I was at the track yesterday and ran some really slow times. It was ~70, humid and a low pressure system had just moved in to the area.

-- The Truth About Humidity and ET Prediction --
Frequently, I get asked about the effects of humidity on ET Prediction. Many racers try to develop their own 'formulas' for predicting ET using a spreadsheet program. It just can't be done. The scientific formulas that we use are much too complex for that. Humidity is one of the hardest of variables to work with because it's effect on ET changes dramatically with temperature. Some racers will say that humidity is the ONLY variable that effects their car. Or, humidity effects my car differently. This is not true! However, if you are not measuring the temperature, barometric pressure, and humidity accurately, or using a program that doesn't calculate the effects of humidity properly, your results may lead you to believe such myths.

The secret to ET Prediction is calculating how much oxygen is in the air presently, RELATIVE to a previous time. And then finding the loss or gain of horsepower due to the specific change in available oxygen. Engine performance then, is only effected DIRECTLY by the amount of oxygen molecules in a given cubic foot of air. Nothing else.

The variables, Temperature, Barometric Pressure, and Humidity are responsible for the change in available oxygen. Barometric Pressure compresses the air and allows more room for oxygen molecules. So, the higher the Barometric Pressure, the more horsepower your engine will produce. Higher temperatures expand the air molecules, and therefore decrease the room for oxygen. So, the higher the Temperature, the less horsepower your engine will produce. Humidity is just water vapor. The presence of water vapor takes up room and therefore, displaces oxygen molecules. So, the higher the Humidity, the less horsepower your engine will produce.

Temperature is the biggest factor in predicting performance, not humidity. However, the higher the temperature, the bigger effect humidity has on performance. If you are not measuring the temperature accurately, then the effect of humidity can not be calculated accurately. And, at higher temperatures this becomes very dramatic. For example, at 70 degrees, the addition of 82 per cent humidity could slow your ET by .073 seconds. At 78 degrees, the same amount of humidity could slow your ET by .100 seconds. Almost three (3) more hundredths due to the humidity alone. About .026 more due to the change in temperature alone. So, if you read your temperature gauge wrong, or your gauge was inaccurate, under high humidity conditions, your Predicted ET could easily be wrong by .060 of a second or more. Garbage in, garbage out.

 

 

The only problem with high pressure is that the increase in barometric pressure would also increase the drag on the car, nearly canceling hp gain from slightly more dense air.