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Altitude Density Tuning - Horsepower In The Air

What every car crafter needs to know about air, pressure, temperature, and humidity-because there's horsepower in the air

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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.
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

Weather Tuning
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.

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.

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