Quote:
Originally Posted by ST2008
OK, but why some state that we should keep vacuum as higher as much possible in order to have higher mpg.
For myself, the vacuum shows in Torque pro are 21psi for idling and 8 to 12psi for driving in a freeway (60mph). And I have no clue how to control the vacuum.
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Are you referring to the MAP gauge? It's impossible to have more than about 14.7 PSI of vacuum because that is the density of the air. The only way to have more vacuum is to have denser air.
Vacuum is simply the difference in pressure between the atmosphere and the intake manifold.
Engine load is inversely related to vacuum, meaning the more of one you have, the less you have of the other. This generally means that a higher engine load is more efficient than running a lower load. Could engine load be what people are referring to when they say you want to keep it high for fuel efficiency?
So far we have been completely ignoring another very important factor in fuel efficiency, which is engine speed (RPM).
You need a BSFC map for your particular engine to know exactly what RPM and engine load is most efficient.
All of this is mostly irrelevant if you aren't using the pulse and glide method, as your engine speed is mostly dependent on how fast you need to travel, or how quickly you need to accelerate.