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Understanding AFR

Relevant Module: The AFR Target Map > Full Load N/A - Transition F/I

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Discussion and questions related to the course Understanding AFR

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So I just had the question 'At high RPM, naturally aspirated engines can suffer from restrictions in the intake tract, what is the result?' and I answered an increase in vacuum which was wrong.

I understand as we near 100 kPa the air filter etc. can become restrictive, causing a drop in VE, making us operate in the 90-95 kPa range. Is this not an increase in vacuum? I.e. at idle the manifold sees high vacuum. As we move through the rev range the vacuum decreases until we hit atmospheric pressure, or 100 kPa. If we drop from 100 kPa to 90 kPa is that not an increase in vacuum? Not a drop in vacuum (which is the correct answer on the questions)? Sorry if I'm being daft but that's confused me a bit.

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