How important is humidity.

alvin l nunley

Site Supporter
Barometric pressure:
assuming 29.235@77° equals 1.000 correction factor that would mean 29.000 at 77° the air density correction factor would read 1.010.
 

fatboy1dh

Member
Barometric pressure:
assuming 29.235@77° equals 1.000 correction factor that would mean 29.000 at 77° the air density correction factor would read 1.010.

It took you a whole page of replies to finally say what you have been wanting to say. Humidity does not change correction factor much, but change in air density does make a huge difference. Often, the change in humidity that people claim they are changing for also comes with a change in barometric pressure. So, they may not be getting to the right answer for the reason they think they are, but they are still getting there. Sometimes you dont need to understand why a change works, you just need to know it does. So, to Paul's point...

When you feel the "good air" come in, adjust your fueling and keep up. Who cares if its barometric pressure or humidity or temperature? As long as you are keeping up, you are doing what you can do.
 

jaymancds

Site Supporter
It took you a whole page of replies to finally say what you have been wanting to say. Humidity does not change correction factor much, but change in air density does make a huge difference. Often, the change in humidity that people claim they are changing for also comes with a change in barometric pressure. So, they may not be getting to the right answer for the reason they think they are, but they are still getting there. Sometimes you dont need to understand why a change works, you just need to know it does. So, to Paul's point...

When you feel the "good air" come in, adjust your fueling and keep up. Who cares if its barometric pressure or humidity or temperature? As long as you are keeping up, you are doing what you can do.
Getting to the right answer, the wrong way, is perfectly fine as long as you don't teach anyone else how to do it :ROFLMAO:
 

alvin l nunley

Site Supporter
Barometric pressure can go up or down. Temperature can go up or down. In either case, neither, necessarily, will have an effect on the other. As barometric pressure increases and/or decreases, it does not necessarily have any effect on the vapor content of the air. Same for temperature.

Relative humidity; the amount of water vapor in the air compared to the actual amount of water vapor the air could hold at that temperature. If the actual water vapor in the air remains constant, and the temperature goes uo, the relative humidity goes down. For instance, and I don't have the exact numbers here in front of me, let's say 60° and 60% relative humidity. It warms up. Now it's 70° and 50% relative humidity. The amount of water vapor in the air has not changed. Not real numbers, just an example of what really happens.

If you're waiting for the humidity to give you an indication of the need to do some tuning, I'm afraid you going to have a long wait
 

Pete_Muller

Moderator
I believe that looking at any one variable in isolation does not give the entire picture, especially if it's warm or hot.

Cool air simply does not have much moisture carrying capacity, whereas warmer air does. Going from around 60F to 100F roughly quadruples the amount of water vapor which can be held in the air.

Drag racers calculate "grains" of water in the atmosphere, and seem to really fight the tuneup over a certain point (irrespective of temp and pressure).

I would think that the same issues would effect the engines being run in LTO racing.

PM
 

alvin l nunley

Site Supporter
The atmospheric ability to hold water vapor increases as the temperature of the air increases. It's an exponential curve. At 100° the air can hold 4% water vapor, a gas, like any other.

When it's hot, you perspire, if the air is at, or near, but in the morning its saturation point (holding all the water vapor it can) your perspiration has nowhere to go, so you sweat. It's a simple thing.
 

alvin l nunley

Site Supporter
In a previous post I mistakenly said "exponential" curve when I should have said "parabolic" curve.

Parabola: a line with a constantly increasing/decreasing radius curve.
 

Pete_Muller

Moderator
Paul,

I believe that it's not that simple. Since the same air density number can happen at different pressure/temperature combinations, higher vapor density (more water vapor) would be possible at whichever density number has higher temperature.

And Al, you had it right the first time: it's exponential, not parabolic. The formula to calculate vapor density contains both a Tc-squared and a Tc-cubed (fractions of both of those, though the exponent is still significant). Tc = Temperature in Centigrade

PM
 

paulkish

old fart
I Added: Yes you can have the same air density at different temperatures and pressures, (from the net)>humid air is lighter than dry air at the same temperature and pressure. <<<< so it gota be true

I figure you all are correct and there must be some left out variable or "constant".
Might you two be reporting about weather in general and what I found on the net refers to a constant volume?

Isn't what I found the reason why rain clouds are up not down like fog?
Or is the vapor in the "outside" air up or down strictly because of temperature.

Dumb backyard thinking more on it if humid air is lighter then wouldn't there be more room in it for fuel vapor?
Yes learning from al vapors are all similar in density if I got it right, then you can control the amount of fuel vapor but you cannot control the amount of air vapor. yep all proly laughable wandering words ... :)

hummmmmm, why don't we carry on board both our fuel and air and not have to deal with what the outside air is doing?
hummmmmm again, dragsters are long could they use their length for drying air to be burned, the blower could suck air thru a dryer beyond air pressure.

Which brings me back to if an engine is mechanically supercharged beyond air pressure isn't the blower sucking air in and pushing it into the cylinders?

dumb but it was fun to write
 
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paulkish

old fart
Another dumb thought.

If a supercharger relied only on atmospheric pressure to push air in then wouldn't it create a vacuum at the air entry point limiting its intake to atmospheric pressure?
If that's the case it has to be able to suck an amount of air in beyond what atmospheric pressure can supply via push.
Seems like then when a vacuum is made though the distance between molecules of air or what ever are stretched apart they remain connected and it simply takes more effort to move them. It takes more effort because as vacuum increases each molecule has to be moved a greater distance to become usable in the compressor. Scooping them up by the compressor intake moves the molecules closer and closer together as the dragsters speed increases.

I wonder if to be accurate setting fuel needs if you have to separate your measurements of fuel and air inputs by what is vapor and what is not vapor?
Maybe fuel needs are really about the amount you compress needed vapors followed by how the burning or exploding vapors are able to cause residual liquid to burn. That is assuming fuel is put into a cylinder in both vapor and liquid form.
The next thing this makes me think about is that additives are to either make fuel vapor easier to ignite or liquid fuel easier to burn.

sorry, where's that next coffee?
 

Pete_Muller

Moderator
Paul,

In simplest terms: pressures try to equalize, period.

Since a complete vacuum is the lower limit (zero pressure), and generally speaking, atmospheric pressure is about 14.7 psi, that is the maximum pressure differential available, period. (at least if "one side" is atmospheric).

Think of a supercharger in another way: imagine having an enormous tank that has zero air in it... absolute pressure is zero. Now have an enormous tank that has 14.7 psi in it. Connect those two with a one quarter inch ID hose that is 10' long. How fast will the air move that is trying to equalize the two giant tanks? Not very fast, right? But now connect them with three 4" holes of virtually no length (like he front face of a blower), how fast will the air move through those holes? (and remember that the "enormous tank" with 14.7 psi is virtually unlimited since it's the size of the surface of the earth).

That's why a supercharger can "take in" so much air that it can feed a 500 cubic inch engine with some ungodly high pressure. (additionally, a forward-facing supercharger inlet at 200 or 300mph is "presenting" a pressure higher than atmospheric to the front face of the blower). The back side can still go no lower than 0 psi though (and it won't be that low).

Once again... here on the surface of the earth, it's only possibly to get a 14.7psi differential to a pure vacuum. Pressures try their best to equalize.

PM
 

alvin l nunley

Site Supporter
PS I, pounds per square. Have you ever seen a 5 gallon gas can have the air pumped out of it, it collapses, and you know there's no way that there was a perfect vacuum in the can. That's tank on the back of the propane truck, I've seen pictures of something like that being collapsed just from the ambient air pressure. It was on a flatbed railroad car. Don't know about the circumstances but it was crunched like that 5 gallon can. In the movie the Martian, I calculated the pressure against that plastic sheet he put over that hole in his living space. Needless to say, duct tape would not have held it.
 

alvin l nunley

Site Supporter
I lost track. Pete's familiar with a 471 GMC lower. How many square inches on the intake side of the low. Then there's the fact that the lower creates a constant pressure in the intake track. Both valves are only open for a limited time of the720° of each cycle but the blower is supplying a constant supply of air, thus raising the pressure on the backside of the valve.
 

flattop1

Dawg 89
PS I, pounds per square. Have you ever seen a 5 gallon gas can have the air pumped out of it, it collapses, and you know there's no way that there was a perfect vacuum in the can. That's tank on the back of the propane truck, I've seen pictures of something like that being collapsed just from the ambient air pressure. It was on a flatbed railroad car. Don't know about the circumstances but it was crunched like that 5 gallon can. In the movie the Martian, I calculated the pressure against that plastic sheet he put over that hole in his living space. Needless to say, duct tape would not have held it.
I have seen steel petroleum tank roofs collapse via not opening the vents when draining . Its a semi common occurance . Especially after painting or testing .
 

Ted Hamilton

Design Drafter / Racer
Anecdotally.... I grew up racing a vehicle similar to a 1/4 midget with a 5 hp WKA stock briggs flatty. I happened to have a remote carb tuner installed and nobody else did. This was my Mark Donohue "Unfair Advantage." We rolled off for the feature, and I was my typical mid-pack position (primitive chassis design vs. SOTA front runners.) But there was a yellow flag mid-feature (18 laps into a 25 lapper.) While we were parading under yellow, a massive fog bank rolled in and air temp dropped 10° or so. I put my unfair advantage to work, and when we got the green, I gapped the field on the first lap by some 10 car lengths. Nobody was able to catch me for the balance of the race, and I scored my first win.

This was proof positive to me that atmo conditions (including humidity) absolutely matter, and anything less than optimal jetting is throwing away power -- and possibly substantial power. Now I'm open to discussion on WHY it matters -- does the higher vapor content expand the volumetric envelope? Does it affect the combustion process? I haven't worked it all out yet. But I also have done a fair amount of running for sports, and on a high humidity day, it's much more work to breathe, and the available oxygen is apparently less. Winter air that's dry and cold is still easy to breathe, but also more oxygenated. This correlates with what my race engines seem to indicate too...

It's a shame Al can't carry this conversation on, but I hope we do... If the theory doesn't match the data, get a new theory.
 
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