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Originally Posted by jeff88
So what you are saying is that with a higher back pressure, the throttle plate opens up a little more, thereby lowering manifold vacuum?
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If there is some extra resistance in the exhaust stroke, then extra throttle (and fuel by the way) is required to overcome it. The extra throttle reduces the work done on the inlet stroke so there's not a one-one fuel consumption penalty to increased exhaust manifold pressure.
I doubt that's the whole reason for any fuel economy benefit for just adding a turbo to an engine but might be part of an explanation as to why there was one.
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I honestly don't know how the EGR specifically effects the intake. Does the MAF sensor recognize a smaller amount of air (because of the increase in EGR), so it opens the throttle more to allow more air in?
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If you hold the throttle opening constant and introduce EGR (any inert gas) into the inlet manifold, the inlet manifold pressure is raised and the pumping work is reduced. The same quantity of fuel and air is consumed but there is less energy wasted in working against the vacuum (inlet man. press.) on the inlet stroke.
For a given power requirement at the crank, less air and fuel is needed, so the throttle can actually be closed more with EGR.