Keep in mind that density altitude was intended for use by aircraft to determine the lift capability at takeoff. It does not care if the change in density is due to humidity changes (which displace oxygen) or due to temperature changes.
Water vapor is lighter than air so increasing humidity reduces density (increases density altitude) and reduces the amount of oxygen in a volume of air by displacing some of it with water vapor. Two identical density altitudes one with very high humidity and one with very low humidity will require slightly different mixtures.
The high humidity mixture will have less oxygen per volume and the water vapor will change the specific heat of the air, so the combined effect will be for the mixture to be slightly richer and burn a bit cooler. An identical density altitude with very low humidity will be slightly leaner due to the increased oxygen content per volume and it will burn a bit hotter due to the low humidity.
I would watch the density altitude as a gross reference point but also track the humidity, and
http://www.airdensityonline.com/ web site pay attention to the two values for
# air density(without water vapor) = 81.98
# air density (WITH water vapor) = 81.24
These are the reported values today here at Bandimere drag strip (5800 ft altitude) and :
# temperature = 50.0 deg f
# corrected barometer* = 29.81 Hg
# relative humidity = 68 %
Note that the high humidity effects the air density without water vapor vs with water vapor. As far as fuel air mixture you need to tune for the actual oxygen content per unit volume in the incoming air charge, but the humidity will effect how hot or cool a given AFR will burn, as humid air takes considerable more energy to increase its air temperature a given amount compared to dry air.
An easy way to see how humidity effects the heat capacity of air is to look at the dry lapse rate and saturated lapse rate values used in meterology.
Saturated air changes temperature at about 2.7 °F/1,000 ft change in altitude, where dry air changes temperature at a rate of approximately 5.5 deg F/1,000 ft change in elevation. As you can see the ratio is 5.5/2.7 = almost 2:1 difference in how much energy it takes to heat or cool a parcel of air depending if it is completely dry or fully saturated with water vapor. That means hot dry conditions are most likely to hurt the engine if it goes lean, compared to hot damp conditions and the same change in fuel air mixture.
Take a look at this web site, he does a good job of explaining how temperature, humidity and density effect engine power and how it compares to density altitude.
http://wahiduddin.net/calc/calc_hp_abs.htmIf you play with his calculators you can see that you can find two different conditions with identical density altitude but the relative power levels will be different.
For example if you input values of 50 deg F, absolute air pressure (station pressure = uncorrected barometer) of 24.3 in-hg, and relative humidity of 60% you get a density altitude of 6482 ft and a relative power level of 82.2.
If you put in a temperture of 51.42, Deg F, and the same barometric station pressure of 24.3 in-hg, and a humidity of 10% you get the same density altitude number but the relative power is now 82.8. The drop in humidity was worth an increase in power (due to increased oxygen to burn) of 82.8/82.2=1.00729
That does not sound like much but on a 400 hp engine it would be a change in power of 2.9 hp. That is assuming you nailed the tune for both conditions. If you hit one and missed the other, the difference could easily be 5-8 hp or more. The variation is much higher in hot temperatures because air can hold much more humidity in high temperatures.
If you use dry conditions 10% humidty, station pressure of 24.3 and temperature of 110 deg F your density altitude is 10093 and a relative power of 77%. If you drop the temperature to 75 degrees and 70% humidity and adjust the station pressure to 22.946 in-hg get to the same density altitude, you have relative power of 73.4%
Power has changed by 77/73.4=1.049 or about 5% between the two conditions even though the density altitude computed is the same.
On a 400 hp engine that change would be 20 hp difference if both tunes were ideal.
Larry
