late or lasts too long, that's when an appreciable omount of the intake charge gets sucked out the exhaust ports. It's a matter of timing. This is what is meant by a header being "tuned" to match a certain cam. If the header is correctly tuned, efficiency is best with no exhaust system other than the header, and we run 3" pipes and straight-through mufflers to try to make the system transparent to the header.
If we don't have a header, we can work with the diameter and length of the pipes. This is why the later double-downpipe and manifold add 7%-8% horsepower and torque over the earlier single downpipe. It's actually a crude sort of header.
Any tuning after that also comes from the diameter and length of the exhaust tubing. What does not have any benefit whatsoever is putting a restriction way far away from the motor, like a muffler. It's too far away to affect the timing (remember, it's the timing that's all-important); all it can do is get gasses jammed up in the pipe and raise the low pressure, which is called backpressure, and which is exactly what we do not want at any rpm.
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