Keeping track of atmospheric conditions will help you learn more about your car as well as
We all deal with engines, so we should know about their relationship with air pressure and density. Query any serious bracket racer, and you'll find he's become a virtual closet meteorologist. What is it they know that you should? Let's find out.
What You Can't See Does Hurt
This amazing planet we live on is surrounded with a delicate bubble of life-sustaining air that breaks down to 78 percent nitrogen, 21 percent oxygen, and another 1 percent of less important stuff. Oxygen is what we crave since it supports combustion. Keep in mind that gasoline won't go whoosh without oxygen. That's why we have to mix 12.9 or so parts of air with one part of fuel to get best horsepower and torque. So it makes sense that stuffing more air into an engine will make more power. But that air has a nasty habit of changing like, well, the breeze. It's not the percentage of oxygen that changes, but rather atmospheric conditions that make this such a volatile subject.
While a simple barometer like this will indicate atmospheric pressure, don't rely on it fo
Soon after Rudolph Diesel cranked up the first internal-combustion engine running on peanut oil, car guys realized that the air they use to make power is in a constant state of temperature, pressure, and humidity change. Let's take a look at pressure first.
Earth's atmosphere extends hundreds of miles above sea level, but 99 percent of the atmospheric mass is concentrated in the first 20 miles and is held in place by gravity, which is a good thing. While air is light it does have mass, and therefore a column of air that extends 20 miles or more into space will have weight. Folks who care about stuff like that long ago established that a column of air 1 square inch extending from the outer region of our atmosphere to sea level will have a weight of 14.7 pounds. This weight becomes what we measure as a pressure of 14.7 psi. That is considered standard pressure. If you were to measure the air pressure in the mountains at 10,000 feet, you'd find it has dropped to something closer to 10 psi because that column of air is that much shorter. Besides a pressure change, with increasing altitude you also get decreasing vapor pressure and temperature. But that's not always a given since the high desert gets plenty warm in the summertime.
When it comes to engine performance, you'll hear a lot about air density. What this refers to is the amount of oxygen molecules present in a given volume of air. Let's say we have a cube that measures 1 foot in each of its three dimensions, which equals 1 cubic foot of air.
The old-school procedure for testing humidity used what is called a sling psychrometer. It
The variables of pressure, temperature, and water vapor all play a part in determining the amount of oxygen present in that cubic foot. It's pretty obvious that as pressure increases, the density will also increase. In addition, as temperature decreases, the molecules are less active, which creates less room between them-so air density in terms of oxygen content increases. Now, let's add in the variable of water vapor. As the amount of vaporized water decreases in that 1 cubic foot of air, it increases the oxygen content and the air becomes more dense.
The combination of these three variables contributes to the oxygen content, and as you can imagine there are a staggering number of combinations possible. The ideal combination for good engine performance is high pressure, low temperature, and zero water-vapor pressure. This combines to create maximum air density that's packed with those great molecules of oxygen that will help burn fuel.
The problem with atmospheric conditions is that you are forced to juggle more than one variable when evaluating air density. Perhaps you've seen an old air-density gauge that calculated density as a percent based on air temperature and barometric pressure. The problem with that gauge is that it ignores humidity (vapor pressure). Pilots were the first to work out a way to juggle two of the three variables by using a system called density altitude. For aviation purposes, vapor pressure is ignored. The formula (which is way too complex to dive into here) takes temperature and pressure conditions into account to come up with a single equivalent elevation above sea level.
We've been using this PerformAire electronic weather station for years to keep track of th
Standard motorsports temperature and pressure are established to be 60 degrees F, 29.92 inches of mercury pressure (which equals 14.7 psi), and zero humidity. Aviation standard temperature is 59 degrees, which is why you will see that figure used sometimes. That is considered zero-density altitude. Any temperature or vapor-pressure increase or pressure decrease will contribute to raise the density altitude. This ultimately means a decrease in overall air density.
According to the folks at Altronics, there is one more variable here that we have not touched upon. The science books tell us that the oxygen content of air is roughly 21 percent. But the reality is that this can vary by a few tenths of a percent. This can have a slight effect on performance, and it's significant if you are a bracket racer where every hundredth of a second must be tracked in order to make the car as consistent as possible. For the rest of us mere mortals, however, keeping track of density altitude will suffice to make us better tuners than most of our friends.
The Weatherman Lies
Have you ever wondered why the weatherman in Denver will tell you with a smile on his face that it's going to be a beautiful day with the pressure at 30.12 when your uncorrected mercury barometer tells you it's actually something like 24.80? The government decided that regardless of the altitude, atmospheric pressure readings from the National Weather Service should be altitude-corrected based on similar conditions that would exist at sea level.
In most cases this would not be important, except that you cannot plug the weatherman's data into your density-altitude calculations because the data is incorrect for the altitude. This is why if you are attempting to do density-altitude calculations, you must use what is called uncorrected station pressure. This is actual atmospheric pressure at a given altitude and can be obtained from the nearest airport by asking for uncorrected station pressure. The higher the elevation, the greater the disparity will be between those numbers and what the weatherman gives you.
This is a close-up view of the screen that delivers the date, temperature, pressure, humid
The Shell Game of Correction Factors
Since internal-combustion engines are directly affected by the constant change in atmospheric conditions, long ago the automotive industry came up with a plan for using correction factors to establish a common ground from which all power numbers could be compared. The original "gross" horsepower correction factor includes the standard of 29.92 inches of Mercury (Hg), which is standard sea-level pressure, combined with a temperature of 60 degrees F with no humidity, or zero vapor pressure. As you can imagine, these are ideal or "gross" horsepower numbers that are not practical in the real world but serve as a common reference point. Through the early '70s, these were the numbers Detroit advertised and the performance industry followed. In 1972, Detroit switched to a net horsepower correction followed by several more changes, the last of which occurred with SAE standard J1349. This current Detroit correction factor uses a lower 29.235 inches of Mercury pressure with a higher air temperature of 77 degrees F and zero vapor pressure. This correction factor reduces the old gross-horsepower output number by roughly 5 percent but is also more realistic. As an example, the new 427ci LS7 Corvette engine rated at 505 hp would probably correct to closer to 530 hp using the performance-industry gross correction factor (C.F.).
To bring this home, let's take a look at a big-block power curve with three different sets of numbers in the chart below. The first column represents the observed numbers generated on a high-pressure day with 30.02 inches of barometric pressure, air temperature of 73 degrees F, and a vapor pressure of 0.35. Using the classic gross correction factor reference, this equates to 1.025 or a 2.5 percent increase over the observed power. The third column indicates power using the SAE J1349 C.F., which uses 29.235 inches of Hg station pressure, 77 degrees F temperature, and zero percent humidity as its reference point. The observed power data came from a dyno operated at very close to sea level. Using the SAE J1349 standard, the correction factor calculated to 0.978, which reduces the observed power by 2.2 percent! This works out to a differential between the gross and SAE J1349 corrections of roughly 5 percent.
If you take the same engine and test it exactly the same way on five different dynos, it's
So which set of numbers is correct? All three are accurate as long as they are properly identified. We guarantee no one uses observed (uncorrected) dyno numbers unless the density altitude that day is a negative number (which does happen-it's like racing down a mine shaft). But if you were an engine builder, which set of numbers would you give your customer? Armed with this knowledge, what questions should you ask your engine builder when reading a dyno sheet on the engine you just paid for? At the very least, you would want to know the inlet air temperature, uncorrected station pressure, and vapor pressure. If he begins to waffle, consider becoming very suspicious.
|Corrections Compared |
| ||Observed ||Gross C.F. ||SAE J1349 C.F. |
|RPM ||TQ ||HP ||TQ ||HP ||TQ ||HP |
|4,500 ||571 ||489 ||586 ||502 ||558 ||478 |
|5,000 ||581 ||553 ||597 ||568 ||568 ||541 |
|5,500 ||582 ||609 ||597 ||625 ||569 ||595 |
|6,000 ||567 ||647 ||582 ||665 ||554 ||633 |
|6,500 ||536 ||663 ||551 ||682 ||524 ||648 |
SuperFlow uses the standard performance-industry (or gross hp) correction factor for all i
So what does all this weather-info and density-altitude talk have to do with your car at the dragstrip? The answer is-plenty. If you're curious why your car runs slower on one day and quicker the next when you made no changes to the car, the answer could be because of atmospheric conditions -specifically the density altitude was probably lower on the day your car ran quickest. This means if you want to try to run a quick number, it's best to run early in the morning or after the sun goes down rather than in the middle of the day.
We talked to Super Stock racer Bob Lambeck, who gave us some valuable insight into the effects of density altitude on performance. Lambeck says that for every 100-foot increase in density altitude, he sees his car slow down 0.01 second, while the handheld electronic weather station claims a 200-foot move in density altitude will result in the same 0.01-second change.
These estimates are generalizations based on experience, and it points out that if you become good at keeping records on your car's performance, you will soon be able to predict it rather accurately based on the effect of density altitude. Temperature and vapor pressure are the two leading variables that most affect engine performance. Lambeck adds that water in the air has a big effect on the tune-up at both ends of the vapor-pressure scale. He says he's seen humidity as low as single-digit percentages in Phoenix, which makes tuning a challenge. Water tends to cool the chamber, but he reports that at levels below 40 percent his engine responds to richer jetting. Alternately, humidity levels above 40 percent reduce air density and generally require less fuel and perhaps more timing. As an example of this, engine builder Jere Stahl recommends trying half a degree of additional timing for every 20 percent increase in humidity. That's not much (assuming you can accurately measure a 1/2 degree), but it reinforces the point. Many racers are now looking at grains of water per pound of dry air, and if you desire to dig deeply into this, you can read more about it on the Altalabs Web site. This is especially valuable information for bracket racers who strive to make their cars run exactly the same from one run to the next.
A logbook is a very useful tuning tool even if you're not a bracket racer. Summit Racing s
For many cars, major changes in density altitude do not necessarily demand tuning changes. If you've done research in this area, you may have run across information that shows that for every 2,000-foot change in altitude, you should lean a Holley jet size by one number. The problem with this idea is that carburetors are velocity-sensitive, not sensitive to air density. Assuming the same air temperature and vapor pressure at both 3,000 feet and sea level, we've tried different jetting and found that our carbureted small-block car did not need a jet change despite the radical change in altitudes. The car ran slower at 3,000 feet compared to sea level because the engine is air-density sensitive, but the carburetor saw a minimal difference in relative velocity.
This is not to say that density altitude is not a worthwhile tuning indicator. It can be useful if you keep a logbook on car performance and use the density-altitude numbers to help you with major changes in altitude at different locations. But if you see a difference in performance in your car with exactly the same density-altitude numbers, the More on Density Altitude section should make this less confusing. This is why it's important to record all three atmospheric variables and not just the density-altitude number.
Log on to quarterjr.com and plug different conditions into the Weather Station and the Dra
More on Density Altitude
Let's get into some specifics to see how density altitude works, and where it doesn't. We took our car to Pomona, California, which is roughly 900 feet above sea level. With 72 degrees F, 55 percent humidity, and an uncorrected atmospheric pressure of 28.72, we had a density altitude of 2,400 feet. This is the equivalent of a 60-degree dry-air day at 2,400 feet of elevation. The next day we traveled to a dragstrip at 2,000 feet of elevation with 49 percent humidity and a chilly 55 degrees F. It turns out this was also exactly the same 2,400 feet of density altitude. While the temperature was much cooler, the air pressure was lower since we were at a higher elevation. Even though the tune-up might be slightly different with the same density altitude, most material out there claims a car should run the same since the density altitude is the same. According to Patrick Hale at Racing Systems Analysis, the car will probably not run the same at both tracks.
On his Web site quarterjr.com, Hale has created two very simple programs that are free for anyone to use. We plugged both track conditions into his Weather Station program to get the density-altitude numbers. Then we used his Dragstrip Dyno program to get an idea of how our 3,550-pound street car with 400 hp and an automatic would run at both tracks. The dyno estimates the car will run slightly slower at the higher-elevation track despite the identical density altitude, cooler air, and reduced humidity. Hale's Dragstrip Dyno shows the difference in performance is 12.18 at 113.60 mph versus 12.13 at 114.00 mph. Part of the output of the Weather Station is a horsepower correction number. At the 900-foot track the correction was 1.079, while the higher-elevation track's horsepower correction factor was a larger 1.092, which means the engine was making less power at the higher elevation despite the lower temperature. Part of the problem with the density-altitude formula is that it was originally developed for aircraft and solving for proper lift with a given atmospheric condition. Unfortunately, engine power does not respond as strongly to changes in temperature as density altitude predicts.
This is the Dragstrip Dyno screen that uses the horsepower correction number generated by
The Dragstrip Dyno program uses inputs from Hale's Weather Station program to estimate power based on pressure, temperature, and vapor pressure. The Dragstrip Dyno then calculates a simple horsepower correction factor that can then be used along with the variables of horsepower and weight to come up with a simplified e.t. and speed estimate. The beauty of this is not that it is accurate for your particular car, but that you can quickly input different atmospheric conditions and see their effects on the performance of any car. As our example above illustrates, lower pressure hurt performance, but the cooler air made up for much of that so the car slowed down only slightly. Had the 900-foot temperature and vapor pressure been the same but at 2,000 feet, the car would have slowed down even more to a 12.30 at 112.60 mph with a density altitude of 3,750 feet-that's 0.17 second and 1.4 mph slower than what the car ran at 900 feet of track elevation. It's all about knowing how atmospheric conditions affect engine performance. We have abbreviated this explanation for the sake of space, but if you want to learn more go to Hale's Web site, which also includes some simple math that will make it all very clear.
NHRA Elevation Correction Factors
Use this chart to convert your e.t. and mph to sea-level performance by multiplying either e.t. or mph performance at a given altitude by the indicated factor. These are NHRA's correction factors for normally aspirated Stock and Super Stock race cars, which equate the best for a typical normally aspirated street car. These factors are used to "correct" back to sea level as a way of establishing class indexes.
As an example of how to use this chart, if your car runs 12.00/114 mph at a 3,000-foot-altitude track, use the correction factors (e.t. x 0.9640 and mph x 1.0381) to generate altitude-corrected numbers equaling 11.568 at 118.34 mph. These are roughly 3 1/2 percent corrections for that altitude. Keep in mind that these factors correct only for track altitude, they do not address the existing weather conditions. This is a way to roughly compare a normally aspirated car's performance at Denver's Bandimere Speedway (at a wheezy 5,800 feet) to your buddy's Englishtown, New Jersey, rocketlike e.t.'s running at a track that is virtually at sea level. It's also worth noting that NHRA uses a less aggressive correction factor for the quicker Competition Eliminator cars. Plus, most supercharged and turbocharged cars (AA/Altered Turbo for example) use half of the Comp correction factor. Alcohol dragster and Funny Cars are not factored.
There are also several computer programs you can use to track atmospheric conditions to he
These conversions come courtesy of the HP Books Auto Math Handbook by John Lawlor. Some data also came from Metals Progress Databook.
|To Convert ||Multiply By |
|Atmosphere to inches of mercury ||29.921253 |
|Atmosphere to kilopascals (Kpa) ||101.325 |
|Atmosphere to psi ||14.69595 |
|Inches of mercury to atmospheres ||0.0334211 |
|Inches of mercury to kilopascals ||3.37685 |
|Kilopascals to atmospheres ||0.0098692 |
|Kilopascals to inches of mercury ||0.296134 |
|Kilopascals to psi ||0.1450377 |
|Pascal to psi ||0.0001450 |
|Psi to atmospheres ||0.068046 |
|Psi to inches of mercury ||2.036021 |
|Psi to kilopascals ||6.894757 |
|Parts List |
|Description ||PN ||Source ||Price |
|PerformAire weather ||PAE ||Summitracing.com ||$599.95 |
|PerformAire weather ||PAE-O2 ||Summitracing.com ||$799.95 |
|PerformAire weather comp ||PA2 ||Summitracing.com ||$399.88 |
|PerformAire weather comp ||PA2-O2 ||Summitracing.com ||$699.88 |
|Summit Logbook ||G5155 ||Summitracing.com ||$4.95 |
|NHRA Correction Factors |
|Altitude Above |
Sea Level (Ft.)
|E.T. Factor ||MPH Factor |
|1,500 ||0.9835 ||1.0171 |
|2,000 ||0.9770 ||1.0241 |
|2,500 ||0.9705 ||1.0311 |
|3,000 ||0.9640 ||1.0381 |
|3,500 ||0.9575 ||1.0451 |
|4,000 ||0.9510 ||1.0521 |
|4,500 ||0.9445 ||1.0577 |
|5,000 ||0.9380 ||1.0661 |
|5,500 ||0.9315 ||1.0731 |
|5,800 ||0.9276 ||1.0773 |
|Pressure Vs. Altitude |
|Altitude Above |
Sea Level (Miles)
|Percentage Of |
|3.5 ||50% |
|10.0 ||10% |
|19.4 ||1% |
|29.9 ||0.1% |
|40.4 ||0.01% |
|49.2 ||0.001% |