The theories of air density and pressure and elevation are above my pay scale.
Anyone have a simpler explanation or even better a simple calcualion like there is 5% less oxygen in the air for every 500 feet of elevation.
why I ask is the very notable performance difference when I rode this weekend at greatly varying elevations. I rode for times at diffences of over 1000 meters. At that high of elevation the top end performance was hurting noticeably. I know enough to know that there is lees air up there so the bike will run fat right. But I am hoping to learn how much thinner the air was ,and then try to factor in the air temperature (since it was much cooler up there) to try to figure out if the elevations rarifying the oxygen is greater then the temp drops increaing the density. In simple words...even though I was high was it cool enough to actually end up bieng denser air ? Which has a greater effect, an elevation increase of 1000 meters or a temp decrease of 25 degrees?

Wow...hurts my head to contemplate this....

A few thoughts...

Relative humidity has a greater effect on air density than temp...

So does elevation....

I think its all too complicated to be able to assign a scale to this....

Too many variables...

Temp, RH, Elevation...even if these are the only 3 properties to consider, the possibilities are ENDLESS...

Any answer would be THEORY.....the ANSWER would be in experience, or a dyno...because in the end, how a particular bike and setup responds to any change in these three things is completely individual....

We all know the likelihoods of effect, but getting it down to a formula seems damned near impossible...

Interesting question, I know when we run at El Mirage and Bonneville sometimes the density altitude can be as much as 9000 ft. I myself have only seen 7800 ft on my time slip, even though we are not actually that high in elevation.

Maybe Aaron will chime in he is pretty smart about all of that stuff.

Scott

This is the best article I have found so far ...its for sleds but it does refer to mikuni jetting.
http://www.snowtechmagazine.com/articles/2001/baseline/baseline.php

Ps . At least my question wasnt as tough as this guys...
Posted: Wed Sep 07, 2005 12:37 pm Post subject: Baro at mile high

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I am searching for a ECU change/upgrade and would like info that would convince me that a 422 unit will handle the changes effected by altitude changes for my application.

I'm in Denver, and within 45 minutes, I can be twice at 11,500ft.(from a baro in the city of approx 83KPa to approx 68KPa).

I am currently running a Haltech F10 with a reference Map for Baro and it is tuned quite well at all loads and speeds for 5300ft. But it doesn't cope well with big changes in altitude, and I have to cobble the mapping for high elevations. The Baro correction scheme actually works against an increase of altitude.

The air system is twin TWM throttle bodies on a BMW 4 cy. with small intake volume. The fuel map characteristics are such from about 75 to 83 KPa (83KPa is our max atm here), the fuel curve is rather steep and from 65KPa and below it has a rather mild slope with a transition zone between 65 and 75 kPa.

At WOT as altitude increases, the engine's fuel requirements do not reduce as much as the fuel pulse is decreased by the ECU fuel curve (the fuel curve is much steeper than a correction curve for altitude, i.e. Atm density change). The change of slope in the fuel curve appears to me to be a result of "change in effective runner length" as the throttle plate opens and a stack effect changes the effective intake volume.

So in traveling from 5300ft where it is tuned, to the higher altitude, the reduced MAP sensor input causes the ECU to select the point associated with 68KPa on the fuel map (and hence reduce the WOT fuel pulse from 6.59ms to 2.85ms (57% reduction)). The atm density change from 83 to 68 KPa is about an 18% reduction. Even with a max of +50% Closed loop EGO range, there is a 35% shortfall of fuel (2.85ms x 1.5 = 4.27ms). With a reference baro map sensor, the Haltech theory is to not require any baro correction, but must I cobble the baro correction map to force a positive 50+% correction (it's more than the 35% bacause I believe the reference baro is actually worsening the correction need).

I have done much analysis on spreadsheet to display my observations.

How does the 442 unit deal with this sort of situation?

I used to live in Denver which is about 5400 feet, aka the mile high city and the first thing you do if you're a flatlander is rejet your carb. Seriously, I think altitude has a greater effect. But if you had fuel injection the problem would be easily solved since the computer would determine the correct fuel/air ratio.

The theories of air density and pressure and elevation are above my pay scale.
Anyone have a simpler explanation or even better a simple calcualion like there is 5% less oxygen in the air for every 500 feet of elevation.
why I ask is the very notable performance difference when I rode this weekend at greatly varying elevations. I rode for times at diffences of over 1000 meters. At that high of elevation the top end performance was hurting noticeably. I know enough to know that there is lees air up there so the bike will run fat right. But I am hoping to learn how much thinner the air was ,and then try to factor in the air temperature (since it was much cooler up there) to try to figure out if the elevations rarifying the oxygen is greater then the temp drops increaing the density. In simple words...even though I was high was it cool enough to actually end up bieng denser air ? Which has a greater effect, an elevation increase of 1000 meters or a temp decrease of 25 degrees?

only if it has closed loop o2 sensors....I think

The theories of air density and pressure and elevation are above my pay scale.
Anyone have a simpler explanation or even better a simple calcualion like there is 5% less oxygen in the air for every 500 feet of elevation.

I don't have a formula, but I have a simple explanation.

The atmosphere gets thinner the further you go from ground.

THINNER AIR, means LESS of everything (Nitrogen and Oxygen mostly right).

You need 02 to burn fuel, so the thinner the air, the less fuel you can burn.

The less fuel you can burn, the less power you make.


Simple enough?

I can post more later, but try looking up "density altitude" you will find all sorts of aviation links, calculations, etc. I imagine that it will help you out. After you read all about density altitude here (http://wahiduddin.net/calc/density_altitude.htm) then you can use the simplified density altitude calculation to get a good approximation...then, every time you do a run, keep a log of DA along with everything else. Once you get it dialed in at a certain DA then you should be able to (again, in theory only, and assuming EVERYTHING else is equal) repeat the performance at that given DA on another day at another actual altitude above sea level. "simplified" equation:

http://wahiduddin.net/calc/images/density/pdb_da1a.gif

where: DA = density altitude, feet
Pa = actual pressure (station pressure), inches Hg
Tr = temperature, deg R (deg F + 459.67)

http://www.csgnetwork.com/dadtcalc.html

This is one of the eisier ones to use. The final answer is the plane will fly like it is a XXX altitude. This is a little dirrerent than the HP you will get is like it is at XXX altitude. Planes need the desity of the stuff going over and under the wing. Motors need the amount of air in the cylinder.

Hopes this helps

AW

Our atmosphere is 21% O2, or .21 times the atmospheric pressure will give you O2 pressure. The higher you go the less the atmospheric pressure so O2 pressure is less also, but it is a constant 21%.

I don't have a formula, but I have a simple explanation.

The atmosphere gets thinner the further you go from ground.

THINNER AIR, means LESS of everything (Nitrogen and Oxygen mostly right).

You need 02 to burn fuel, so the thinner the air, the less fuel you can burn.

The less fuel you can burn, the less power you make.


Simple enough?

That part I already know...and I know that as you go up in elevation it is inherently chillier due to lower density of the air and the subsequent lowering of heat induced from molecular friction. So higher is always colder...and higher is also always known as thinner...so since higher is known as always thinner even though its also always cooler it seems that elevation s effect of thinning is greater then the cooler temps effect of on density....

Linkin, thanks ,I hadnt thought that the o2 in air stays constant and its thewhole mix that just thins out. I figured that the different ingredients in the mix rarified at different rates.

That part I already know...and I know that as you go up in elevation it is inherently chillier due to lower density of the air and the subsequent lowering of heat induced from molecular friction. So higher is always colder...and higher is also always known as thinner...so since higher is known as always thinner even though its also always cooler it seems that elevation s effect of thinning is greater then the cooler temps effect of on density....

Linkin, thanks ,I hadnt thought that the o2 in air stays constant and its thewhole mix that just thins out. I figured that the different ingredients in the mix rarified at different rates.

Well, you SAID SIMPLE - ;)

But if you knew what I said, then how come you missed the part in bold?

That's what I said when I said "less of everything" ;)

Yes, although cooler air is more dense than the hotter air AT THE SAME ELEVATION - the higher up you go the air still thins out.

It's possible that at slight elevation differences the temperature difference may make 02 content close to equivalent at times - you'd have to work the forumulas and take into account weather changes too.

Another way to think of it is why we use the term atmopsheric PRESSURE - because the gases are sitting on the surface, pushing downward, and just like the deeper you go when diving the more the water above you pushes down on you and causes more PRESSURE, so is the case with the atmosphere.

The higher up you go, the less "atmosphere" is pushing down on you, i.e. less pressure, less gas.

Kev

Did cattch that about everything ,just wanted to spread the kudos around mate.

Did cattch that about everything ,just wanted to spread the kudos around mate.
lol - ok, as long as you got what you needed (and it looks like you got a lot from some of the other dudes)...

K

Relative air density calculator, easy.
Follow the link, http://www.dvbrc.com/rad.htm


: Mike :tour