And here I thought the MAP gauge indicated the pressure in the intake manifold... silly me! Now, where did I get that outrageous idea from? Would that amount be in mass or volume?
I have a feeling this picture will go a long way for those who got lost somewhere in the calculus:
Power delivered is the area of the power loop minus the area of the pumping loop.
Simple calculations of the pumping losses land in the ballpark when assuming the pumping pressure delta to be equal to the atmospheric pressure delta. If we included the fact that the exhaust gasses pass through an orifice on their way to atmospheric, I think we'd get close. IIRC, the combustion chamber pressure and the atmospheric pressure will be at a fixed ratio determined by the fixed mass flow through the exhaust valve, so the lowering of the exhaust back pressure will be multiplied in the calculation of the pumping loss in the exhaust cycle.
For example, lower the exhaust back pressure to 0.75 of atmospheric, 750 hPa instead of 1000. The exhaust stroke combustion chamber pressure would be, say, twice the exhaust pressure, so it'd go from 2000 hPa to 1500 hPa, giving a relatively larger reduction in pumping losses measured in kW/bhp.
I really hope someone who deals with this for a living will come along and set us straight eventually. That someone is, unfortunately, not me! Born 50 years too late for Real Engines.
