Changing backpressure is like changing cams.
When the exhaust valve is about to close the intake opens. This is called valve overlap. With the exhaust valve open a new fuel mixture is sucked into the chamber.
Two much overlap can be caused by (No backpressure or low exhaust manifold pressure.) The air entering the cylinder flows too fast or too long, causing lost torque, less efficient.
At higher RPM pressure will increase, giving some power back.
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Frank
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