Humidity affect running that much?

[Birken Vogt]

Hello all, I live in Northern California where it is very dry most of the time and have an XR600. Well this last weekend it was raining at about 65 degrees and I thought I would ride in it. The riding was nice except for the puddles of muck, but the bike lit off fine and I rode it a short distance, after which it started running terrible. It would buck 5 or 6 times when rolling the throttle from low RPM and had general lack of torque all over the low end and was constantly coughing and dying. It was miserable riding the twisty mountain roads and hill climbs.

It has a stock exhaust, but seems to have been jetted for the baffle removed. I had started the day with the baffle out. Then I put the baffle in hoping it would run better but there was not much difference. In ordinary circumstances the baffle tends to make it run rich and load up (not bad though) but there was not much change this time, being as how it was already running so awful. It would run OK under wide throttle openings such as going uphill.

It was not raining to speak of, just spitting a little bit here and there, and there was not much standing water to run through. But the humidity was up around 100%.

Now when I got the pig home I went to ride it around the field, baffle still in and it was running fine. I replaced the spark plug just for the heck of it. The old one was black but not fluffy or bad looking. I had to redo both the spark plug connection where the boot screws into the wire and the brass contact where it connects to the spark plug (added a little blob of solder to compensate for the wear) but these were minor and I don't think the problem.

The only thing I could think was different was the humidity. But it doesn't seem to add up. What do you think?

Birken

 

[JasonJ]

Was there a big change in altitude as you were riding? When the humidity is up very high it can be the same as when your up at higher altitudes as the water in the air displaces O2 and the air preasuere is usally very low to begin with so air density is low. My WR ran fine this last Sunday in the warm humid rian with plenty of standing water and river crossings. I often have to adjust the idel through out the day as condidtions and terrain change but no more than a 3/4 of a turn at a time. My street bike however is funny, it actually idels up when its humid, funny.
My answer is yes humidity dose affect the bikes running IMHO but if it was that that made your bike run bad it sounds your on the edge of a jetting issue and the weather is enough to cause it to surface more pronounced.

 

[Rich Rohrich]

The effects of humidty tend to vary with temperature. High humidity with low to moderate temperatures (say below about 50) will tend to make the engine run a bit rich, but the effect is pretty minimal. While high humidity at high temperatures will cause rich running and a loss of power.

Why a difference? It has to do with the saturation pressure associated with specific temperatures. Basically when water is a vapor in the air it exerts a pressure that becomes part of the total atmospheric pressure. So lets say it's your basic Chicago day in July it's 100 degrees (F) and it's 100% relative humidity . Because we are at 100% humidity it means we've reached the point where no more water will evaporate into the surrounding air (i.e the saturation point. That water vapor in the air will exert pressure (it's a gas now) of 1.93" hg. So if we have a barometric pressue reading of 29.00, only 27.07" hg will be exerted as AIR pressure. The lower density air would require less fuel to hold our air/fuel ratio at best power 12.5:1

If we left every thing else the same but dropped the temperature to 60 degrees, the water vapor would only exert a pressure of about 0.5" hg, so our actual air pressure would be 28.5" hg. The available air density would be MUCH higher at this temp. Higher density air means we need more fuel to hold our air/fuel ratio at best power 12.5:1

For a given temperature, as the relative humidty goes up the air pressure we can use to make horsepower goes down.
Basic rules HOT and HUMID lean it out a lot, Cool and humid MAYBE lean it a little. But keep in mind as the air temp goes up the ignition delay period is reduced and engines become more knock prone, so tread lightly.

Keep in mind I generalized the hell out of this for the sake of clarity, but hopefully you see the basic connection. I really think this is easier to understand than it is to explain
There are standard charts for relating relative humidity and temperature to pressure for those of you who want to play around with this stuff. A lot of good drag and road race tuners use baro, temp, & humidity to form a density altitude (i.e pressure) value to tune from. I use the PerformAire computer from Altronics rather doing the calculations by hand with a little pocket computer. The Altronics unit calculates density altitude based on temperature, true barometric pressure, humidity, vapor pressure and dew point. It's a slick little piece and easier to handle than a fragile barometer and psychrometer.

The ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) is a good starting point if you want to really learn the science of this stuff. http://www.ashrae.org/ . Aviation sources will have similar info.

I hope this helps.

 

[steve125]

Rich, on this PerformAire computer. Could you tell me a little more about it and where to shop for one? thanx

 

[Rich Rohrich]

Steve - You used to have to buy the device direct, but Summit started carrying them so it's way easier now. www.summitracing.com

It basically produces a Density Altitude correction number from Temp, Altitude, Baro and Humidity. They now have a version that uses an O2 sensor to get more accurate corrections. It has a bunch of drag race specific features that aren't of much value for us, but overall I've found it to be a very useful tool. I sure wish I had this when we were road racing :)

 

[Birken Vogt]

Thanks for the reply, Rich. Generally when it rains here it is very cold, so I was not sure what to think. If it keeps on running good I will never know for sure I guess. This kind of event is very rare here. The first time it has ever rained on August 2.

I was thinking it earlier that day, and am still wondering...what percent of the air is water vapor anyway? This obviously depends on temperature and RH. But when "they" say that air is 78% nitrogen, 21% oxygen, and 1% other stuff, I wonder how much water vapor would be there. There must be a relatively simple formula for this?

Birken

 

[dirt bike dave]

Birken, check out this http://www.bom.gov.au/lam/humiditycalc.shtml it has a table showing that the weight of water in the air can vary, from almost nothing (extremely cold & dry) to as much as 50 grams per cubic meter (extremely hot & humid).


http://www.csgnetwork.com/relhumhpcalc.html is a link to a calculator that you can use to see how different weather conditions effect power potential.   I think the correction factors indicated by the calculator assume you are rejetting to maximize power for the conditions.  Actual power losses could be much greater if you were already rich and the air got worse.
 

 

[steve125]

Rich walk me through this a bit more :confused: Now that I have this density altitude correction number. Now what? Is there a math formula like in Erics air density guage he used to sell, still sells? that will work in the same way?

 

[Birken Vogt]

Absolute humidity, that was the term I was looking for.

Birken