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Wrench spinner
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Location: Leigh On Sea
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Assuming that the overlap is symmetrical about TDC then it means that the inlet valve opens around 24 degrees before TDC as the piston is close to the head and the vast majority of the exhaust gases have left the cylinder - the momentum of the gasses passing into the manifold will continue to pull gas out of the cylinder even though the piston has pretty well stopped pushing.
Opening the inlet valve at this time does two things - it allows the exhaust gases to continue flowing under momentum and it allows the fresh fuel mixture to be sucked in under the now lower than inlet manifold pressure that exists in the cylinder.
As the piston passes over TDC the chamber is at its smallest and the last of the exhaust gasses is being replaced by fresh mixture - closing the exhaust valve is timed to coincide with the fresh mixture reaching it at around 24 degrees after TDC, just as the piston is starting its induction stroke.
So the valve overlap actually helps fill the cylinder with the mixture for the next power stroke.
The rate that the exhaust gasses leave is proportional to the difference in pressure between the cylinder and the manifold - anything that can be done to reduce the manifold pressure will help engine efficiency - absence of exhaust system restrictions and tuned lengths to generate reflected pressure waves both do this.
So "back pressure" is making the engine less efficient than it could be and is largely generated by poor manifold design (cheaper to produce than properly tuned ones) and by the need to quieten exhaust noise (efficient noise reduction causes restriction in the system).
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