Effects Of Elevation & Air Pressure And How To Jet Accordingly

Blaner

Your Friendly South African Ambassador
Mar 26, 2008
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East London, South Africa
There are a handful of articles on this forum that are focused on jetting and have excellent information on the steps and procedures that should be carried out in order to get your carb jetted right. What I have noted however is that all of these as well as most advice given on here suggests that everybody’s jetting will be different depending on the air volume demanded by the motor. Obviously, motors in different states of tune will require different volumes of air to run correctly and hence different jetting for each motor is required.

However, at a recent ride in early 2011, my baster seemed to be running too rich and wouldn’t idle during the ride. It was fine at home before we left and fine again upon returning home. I immediately knew that the higher altitude and chilly temperature was the cause of this change I looked into the specifics and found that the area that I had ridden at is on average 900m above sea level. Sea level being the elevation at which the bike was jetted at. So I realized that altitude as well as air pressure plays a much bigger part in jetting your carb than most of us care to know.

Why is this so important? Well, for me, having my bike jetted at the coast, the effects of moving to a higher elevation was not terminal as it caused me to run rich, BUT for anybody who has their bike setup to run at high altitudes is potentially threatening their motor upon using the bike at coastal altitudes. The effect will be the opposite of what I experienced and will result in lean conditions which we all know to be the ultimate killer of engines!

So this article serves as a detailed guide to the things you need to know to understand the effects of altitude on air pressure and hence the jetting changes that need to be made when changes in altitude are made.
I consulted a Professor Deon Raubenheimer, a professor of physics at the Nelson Mandela Metropolitan University at which I study. He has been instrumental in my understanding and putting together of this report.

To start with we need to know a few basic things about air.

The Composition of Air:
Air consists of various gases, namely Nitrogen – 78.09%; Oxygen – 20.95%; Carbon Dioxide – 0.03; Hydrogen – 0.00005% and Argon – 0.933.
For our purposes, we will round this off a bit to Nitrogen = 78%; Oxygen = 21%; Others = 1%

airfractions-1.png


  • The water or vapor content in air varies. The maximum moisture carrying capacity of air depends primarily on temperature – this is what we know as HUMIDITY
  • The composition of air is unchanged until elevation of approximately 10000m
  • The average air temperature diminishes at the rate of 0.6˚C for each 100m vertical height
  • Air pressure is likened to the weight of the entire atmosphere pressing down on us under the influence of gravity.
  • "One Standard Atmosphere" is defined as the pressure of all the air above us pressing down on us at sea level and at standard gravity this is 1.2250 kg/m3 or 1 Bar or 100kPa (I will be using 100kPa for the calculations)
  • At sea level, the availability of oxygen is 100%.
  • The composition of air doesn’t change with altitude, therefore at 5000m there is still 21% oxygen in the air, but there is less air as the molecules are spread further apart and hence are less dense therefore the pressure is lower. So although there is 21% oxygen, that 21% is made up of less oxygen atoms than at sea level – this is known as Boyles Law.

The relationship between Altitude and Air Pressure.
I must make it clear that it is not altitude that is responsible for the changes in air but rather it is the air pressure that will affect the running of a motor. Air pressure is always a variable and changes with changing elevation, but it is not limited to changing altitudes, for example, the air pressure can change at a fixed altitude, this is what weather is! We can have pressure lower than 100kPa at sea level due to low pressure systems and have pressures higher than 100kPa at sea level.

For our purposes we will simplify this and not get carried away. We will assume that at sea level, the pressure is set at one standard atmosphere or 100kPa.

So what happens when altitude increases, well, as altitude increases, the air pressure drops. The rate at which this happens is summarized in this graph:

elevationchange.png


From this, we can see that the Oxygen concentration relative to seal level decreases obediently with the decrease in air pressure as altitude increase. We can also see this is a linear regression, therefore making it very easy to calculate our jetting needs!

Variables of Air
There are a number of variables of air that we need to consider in order to fully understand the effect of atmospheric conditions on the correct running of a motor, these are humidity and temperature and obviously elevation which I have already dealt with.

So, I’ve mentioned that for every 100m, air is 0.6˚C cooler than at sea level, so what does this mean? Well, like pressure, cooler air is more dense than hot air, meaning the air molecules are packed together more tightly, hence you will be getting more oxygen into your motor when it is cold compared to when its hot making your bike run rich and vice versa.

Moving on to humidity is where I had the most fun! My logic dictated that moist air would be denser because of the addition of water vapor to the mix of gas. I then decided that more O2 would be available in moist air as opposed to dry air.

I did a bit of research on this and consulted Prof Raubenheimer, he directed me towards the Ideal Gas Law, which states that says a certain volume of gas at a certain pressure contains a certain number of atoms/molecules; that means since liquid water is more dense than dry air one may argue that moist air is more dense than dry air. But this argument is irrelevant since the law above is the IDEAL GAS LAW, and not the IDEAL LIQUID LAW. We may therefore not compare these different phases with one another in the same equation – it simply confuses the issue.

Explanation: You see the air surrounding us is made up of mostly nitrogen and oxygen molecules with a bit of water vapour as well. Now N2 has a molecular weight of 14 x 2= 28, and O2 a molecular weight of 16x2=32; BUT, H2O however has a molecular weight of only (1x2)+16=18 - that means it is less dense than either of the other two mentioned.

If you have a certain volume of air which contains some water vapour (i.e. moist air), it will therefore weigh less than the same volume of “dry” air because Density = Weight/Volume Therefore moist air is thus less dense than dry air – meaning in humid conditions, relevant to that in which the bike was set up, your bike will be running rich and vice-versa - Aweh!

Now once I had that all sorted out, I pondered the likelihood of the Hydrogen in water vapour adding to the combustibility of air, thus, my question was, if moist air is less dense would not the increased presence of Hydrogen be conducive to enhancing the combustion process thereby essentially nullifying the decrease O2 pressure in moist air?

I again chatted to the Prof who pointed out that sure, Hydrogen is highly flammable in air (remember the Hindenburg?) but I was trying to use the reaction backwards, you see, H2O is a by-product of hydrogen combustion, I was now trying to get my H2O to combust backwards- what would the end product be? – It’s never going to work! After a lengthy explanation on the enthalpy of combustion, that theory of mine was destroyed!

So, to put it all together, increased humidity decreases the air density therefore providing less O2 for you motor to burn – Exactly the same effect as increased altitude has on air pressure. Added to that cooler temperatures provide more O2 for your motor to burn as opposed to hot air,

An important thing to remember her is that it is not the fraction of O2 that changes out of the air, that is ALWAYS 21% until you reach 10 000m but rather it is the PRESSURE of the air that changes. Remember High pressure is like packing more air into the same space as you had when you had half the amount of air.

Jetting Correction Methods:
All this pressure talk is all well and good but unless we can do something about it, out jetting will always be off when it changes.
This is where jetting correction factors come in, we need to calculate a correction factor and use it to calculate the size of the jet we need to run when we change our riding elevation, temperature or humidity

I have come across Two methods of solving this problem. The first is via a calculation Prof Raubenheimer advised me on.

Poisuelles Law

jetting5.png


*NOTE: that is the 4th root of the whole function p2/p1 and not just of p2

The correct jet diameter d2 is sized according to the fourth root of the ratio of the air densities/air weights. You see according to Poiseulle’s Law the flow rate through a jet is proportional to the fourth power of the diameter of the jet opening - make sense!

That means if the jet size at sea level (where P1 = 100kPa) is a d1 (or 260 - the jet number being a measure of the jet size) - then the jet d2 at another place on earth can easily be calculated if the air density (pressure) p2 is known!

So to use this in a real example: I am using a 260 main jet running correct at sea level. So my d1 = 260. And p1 = 100kPa. Now I want to ride at a spot 2000m above sea level, so I look at the graph I inserted earlier and see that at 2000m, my air pressure is roughly 80% that of sea level and 80Kpa. So p2 = 80kPa

Now I use the formula to work out the new jet size I need:

d2/260 = 4√80/100

Therefore

d2 = 260 x 4√80/100

d2 = 245.89 ----so our new jet will need to be a 245.89 or rounded off to the nearest size, a 250 jet.

This method is extremely accurate with regards to pressure changes but one shortfall is that you will need the pressure of the region you plan on riding, temperature and humidity are not directly included in this formula but technically they are because air temperature and humidity both act to affect the air pressure, hence this formula uses the final pressure figure which is the net result of all the atmospheric variables, so it is the perfect method of determining your jet sizes!

This can be used for both main and pilot jets!

Correction Factors

The other method is a bit more prone to flaws, it is a system developed that uses a percentage fraction multiplied by your current jet size which gives the new jet size.

N = B x P
Where:
N = new jet size
B = Base (original) jet size
P = Percentage

This method also relies on your jetting at your “normal” pressure/elevation to be correct. It also puts a strong emphasis on temperature. Each person will have their own personalized chart, this makes it tricky to explain but I will use mine to explain. My jetting is correct for my bike at sea level (0m) at 15˚C, hence this is my 1 or 100%. (See the table below, yellow block) you can see as you look around the table, the figures vary from 0.86 to 1.04 these numbers are the CORRECTION FACTOR and are a PERCENTAGE of your jet size. So at 15˚C, 0m elevation, I have 1, as mentioned, meaning my jet size is 100% of my installed jet size (duh!) so 0.95 means I need a jet 95% the size of the one currently installed at my “Home Base” of 1.

jetting1.png


So, at 2000m and at 15˚C, I need a jet sized 0.94 or 94% the size of the jet in at 1, so I use the formula

N = B x P

Where:
N = new jet size
B = Base (original) jet size
P = Percentage

To work out my required jet size at 2000m 15˚C. So,

N=260 x 0.94
N=244.4 - - -my new main jet size!

EG:
jetting4.png

NOTE: in the Picture Eg, i used a 250 main, whereas i have descibed a 260 main in the text

Compare this to Poiseulles formula and we see a connection: 245.89 vs 244.4 –very close!

There is however a discrepancy between the two methods of 1.49, this can be attributed to the inaccuracy of the correction factors in the graph. The increments of increase/decrease in the correction factor table are set to 0.01 or 1% for every 10˚C, I do not know if this is accurate as one could use 0.005 or 0.5% per 10˚C and have it more accurate, either way, I maintain that Poisuelles Formula is the best way to go.

Again, this method can be used for both pilot and main jet.

Here is an example of a fully calculated chart based on a main jet of 250 at sea level at 15˚C:

jetting3.png


Setting Up Your Own Correction Factor Chart:
So what if I live at 2000m above sea level? Your chart will be different to this, your 1 or 100% where your jetting is spot on at let’s say 20˚C will be in the block lining up from 2000m and 20˚C. once you have established that, fill in the block moving at 0.01 or 1% increments for every 10˚C and likewise for every 500m elevation, anything cooler and lower than your “home base” will be bigger than 1 and everything warmer and higher than “home base” will be smaller than 1. (This is exactly why Poisuelles formula is much simpler to use!)

Other Carb settings:This little table shows you what other carb settings must be made based on correction factors.

Jetting2.png


So I hope some of you who took the time and effort to read and understand these benefits from this post and are able to gain a better understanding of the calculations needed with respect to jetting and appreciate that the atmosphere in which we live is just as important to our bikes optimal performance as any other engine mod.
Please feel free to ask any questions you may have!
I am available for helping anyone interested in working out their own correction factors, just PM me.
 
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Rep fully, this is more than an awesome right up and much needed for those such as myself who are constantly jetting/effected due to the elevation I live in.

Sticky.........................Sticky
 
Hi guys, LUNI has asked me to fiddle with these formlae to help get him in the ball park jetting wise. We run a similar setup but i run a OKO30mm and he uses a PWK28mm.

I have used the jetting i have found to work for me AT SEA LEVEL and have punched them into the system to see what he should use at 8000ft (2500m)

Here we go!

I run a 165 main, 58 pilot and a JJH needle, bottom clip.

because he is at 8000ft/2500m, his air pressure is roughly 75kPa

POISUELLES FORMULA:

D2/D1 = 4√P2/P1

D2= Luni's new jet size
D1= my jet size - 165 main
P2= Luni's air pressure - 75kPa
P1= my air pressure - 100kPa

SO: D2=165 x 4√75/100
D2= 153.54 MAIN JET

punching in the same formula for the PILOT = 53.97



Using the Graph Method:

LUNI.png


The control is set at the yellow highlighted box, with 1 in it, this means my jetting is correct at sea level. he correction index at 2500m is 0.93.

So: 165x0.93= 153.45 MAIN
58x 0.93 = 53.94 PILOT

you can see this as the blue highlighted boxes!

So LUNI's jetting should be: 154 MAIN; 54 PILOT. Obviously, you will need to use the next available sizes.

As for the needle, probaly one leaner than the JJH would do.

Hope this helps!
 
I have man. I started at a 165 and worked my way down. I think Im now at a 140 with the dial a jet (it compensates up though if you need it)

I think my pilot is pretty low though. I know its too rich though, cause it fouls plugs out at higher elevations. I believe Im at a 38 on the pilot.

Theres more to it than that calculator can calibrate for. But I agree its a decent ballpark.
 
i have a 2005 blaster just redid the whole thing.its a stock motor 10 over shear pipe carbon reeds.what jets should i have seems to be lean.i live in detroit michigan wich is 850 ft above sea level.have a 310 main 32.5 pi not sure on needle yet.
 
i have a 2005 blaster just redid the whole thing.its a stock motor 10 over shear pipe carbon reeds.what jets should i have seems to be lean.i live in detroit michigan wich is 850 ft above sea level.have a 310 main 32.5 pi not sure on needle yet.

Hi, firstly this post is not designed to help you set up your jetting at your "homebase" location. It provides info on how much to change your jetting from your homebase optimal jetting to a new jetting when you take your bike and ride at a different location.

You will need to perform normal jetting procedures in order to work out your homebase jetting such as plug chops.

but for the record, a 310 seems fine and the 32.5 pilot is also fine. Id suggest having a look at the needle and drop the clip one position.

Do a plug chop to check your main jet.