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# MAP vs performance

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I'm sure my question is based on fundamentals that I'm missing, but I'll ask anyway.

I've been reading around looking for data on how to interpret manifold vacuum on an NA setup. My original thought was that 0.0hg meant that there was no restriction in the intake system and would mean you're getting the most potential (ignoring any tuned intake benefits). Wouldn't 0.0hg mean that pressure inside the manifold is equal to the BARO? If a pressure differential is required for air to move, wouldn't 0.0hg reading mean no air is moving or am I not understanding something?

If vacuum is reading 20hg with closed throttle meaning it has massive restriction, and 0.0hg at WOT means no air is moving, what would the optimal reading be? Is 0.1 better than 1.0 or vice versa? What are the variables for determining optimal vacuum at WOT?

I have only found data from carb'd setups in which vacuum has different meanings. I'm specifically looking for fuel injection MPI and GDi setups.

In a natrurally aspirated engine the manifold pressure at WOT 'should' be equal to barometric pressure if the inlet isn't posing a restriction. It's the difference between the pressure in the manifold and the pressure in the cylinder (much lower than the manifold) that causes the air to move into the cylinder.

It's quite common for the manifold pressure to drop below barometric pressure as a result of the combined restrictions in the inlet system. This often gets worse as the rpm increases. If you're seeing this situation in a log file then it suggests you have the potential to see improvements.

Even with throttle full open, as rpm increases there will be some drop in manifold absolute pressure due to the restriction of the air induction system (filter, piping, etc). At low rpm that restriction isn't a major factor because there is very little airflow through the engine at low speed.

If you install a pressure sensor in the intake runner near the intake valves, and measure it at high speed you should see pretty significant vacuum during that cylinder's intake stroke. The intake manifold usually has a plenum chamber that dampens those vacuum pulses, and the MAP sensor is usually installed near the throttle (not near the intake runners). The pressure drop at the MAP sensor might be too small to notice on a gauge, but if you look at the data closely you will probably see that the manifold pressure is a little lower than atmospheric at full throttle high RPM.

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