[FED 387]

GRAINS OF WATER-is a measurement that has to do with how much oxygen has been displaced in a cubic foot of air in relation to the maximum amount of oxygen that is normally present in that same cubic foot--a normal cubic foot of air has about 8000 grains of water & 21% of oxygen so if the amount of grains decreases to say 7500 grains then you will have a larger percentage of oxygen available to the engine and the result is more power --- more grains of water means less oxygen available and results in a corresponding decrease in power.
WATER VAPOR PRESSURE- has to do with humidity and barometric pressure---generally when you have high humidty you will have a lower barometric pressure reading but part of this is related to ambient air pressure too--- so if you have a warm humid day you will have a lower barometric pressure reading which generally results in decreased performance--- you can also have a high humidty cooler temperature situation but have an increase in barometric pressure and still have a lower engine performance. That's because of the absence of available oxygen due to the increased grains of water in the air
Basically Grains has to do with the percentage of water in the air and pressure is the weight of the water in the air

Hope this helps--- google it and read up on it you will learn quite a bit about how & why Grains affects engine performance --also look at Vapor see what it has to say