This post reminds me of a bob marley tune............I think it went something like

"I smoke 2 joints in the morning, I smoke 2 joints @ night"

I have seen a loss of 14hp when going from 60%Rh to 80%Rh with an inlet temp of 25c

 

Unfortunatly even theoretically he isn't really that correct. i reccomend a couple of classes to take and then you can return to this subject prepared. First, thermodynamics, understand why heat does what it does and then take one in fluids that way you can understand what happens as the air from outside is sucked into the turbine and impacts the fins. Humidity affects cavitation (I'll have to check spelling, its been a little time) in a fundamental way. Plus, as you said before, humid air retains more heat then dry air... so.. you take heat retaining air, push it into the engine via fins designed to cavitate dry air (in the process heating the air) and you end up with much less oxygen and a charge that holds its higher temp easier than with dry air. Let me see if I can find my old textbooks somewhere.. I'll scan something in....

 

Humidity good = junk science. Period.

Part of what I do every day is engineering for cooling tower sizing, which requires that I have a firm grasp of weather and psychrometric analysis. Some of what is being said in this thread is just plain wrong, and here’s why:

For one thing, humid weather is more often than not accompanied by low barometric pressure. Look up a formula that racers use to calculate Density Altitude (DA). You'll find that humidity is a negative factor in such calculations. With low barometric pressure, you will have less oxygen in the air, and less oxygen to compress, and thus combust. It is also common to have higher temperatures with very humid weather, and although not directly related, the two together is obviously very bad for turbo car performance.

Humidity is additional H2O in the air, and with 2 hydrogen atoms to one oxygen atom for each water molecule, which will also displace oxygen, thus providing less to oxygen to combust.

High humidity also decreases the efficiency of an air/air intercooler in a similar manner to how it impedes the evaporative cooling process cooing towers use.

The reason that some of you see dew on the grass on a nice cool morning is not humidity, but the dew point. Remember, the cooler the air, the less water vapor it can contain. The dew point is the temperature of air where water will condense from it (water vapor will no longer be suspended in the air). Cliff Notes: Just because you see dew on the ground, during a cool morning, that does not mean there is high humidity.

Higher Humidity is going to be bad for turbo car performance compared to lower humidity (providing all other weather conditions are the same), no matter how you tune for it. Of course, those of you that feel otherwise, and feel that it only takes "tuning for it" to make more power than in low humidity weather, can put your theories to the test. Go fill up with 91 octane, advance that timing, turn up the boost, and go for it since you've got that good 'ol natural water injection. Make sure you have a buddy follow you with a catcher's mitt so he can catch the pistons and rods as they exit the block.

Don’t compare weather conditions with water injection, which is just a wet blanket to inhibit the onset of detonation. Water injection may help, but it’s not quite an even trade for nice, cool weather with high barometric pressure, and low humidity.

 

After reading these posts and doing some more research, I have to admit my initial statement isn't necessarily true. I should have said humidity is a natural octane booster.
"An increase of absolute humidity of 1.0 g water/kg of dry air lowers the octane requirement of an engine by 0.25 - 0.32 MON [27,28,38]."

This whole thing is much more involved. It all goes back to charge density, octane requirements and detonation.
Generally speaking, cool dry air will provide the best performance if you are tuned for it. You are getting the most O2 you can, adding max fuel and producing the most power.
There are many factors that can cause detonation, increasing the temperature if the intake air is the main one we think of. Higher boost adds heat. Unscavenged exhaust adds heat. High abient temps add heat,
But you can also get detonation from a very dense, cool inlet charge producing a massive increase in heat energy as a result of all the fuel added to the dense charge to provide the correct afr.
Sounds funny, but think about it.
If you significantly increase charge density, and place the engine under load, you need additional fuel octane to fend off detonation. So, cool dense atmospheric conditions could produce knock. A big drop in humidity could lead to knock.
It's a delicate balance: a slight increase in charge temp or density isn't a problem, but a big increase in either will require more octane.

 

Yes... it is a known fact that water in air makes it more dense and lowers the octane requirement. This is the premise behind water injection. That is not debatable. The debate is the humid air being forced throught the entire turbo setup.... Which is not in any way beneficial. I still haven't found the book I was looking for but I'll find it and post a picture of what happens when damp air (water droplets) impact a compressor spinning at high rpms... its... not pretty and not efficient...

So basically.. in a nutshell.. yes.. water in air increases density. Leave it that and quite while you are ahead Humidity in outside air is NOT a viable source of increased density that would lead to benefit. Oh.. another thing to consider.. if it has water in it, it has all the environmental crap it picks up from the air around it even more, so you would be pushing more dust and crap throught the engine, too.. things I didnt' even consider in my calculations..

 

I realize that you will make more total power (more heat energy OUTPUT) with cooler air, but a cooler intake charge will actually inhibit knock, so long as the fuel/air ratio is properly set by the ECU, and you don't run too lean.

 

i wouldn't imagine ever being this tedious with my times at the track... i wouldn't use water injection either because i'm not trying to break world records with my evo...

 

Guys,

water injection works.....However, humidity and water injection are not nearly the same thing. I will try to explain this. Water injection works because the water is injected as a mist (still a liquid, just small drops). Water has to absorb alot of energy (latent heat) to change from a liquid to a vapor. This absorption of heat effectively raises the octane (reduces burn speed) and allows further adjustments (timing, boost, AFR) to attain better performance.

Humidity....However, is already a vapor and is already displacing oxygen. Therefore, it cannot absorb as much heat (like the previous phase change). It just heats up, displaces oxygen and reduces performance.

So, in reality water injection injection does work due to a phase change of the water and not the actual vapor presence itself.

I hope this information helps.

HiVolt

 

Mr MayhemSi, moist air is LESS dense than dry air, basic physics. Moist air is lighter than dry air. It has less O2 and (relatively) more water molecules suspended between all the fuel and O2 molecules. This is the basis for the increase in relative octane requirements. With moist air, the combustion process is slower, which is the same thing higher octane fuels do chemically. Overall power will be down compared to dryer, more O2 rich air.
For me out here in my situation, humidity helps.
I'm running 91octane (****), with daily temps around 110F+. My ECU is sensing knock and pulling timing all the time. Last week the humidity went up to around 35-40%. This increased my overall octane, thus, less knock, less timing being pulled, all this made more power. This was only verified by my *** dyno and the feel of acceleration will no timing being pulled.
Being that I have no experience with moist air and compressor fluid dynamics I can'y argue that moist air doesn't compress well in a turbo.
So I guess everybody is correct. Unless you have my exact situation, any increase in humidity will lower performance, but with me (only in the summer here) gain overall perfromance with increased humidity.

 

ok.. I'm done.. you obviously have no idea what I'm talking about and cannot grasp the concepts I'm trying to show you. Moist air is more dense then dry air. It has less oxygen in it, yes. For all you know your experience could simply be a better tank of gas. The *** dyno means nothing, and all real world numbers point to your idea being wrong.

 

Lets build a dyno inside of a controled chamber where we can control, temperature, barometric pressure and relative humidity and test this theory.

 

MayhemSI:
I thought you were an engineer? Any enginer has had basic physics. One of the 1st things I learned during my education as a meterologistis is that moist air is LESS dense dry air:
"Dry air has a molecular mass of 0.029 kg. It is denser than water vapour, which has a molecular mass of 0.018 kg. Therefore, humid air is lighter than dry air. If the total atmospheric pressure is P and the water vapour pressure is p, the partial pressure of the dry air component is P - p . The weight ratio of the two components, water vapour and dry air is:

kg water vapour / kg dry air = 0.018 *p / ( 0.029 *(P - p ) ) "

So, your professional opinion is out the door and you refuse to concede that I am correct....which is your right as an american. (which I have defended for the last 19 years)
I can grasp almost any concepts, but you haven't shown me anything here to indicate I am wrong.
Now, if you are just one of these guys that posts on forums for the sake of arggument, I refuse to play.
I have already re-tracked my initial statement and re-stated my claim.
I am not a stupid guy sputing off without reason.
I am an educated man, with lots of experience and tons of research.
I used to be an old school motorhead, big block Mopars, with big carbs, hogged out heads, and all the other old school crap.
Since I decided to get my Evo, I've had to re-learn tuning.
But as this point falls into my profession, I would have to say I am 100% correct.
You prove me wrong.

 

Let me clarify my use of the word density for you... obviuosly it was misunderstood.. the density of humid air when speaking about compression is more dense and this is proven in its resistance to being compressed at all... by cubic foot at the same temperature humid air is lighter then air.. yes..

I am merely speaking about density of the air with respect to compression. This is actually called the Density Ratio (I left ratio out) which basically is the final true pressure seen in the cylinder as a product of the compressor system. When incorporating humid air into these equations, it is more dense.

 

1.) Density = mass/unit volume.

2.) Higher humidity means lower air density. There is less O2 per unit volume of air, so you'll make less HP. All drag racers know this, there's no question about it.

3.)But a little water in the air can provide a cooler discharge, due to evaporation. A cooler discharge means you can run more timing (=more HP). This is the concept behind water injection.

So the question becomes whether you can "tune out" the negative effects of humidity. I haven't seen any evidence of this in this post. Only thing said was more or less a reworded version of (3.) above

 

I have the answer!
My nine year old has a squirt gun. I'm going to tape a child seat, (with 100 mile an hour tape of course) to the front bumper. Then I will data log runs with and without him squirting water into the intake. This will simulate higher humidity. Then I will remove the drag he effects on the car and should come up with the answer.

I'm through here. It really doesn't matter since this only effects guys in my particular situation, (which I have learned since this thread started).
Humiditry is good or bad depending on your situation.

Thanks, and have a good day.