Grains of water
Grains measure how much water the air holds: grains of water vapour per pound of dry air, with 7,000 grains to the pound. Unlike relative humidity, which swings with the temperature through the day, grains say how much water is actually there. That is the number that changes how an engine runs and how a tune behaves.
The cool evening in our density altitude examples carries about 35 grains; the humid 95 °F (35 °C) afternoon about 132.
Vapour pressure
The same water, as the share of the air's pressure it makes up, worked out from the dew point. It reads in inches of mercury on most weather stations: the cool evening about 0.240 inHg, the humid afternoon about 0.881 inHg. Water vapour takes the place of oxygen, so the higher it is, the less power.
The correction factor
The correction factor folds temperature, pressure and water into one number: what a run's horsepower is multiplied by to give it in standard air. Above 1.000 the air is worse than the standard; below, better. On drag-strip pages we show two standards, SAE J1349 (77 °F and 29.23 inHg of dry air) and the older STD (60 °F and 29.92 inHg):
- The cool evening: SAE 0.953, STD 0.998.
- The humid 95 °F (35 °C) afternoon: SAE 1.028, STD 1.064.
Both are for naturally aspirated engines; boosted engines have corrections of their own. Like every number here, ours comes from the forecast, not a sensor at the strip.
Questions
What is a high grains number?
Over about 100 grains is humid air, and summer evenings in the South can pass 140. Dry desert air can sit under 30. What matters most is the change from one run, or one day, to the next.
Is 7,000 grains to the pound right?
Yes. A grain is an old unit of weight, and 7,000 of them make a pound. Grains of water per pound of dry air is the humidity measure most drag-racing weather stations use.
Which correction factor should I use?
The one your logbook and your weather station already use, so the numbers compare. Many drag racers still use the older STD standard; dyno shops mostly use SAE J1349.