I'm about to put myself in the "WTF do you mean" category again!

Here I sit wondering about life and what the hell 15 #/in.^2 really means?

I'm almost affraid to read the replies in fear of looking like a complete blonde!

So, tell me, am I lifting ~ 15 pounds of 'air' per inch squared at sea level as I
raise my hands to the sky?

Is atmospheric pressure measured in a different type of POUND than a weight lifter's
barbell?

Maybe there is 15 pounds/in.^2 around me and I'm able to displace this air
with ease, but I'm not feeling the "weight" because there is equal pressure
around me?

 

you feel the weight as pressure on your skin. its pushing on you from every side, so you just dont have a point of refrence to compare it to.

while it is applying 14.7lbs of pressure per square inch on you. you wont feel "weight".. but you will feel resistance if you try to "push the air out of the way" with motion...


if you want a point of refrance, just stick some kind of vacume on yourself.... athough the pressure will cause the blood to rush there..... just tell everyone its a hicky. :P

 

http://school.discovery.com/lessonpl...rcesandmotion/



-Mindgame

 

I always understood that a lower barametric pressure or lower pressure, Will always help your car run faster. The lower the pressure, The easier it is for your car to move through the air. If there is less pressure then it would be easier for a car to move through the air, Almost having less resistance. At least, This is how I understand it.

 

You couldn't be more wrong. The amount of air that flows into the cylinder when the piston drops is directly proportional to the barometric pressure. Add 10% more pressure, and you have 10% more air in the cylinder, add 10% extra fuel, and you make 10% extra HP.

The effect of the air pressure (density) on the aerodynamic drag pales by comparison. Take your 14-second car to Denver, and see how quick it runs the 1/4-mile in that nice thin, low pressure air....

 

We live in a "sea of air" which goes from the surface to maybe 60 miles or so high. Because air is a fluid (gas) the upper molecules push on the lower ones so the total push or pressure at sea level (earth's surface) is the combined weight of all the molecules stacked about 6o miles deep. On a standard day, that's about 14.7 lbs of "push" or force per square inch. IOW, a 1 inch square column of air from sea level to the upper end of the atmosphere weights 14.7 lbs. This is barometric pressure.

You've felt the pressure change dramatically when you swim under water. Because water is denser than air, a few feet of water (I'm too lazy to do the conversion now) can vary the pressure 14.7 lbs.

Now about half of the air (and therefore pressure) is in the first 18,000 ft. above sea level. There's really not much air above 80-90000 ft. Some aircraft can fly at 80,000, but to get enough lift from the sparce number of air molecules they have to go fast so that lots of molecules pass by the wing.
The closer to sea level the more densely the molecules are packed together.

Yes, that 14.7 psi pushes in every direction. It's also inside you pushing out, or else you'd collapse.

An NA engine inhales air by creating a slightly lower pressure inside the cylinder during the intake stroke. The higher (relatively) atmospheric pressure forces the air into the lower pressure cylinder to balance the pressure. The engine doesn't really suck air in, it just lowers the pressure inside a little and the atmosphere does the rest. Your lungs do basically the same thing.

Global weather systems have high pressure and low pressure areas. Guess what: air flows from a high pressure area to a low pressure area. We call that airflow.......what? (2 point question)

FWIW, higher pressure means more molecules per a given volume, so higher barometric pressure means more air molecules injested and more power. We normally correct dyno data to some given pressure like 29.92 in. hg. which is about 14.7 psi. If barometric pressure in the test cell is higher than the standard pressure, corrected power is lower than measured and if test pressure is lower, corrected power is higher than observed.

The lower pressure air does decrease aero drag, but generally the lower power it provides makes a bigger difference and makes the vehicle slower, not faster. [oops! Injuneer beat me to it.]

MG is right; this should have been covered in earth science or some such subject where many of us were reading car magazines. (negative 2 points if that was you)

FWIW, those "little pages" of Rod & Custom of the 50's and 60's easily fit inside almost any text book. Even the nuns didn't find them.

My rambling $.02

 

I think Im thinking backwards, More barometric pressure means more air in cylinder right? So you want a higher pressure?
Alright, Another thought, Pressure relating to altitude and Map sensor readings?
Will a map sensor read closer to 100 kpa if the pressure is higher, In other words, The higher the altitude the lower the Barometric pressure? So if you were at 4000 ft. elevation like me, You would only pull 88-90 kpa due to the fact of thinner air and less pressure, Meaning less air and fuel in the cylinder. Explaning why cars run better at sea level, Higher pressure = more air and fuel in cylinder.

 

I can attempt to answer Kryckter's question.

Think of higher pressure meaning, more weight of air. If the barometer is high,
imagine 60 lbs of air sitting on the piston at top dead center (TDC), as opposed
to 30 lbs of air..

Once the piston moves downward and the valve opens, there is a low pressure
area created in the cylinder and the higher weight of air comes crashing into
the chamber.

This low pressure area depends on several things, including intake runner
volume, piston speed, displacement, RPM, engine resonance, etc.

The lower the pressure inside the cylinder, the greater chance of cramming
more air into the cylinder.

If the pressue inside the cylinder was higher than 14.7 PSI at the piston moved
downward at sea level, there would be little to nothing gained (which technically
should never happen unless the exhaust valve went on vacation).

As for my third thought in the first post, I now see the light thanks to many of
your replies!

(OldSS: My car magazines were substitued for Playboy at the time! )

 

The motor will ingest a certain VOLUME of air over a given time. Air is a gas, so it compressible. When it is under pressure, there is a greater MASS (greater number of molecules) in the same volume. So, higher barometric pressure = more air molecules per volume of air = more oxygen molecules to combine with more fuel molecules = more power (if the fuel needed is provided).

Ever think about why it's a MASS air sensor and not a VOLUME air sensor? What counts is the mass of air the motor takes in, not the volume.

Rich

 

So, In theory, People will have more Barometric Air pressure at sea level vs. me at 4000 ft. elevation? Cause from what I am understanding , The MAP sensor will read closer to 100 kpa at sea level if everything is right vs. me running 88-90 kpa at my elevation. Is most of this due to thinner air and the lower Barometric pressure reading? In other terms, This is why cars run better at sea level. More air molecules in the air, Denser air. And This would be why most cars at higher elevation will still pull vacuum in the intake when WOT due to thinner air and less air molecules. In turn...Less HP!

 

More air the better.

 

What if we were driving/racing an electric powered car? It seems to me that an electric car might actually run quicker up in Denver than it would down here in Houston due to less aero drag.

 

That was my original view on the subject. But The HP amount of the denser air is far more substantial than the denser air slower you down.
Take in example for the electric car idea being faster in Denver, Look at baseball, it is alot easier to hit a home run in Denver than it is Houston, Because of thinner air. But when you compare the thinner air to the amount of HP you get from denser air, It is not comparable. More air molecules in the engine will help you out alot more than the thinner air letting the car move easier.

 

Yeah, but trying to stretch a single into a double about winds any lowlander player! A few miles south of Denver at the top of Pikes Peak (14000 or so ft), just walking acros the parking lot is a challenge!

 

Ahh the joys of racing at altitude. The engine makes less power and you don't break as many parts.

Yes you want high barometric pressure but you also want low humidity and cool temperatures. These 3 plus the altitude create a density altitude number. Someone says they raced a couple of weeks apart and the temperature and humidity were the same but the ran slower. Chances are the barometric pressure was much lower causing the slower runs.

There are pros and cons to high pressure. Although there are more air molecules to burn, the air is also denser meaning it's thicker to push through. You make more HP from the dense air but you eat up some of that extra HP just trying to push through it.

Humidity is another factor. Water molecules will take up some of the space the air is trying to use. There is a lot more water in the air at 50% humidity and 80* than there is at 50% humidity and 60* because warm air can hold more water. You need to know what the dew point is to really know how much water is in the air. Basically if the humidity is high and the temperature is high, the engine is trying to burn a lot of water vapor along with the air.

Using a power adder sort of eliminates the need for high barometric pressure since the power adder is forcing more air into the engine, it doesn't rely on the engine's ability to suck available air in to mix with the fuel.

Remember superchargers and NOS were used in WWII so that the fighter planes wouldn't lose power at high altitude for the same reasons.