WO

Hey all... did some long driving today in really hot, humid weather with A/C on and noticed a few things... first off, with the A/C running, I notice that my A4 1.8T tends to "vibrate"... especially noticable when stopped... you can see the stick shift vibrate when idling. Also, I also noticed that when shifting into first, I have to give it quite a bit more gas, otherwise it'll stutter a bit before accelerating... is all this normal effects of the A/C and humid weather?

/// (NT)

Humidity generally makes the car run better (denser air)- A/C however takes around 10hp to run-hence the lag.

T2

The higher the humidity, the lower the density of air/water vapor mixture. Water vapor is actually less dense than air. When I was young I remember baseball announcers stating that the ball would travel less far in humid weather - not true, the drag on the ball would actually go down.<p>Colder air is more dense.<p>Regarding my AC and my 1.8T, there is definitely more noticeable vibration at idle with the AC on and a reduction in power. The vibration appears to vary quite a bit from day to day.<p>Tom<br>1.8TQMS Silver/Ecru

nt nazi

&quot;nt&quot; should end that sentence

apologetic nt nazi

oops, there it is under the post

ALD

Humid air is less dense and denser then non-humid air. Now that I've gotten everyone's attention and have confused everyone totally, let me explain the first part.<br>When you have a humidity of lets say 95%, you have more water in the air then air, and it becomes more dense, and feels alot heavier then air around 50% humidity. But it has less air molecules (molecules other than H20 in gas state) (Which is normal for NYC region in the summer to a point). If you ask jet pilots or even private pilots, they will tell you when the humidity starts to go up, it will take longer for the aircraft to get airborn, not because the air is less dense, but because there is more air to push through.<br>No for the second part, Humid air is denser then less humid air. Again, the water molecules add more 'weight' to the air. and read the section above.<br>Now how does this affect cars with turbos? Well, best way to explain it, you have less air entering the engine, and less air going through the turbo, so the car will feel a little bit more sluggish then it will. I hope that after confusing eveyone totally, I have unconfused you. I knew those classes in meterology would come in handy one day.<p>ALD<p>

Conker

that denser, cooler air is better for power production, and that humid air is worse for power production. I would prefer cool, dry air for my turbo. In addition, I believe that when you floor your 1.8T, the A/C compressor switches off. I am certain mine does, because the air gets "not cold" in a hurry during and after a hard run. Also, I believe my owners manual states this. <br>Conker 98 1.8TS

///

....that's interesting because I've noticed my car always runs better on humid days. Maybe the water vapor has a cooling effect in the car? I don't have a turbo (328), and wasn't aware that outside conditions were pertinent to a turbo's function - I mean it's using exhaust pressure after all ( I may well be wrong). In any event, the A/C will definitely rob low torque unless, like you said, you floor it.

Darryl W

Humidity values are often expressed as a percentage, but you seem to believe that this percentage is the amount of air that is water, so that a 95 % humidity value means that the air is 95% water - this is absolutely false. The percentage represents the amount of water in the air as compared to the maximum amount the air can hold at that temperature and pressure, so that the 95 % value means that the air has 95 % of the water that it can hold. You certainly _never_, at any humidity, at any temperature even close to the boiling point of water, have more water in the atmosphere than "air," which is about 79% nitrogen, 20% oxygen, and the rest other stuff (humidity = 0). <p>All liquids have a vapor pressure, that is the pressure that would exist in a closed vessel with only that liquid being present (i.e., otherwise a vacuum without said liquid) in the vessel. Now, this pressure is temperature dependent for all liquids. The vapor pressure represents the maximum pressure (remember, a vapor is a gas) that can exist in the atmosphere of the liquid without condensation occurring. So, air that is 50 F and 95 % humidity has significantly less water in it than air at 95 F and 95 % humidity, right? I'm totally guessing off the top of my head that the vapor pressure of water at 95 F is about 20 torr, making the air at 95 % humidity 2.5 % water - quite a bit, but not more water than 'air'. Make sense?<p>Now, your statement that humid air is more dense than dry air may or may not be true - certainly it is but I don't know if it is enough to be significant. feels like it is playing soccer on a hot, humid day here in texas.<p>Anyway, humid air is less useful to your engine because it has so much water in it, and water doesn't burn very well, so that humid air should reduce the efficiency of your engine. <p>Darryl W<br>98.5 1.8T qmsb, faster in the winter!

Greg

This is a common stumper in pilot training. Common sense tells you that humid air is denser and heavier than non-humid air (after all, we just added water to the air, right?), but it turns out to be the opposite: humid air is lighter and less dense than dry air. Honest. Adding humidity to the atmosphere reduces the density and atmospheric pressure.<p>Boyle's (?) law: PV=nRT.<p>At a constant pressure and temp, a gas contains a constant number of molecules/unit of volume, regardless of gas composition. At constant pressure/temp, the density (mass/volume) depends on the the molecular weight of the molecules that make up the gas. Humid air is _less_ dense then dry air, because the (relatively light) water molecules replace heavier O2, N2, CO2 molecules (if I remember my periodic table correctly, H2O=18, O2=32, N2=28, CO2=44, but don't quote me).<p>A plane's takeoff/landing performance goes down in humid weather because humidity reduces the density of the air and therefore reduces the lift created at a constant speed, raising the stall speed and requiring higher speeds to take off and land. Also, the lower concentration of O2 molecules in the air reduces the engine power output, making the problem worse.<br>