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Air Density: Explained

Bob Chiras
Crew Chief
AllKart International

#1000 - RaceAir Pro™

RaceAir1000 Price: $479.95

Air Density is a term that we hear around the race tracks all across the United States and when I ask what the person means when they say the words Air Density I get a host of answers and I would venture to say that most of the answers have nothing to do with the meaning of the term.

Air density is a combination of two factors: barometric pressure and temperature. At 85 degrees Farenheight and at 29.92 inches of mercury for barometric pressure (at normal sea level) air density is considered to be at 100%.

Most racers consider their motors to be air pumps and many believe that air flows into the cylinder because it is being sucked into the motor through the action of the piston. A complete fallacy. Air flows into the motor because air has weight. The weight of air is called air pressure. It is 14.7 pounds per square inch at sea level.

If your motor is a Briggs and Stratton and the allowable bore diameter of your carburetor is .695 inches, which translates into .380 square inches. The math is 14.7 X .380 =5.59 pounds of pressure assuming that we had a perfect vacuum in the Briggs motor.

We can do a similar calculation for a 2-cycle motor. Let’s take a Yamaha with a Walboro carburetor. The allowable diameter of the inlet bore is .950 inches, which translates into .790 square inches. The math is 14.7 X .790= 10.42 pounds of pressure assuming that we had a perfect vacuum in the Yamaha motor.

You can do the calculations for the motor you use and find out how much pressure is being forced into your motor.

Altitude and weather systems change the air's pressure. As you go higher, the air pressure decreases from around 1,000 millibars at sea level to 500 millibars at around 18,000 feet. Most of us race at less than 1000 feet of elevation. Weather systems that bring higher or lower air pressure also affect the air's density, but not nearly as much as altitude. Air density is lowest at a high elevation on a hot day when the atmospheric pressure is low, say in Denver when a storm is moving in on a hot day. The air's density is highest at low elevations when the pressure is high and the temperature is low, such as on a sunny but extremely cold, winter's day in New Hampshire. Humidity and air density Most people who haven't studied physics or chemistry find it hard to believe that humid air is lighter, or less dense, than dry air. How many times have you heard someone tell you to add more gear on a hot humid day because it is harder to push the kart through the hot humid air. The inverse is really true, the kart flows easier through the air but the pressure needed to fill the cylinder with the proper air fuel mixture is lessened by lowered air density.

A proper air fuel ratio for our race motors is 13 parts air to one part fuel for the Yamaha 2-cycle gasoline motor and 6 parts air to 1 part alcohol for the Briggs motor. In any internal combustion motor the key is attaining the correct ratio of fuel and air to achieve the most efficient burn. The result of the optimum burn is that the piston is pushed down fast and with as much energy as possible in order to produce maximum horsepower and torque.

It is a lot less math and a lot easier to look at an air density gauge than to do the math necessary to calculate changes for each day. Our air density gauge is from Intercomp, of Minneapolis MN. It is a model 360093 and is priced at $117.00

The goal that we all strive for in racing is to achieve the perfect air/fuel mixture to get the optimum performance from the motor.

Each intake stroke allows a volume of air into the cylinder but density or weight of that volume will vary greatly from day to day. If the density or weight increases and we do not increase the fuel sufficiently the mixture will be lean and this usually means that we stick pistons. If the density decreases and we do not take away fuel we run rich and our performance is flat.

Using an Intercomp air density gauge is very simple:

1.   Open the container; place the gauge in a horizontal position in a shaded area.

Our choice is on top of the toolbox in the trailer.

2.   Let the gauge stabilize while you do your normal race preparation. (1/2 hour)

3.  Using the information provided, select the correct jet.

Well use an example for a Briggs

The last time you ran the kart the air density was 94% (you did write this down with the other notes that you keep). The jet that you ran was a .60. (this also came from the notes, or did you have to remove it from the carburetor to tell). Today the air density gauge is reading 86%.

The formula is 86-94= -8/94= -. 085  (or minus 8.5%)

The minus sign tell us that there is less air density, we know that we need less fuel for lower air density. We need to reduce the main jet by –8.5%.

Intercomp has made this easy for us they provide us with a table that accompanies the instructions. The table tells us that .60 jet has an area of .002827 and that to subtract –8.5% is –. 000240 so the ideal jet is .002587. The closest jet to this size is a 57jet. It becomes as simple as reading the gear ratio chart after a few attempts.

If you are a 2-cycle racer the answer is that you close the needles on your Walboro carburetor. This is where an experienced tuner can adjust the needles to attain the same desired exhaust gas temperature and can examine the spark plug and determine that the correct adjust has been achieved.

When I started karting my mentors were convinced that relative humidity had a great deal to do with performance and for years we listened to their theory on the impact of relative humidity. When we acquired a Land and Sea Dynamometer and began to read the manuals relative to correction factors I came to realize that we were placing far too much emphasis on relative humidity. For most of us the normal range of humidity ranges between 55% and 85%. The correction factor for humidity only changes .009 from 55% to 85% humidity. Even taking 200 data samples per second it is very hard to see the impact of humidity on your motor on the dynamometer. 

The horsepower and torque available from a normally aspirated internal combustion engine are dependent upon the density of the air... higher density means more oxygen molecules and more power... lower density means less oxygen and less power. Remember the relevant term is air density and not relative humidity.

The relative horsepower, and the dyno correction factor, allow mathematical calculation of the affects of air density on the wide-open-throttle horsepower and torque. The dyno correction factor is simply the mathematical reciprocal of the relative horsepower value.

A common use of the dyno correction factor is to standardize the horsepower and torque readings, so that the effects of the ambient temperature and pressure are removed from the readings. By using the dyno correction factor, power and torque readings can be directly compared to the readings taken on some other day.

If you take your engine to the dyno on a cold day, it will make a lot of power. And if you take exactly the same engine back to the same dyno on a hot day, it will make less power. But if you take the exact same engine to the dyno (where the motor builder factors temperature, humidity and pressure) on those different days, it will always make exactly the same power.

Sometimes you may want to know how much power you are really making on that specific day due to the temperature, humidity and pressure on that day; in that case, you should look at the uncorrected power readings.

But when you want to see how much more power you have solely due to the new pipe, or the new cam, then you will find that the corrected power is more useful, since it removes the effects of the temperature, humidity and atmospheric pressure and just shows you how much more (or less) power you have than in your previous tests. 

The Society of Automotive Engineers (SAE) has created a standard method for correcting horsepower and torque readings so that it will seem as though the tests had all been taken at the time on the same day where the air pressure, humidity and air temperature are a constant.

The equation for the dyno correction is contained in SAE J1349 JUN90, is: 



Where: cf = the dyno correction factor
Pd = the pressure of the dry air, mb
Tc = ambient temperature, deg C

The pressure of the dry air Pd, is found by subtracting the vapor pressure Pv from the actual air pressure.

Relative horsepower is simply the mathematical reciprocal of the correction factor.

Horsepower and Torque:

Power is the rate at which work is done. When the engine torque is turning the crankshaft and power is being delivered, the resulting horsepower may be expressed as:



 which can be simplified as



where: hp = horsepower, hp
t = torque, ft-lbs
rpm = engine speed, revolutions per minute

This is a great formula. Basically it says that if you can keep the same amount of torque, then the more rpm you can turn, the more horsepower you get.

Now you have a picture of why jetting and carburetor tuning is so very important. Tuning carburetors with fixed jet sizes such as a Briggs, Rotax, or a Shifter you should plan a change of ½ a jet size for every 2.5% change in air density. Don’t get all upset over the fact that you need a large selection of jets. First Jets are cheap, However do not buy blanks and drill the jets on your shop drill press. Jets are not drilled they are honed and they have to have a very smooth bore to get a effective fuel flow. A jet with a flaw in the bore will flow less fuel than you anticipate and may result in a lean condition that consumes the piston. Contact your local dealer and he can source jet kits for you. We have a large box of every possible size that Bill Price Racing sourced for our carburetor and it really pays to be right on with  jet size.

Carburetors having external needles that can be adjusted during the race are ideal for tuning. The driver has two tools at his disposal an EGT (exhaust gas temperature) gauge and the carburetor needles to tune the motor while running. When tuning with EGT it is best to have a conversation with the motor builder and also to understand the fuel that is being used. For 2 cycle motors we typically run fuel like Power Mist T113 of T114 for Briggs Motors we use Power Mist Methanol. For these fuels we tune the 2 cycles to 1250 degrees of EGT and the 4 cycle we tune to 1050 degrees of EGT. If you run different fuels or if you are adding a hydroginator to the fuel you will likely run at an elevated temperature. The increased temperature may be as much as 100 degrees above the normal temperatures. Just be very careful with mixing fuel additives yourself, as some fuel additives while producing more power are very harmful. There are several common fuel additives that are known carcinogenic materials. There is absolutely no sense in introducing a cancer-causing chemical into a motor and risking the health of yourself or another competitor.

You can contact Intercomp at: www.intercomp-racing.com, Intercomp Company, 14465 23rd Avenue North, Minneapolis, MN 55447, 800-328-3336, 763-476-2531, FAX 763-476-2613.

Power Mist contact information is: Power Mist Racing Products, 67 Stickles Pond Road, Newton, NJ 07860. Phone 973-383-1061, technical info 973-579-9511, FAX 973-579-5185.

Bill Price racing contact is: bpracing@worldpath.net, Bill Price Racing, 86 Shannnon Road, Gilmanton Iron Works, NH 03837, Phone 603-364-2455, FAX 603-364-7825.