After going OT in another thread, I made a comment about how increased hummidty is not beneficial in generating horsepower. While I know this (including temp and elevation) to be true in a general, shallow sense, I didn't know to what degree, until I found this:

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

Given these inputs...

Air Temp = 90F
Altimeter = 29" Hg
Relative Humidity = 30%
Altitutude = 90'

Relative Power = 95.6%

Leaving all input variables the same, while jacking-up the humidity from 30 to 90% makes the relative power drop to 92.3%. If I'm interpretting this correctly, that's over a 3% drop in power, or more than 6 hp on a 200 hp engine. Is that correct?

Wow, look what happens when one goes-up to Webb elevation!:

Air Temp = 90F
Altimeter = 29" Hg
Relative Humidity = 30%
Altitutude = 5280'

Relative Power = 76.3%

Again, is that right? I know that elevation is huge, but that's a very big delta (nearly 20%), all else being equal with the first example...

 

I do almost all of my driving between 4000 and 6000 feet and when I went to Monterey the other weekend I could feel a big difference in performance. But that delta does seem large.


Nik

 

I highly doubt the delta could be that great. Granted, engine performance in the vette drops a small amount during my drive to yosemite (2000ft to 6500ft). The drop is noticible but not anywhere near a 20% loss in power.

 

Having been to Yosemite a butt-load of times, there is usually a nice temp drop as well. As a mountain climber, I'm aware of a formula of sorts to calcuate the temp on the summit based upon the temp at base camp - a drop of so many degrees per 1,000'...

 

Not only do you have the loss of air to burn, you also have less air to cool the intercooler. The same loss in air to burn also means less air cooling! A double whammy.

It is not all bad:
*There is the better exhaust flow from less atmospheric back-pressure.
*There is less air resistance as the car moves through the air.

Either way there are definitely power losses. Exhaust driven Turbos have lower losses, but still losses.

 

ouy of curiosity, since dynos can compensate for humidty and barometric pressure...what correction factor do they use?

 

This is a great thread. That delta seems so high.

Bomboasy

 

We spend time trying to make the air cooler, which helps, namely post SC, but there are other factors as well, if we can reasonably influence and control them, like taking moisture (humidity) out of the air...

Oh, my weather station alarm just alerted me that humidity dropped below 30%, and now is an ideal time to drive to work .

 

For the SAE J1349 relative horsepower calculations, the standard reference conditions (100% relative horsepower) are:

Air temperature = 77 deg F (25 deg C)
Actual pressure = 29.235 In Hg (990 mb)
Relative humidity = 0%

Changes in humidity have the least effect on relative horsepower (RHP). For each 10% rise in relative humidity, RHP only drops 0.37%. For each 10 degree rise in temperature, RHP declines 1%. Altitude has the largest impact on RHP (as all pilots know): at 5000 feet, RHP is only 79%.

If you really want to lay down the horsepower, head to Canada for some winter testing. Try Winnipeg in January. -40F, 50% humidity, 750 ft elevation gives a RHP=112%. Who needs mods!!?