Quote:
Originally Posted by MechEngVT
I don't know how good Andyman's formula is, but his variables are inconsistent. The PA cannot both be manifold pressure and atmospheric pressure. The VAC is actually your manifold gage pressure. Volumetric efficiency varies with engine speed and throttle position. Even if the formula does work over a certain range of variables, it would only be meaningful at steady-state vehicle operation, and would have to be recalculated for a different set of parameters and would most assuredly yield a different answer.
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When I said the PA variable is intake air pressure I meant the pressure of the air before it goes through the air filter and throttle, not manifold pressure. The VAC variable is supposed to be the number you read on a vacuum gauge connected to the intake manifold. I was inconsistent with the units to measure it in the two formulas. I decided to use mm of mercury for the VE calculation because a vacuum gauge is normally calibrated that way. As an example, if you have 12 inches of mercury vacuum in the intake manifold then the VE should be about 1-(12/30) = 60%. There probably are some standard variable names for the parameters but I don't know what they are.
I don't claim the formula is perfect. It comes from bits of data of collected from various places. For example, a pound of gasoline is supposed to make 210 cubic feet of fuel mixture after mixing with the correct amount of air. Because of changes in the composition of gasoline, my numbers might be a few percent off. With alcohol added to the gas, the volume of air mixed with a pound of gasoline would be decreased a little and the fuel GPH would go up. I could make a more complicated formula to account for temperature and fuel type but I thought it was best to keep things simple so the formula would be easy to use and easy for me to create.