How does humidity affect engine performance? On the one hand the air is denser so one might think that it's a good thing, but on the other hand the air is only denser because it's full of moisture which really shouldn't add and O2 to the engines equation. So what really happens?

Ivory Coupe 5AT
Z tube, K&N drop in.
Crawford Plenum and Fujitsubo Exhaust soon to come...

 

http://www.hort.vt.edu/faculty/willi...iditytable.pdf

Engineers use grams per second of air flow into engine but really how much of air is Oxygen is all that counts.
http://www.shorstmeyer.com/wxfaqs/hu.../humidity.html

Dew Point is a much better gauge than humidity%

http://wahiduddin.net/calc/calc_hp.htm

 

Humid air is less dense, not more dense. It just "feels" more dense to your body because the air feels "heavy", but it's actually LESS air and MORE water. The air is being displaced by moisture, and like Q said, oxygen is what matters to your motor. Your engine performance will decrease, holding everything else constant, when the "Relative Humidity" is higher.

Here's a good link to see just what it does to your HP, and to note the relationship to dew point:
http://www.csgnetwork.com/relativehpcalc.html



2004.5 Coupe/5AT/Ivory/Everything but front spoiler/Nismo CAI/Crawford V5/Crawford Cats

 

Understand that MAF cannot differentiate between the causes of density changes and no sensor is included to measure humidity [dew point]. Much of the published error +- 3% comes from changes in humidity.
http://www.tsi.com/flowmetr/app_note...ty_Effects.pdf

http://www.sensorsmag.com/articles/0...ity/main.shtml

Since the software in ecu ALWAYS ERRS on the side of richness it doesn't matter much. As the errors from 30F dewpoint to 90F due point are 1.5-3.0% at same temp and barometric pressure.

11F temp change is 1% as is 333 feet altitude much more significant than humidity unless you are being extremely precise.

DUE POINTS: "one of the highest dew points ever recorded is 93 degrees F in Sharjah, United Arab Emirates (25 degrees 20' N, 55 degrees 31' E), near the eastern end of the Persian Gulf. In Assab, Eritrea (13 degrees 00' N, 42 degrees 42' E), at the extreme southeastern end of the Red Sea, the average afternoon dew point in June is 84 degrees F. ...............
United States has very high summer temperatures in the Desert Southwest and, at least occasionally, very high dew points from the Mississippi River Valley east but never the two in combination.
Each summer, dew points in the low 80s can occasionally be found from the Gulf Coast to the upper Mississippi Valley. "
http://www.weatherwise.org/qr/qry.02...ntextreme.html

 

Actually the now integrated air temp sensor in the MAF housing can at least tell the ecu that the abnormal change in density is from barometric and or humidity changes not which one.

 

Higher humidity should cool the flame and cause less knock for the same temp. Less knock = more available power.

For density, N2 = 28, H2O = 18. More H2O in the mixture will lower density.

2004 G35C 6MT Black. Killer.

 

You are confusing light load to heavy load low rpm transition knock which the knock sensors are primarily designed to suppress with WOT high rpm knock [around and above torque peak rpm], which is already supressed by excess richness.

The 0.5-1.5% water vapor in saturated air is relatively insignificant in cooling the charge...........by mass.

O2 has a molecular weight of 32 while water vapor is 18 so the higher the humidity the lighter the air is. Humid air is lighter than dry air.

It usually takes 5-10% water vapor [supercharger applications] to materially cool charge! Just as we use 10 AF instead of 14.7 AF to cool combustion............4.7AF points is 47% excess fuel..........granted water is better at cooling than gasoline.but not 100 times better.
http://hyperphysics.phy-astr.gsu.edu...vappre.html#c5