Quote:
Originally Posted by ST2008
So, can someone explain to me what is the deal about the Vacuum and the mpg ? and what is their relationship ?
Thx
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Engine vacuum is generated by the pistons descending in the cylinders, pulling in air. But at less than wide open throttle, there is resistance and the cylinder is pulling a vacuum behind the throttle plate. Generally speaking, there is a direct relationship between how deep a vacuum vs. how much load there is on the engine: more vacuum means a smaller throttle opening, means you are asking less of the engine, means less fuel burned.
NOTE: not being controlled by a throttle, this relationship does not exist in vehicles with diesel engines.
Vacuum readings are only relevant to the car in which they are taken. 15" vacuum in one car doesn't mean the same thing in someone else's car also showing 15" vacuum.
The vacuum gauge's main advantage to a hypermiler is showing, moment to moment, approximately how much demand he is placing on the engine and allowing him to alter his behavior in that moment to maximize his fuel economy.