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Don't know where to begin, but here it goes:
High RPM engines have very stiff valve springs. Or else, the valve springs set themselves into resonance within the operating ranges of speed. But when you use very stiff springs to over the resonance issues, you end up losing a lot of efficiency at mid-range. At full throttle, there is plenty of power to overcome the mechanical losses, but at partial and low throttles, where the engine makes significantly lower power, the mechanical losses due to valve spring compression become dominant. For this reason, the exhaust backpressure during mid- and low-range RPM creates enough negative pressure to help ease the effort it takes to compress the springs. The motorcycle industry relies on ex-up valves that can be adjusted on the go. I'm no longer involved with the exhaust mechanisms in the auto world, so I don't know if the current automotive technology uses this kind of valves, or still the exhaust pressure from other cylinders.
TLDR: You need backpressure in low-and mid-range to minimize mechanical losses from the valvetrain.
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