which is greater: pressure from the piston on its exhaust stroke or pressure from combustion? They are both plumbed into the same region. Any, even small amounts, combustion gases passing the valves will add to manifold pressure. - there is always a cylinder burning while another is exhausting, combine them.

Surely, it is negligible. Agreed, that the pressures in the manifolds are exhaust back pressure / turbo boost and are the same pressures working on the valve seals.

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