1qk4mula

How does sea level affect my time?

example ; My local track is located 600 ft above sea level. (Toronto Motorsports Park)

If i run the same car at sea level will it go faster or slower? I'm guessing higher altitudes less density of air = run slower.. Does this make sense?


What altitudes are your tracks at?? and what city state?

AlkyIROC

Altitude of the track is only a small part of it. Calgary's track has an official altitude of 3380 feet however a typical race day has the density altitude around 5000'. Anything under 4000' is a good race day.

Denver has an altitude of 5800' but usually has a DA of 8000' and has gone over 10,000'. Places like Mission BC is 24' and in the spring and fall when the air is cool, they usually have a DA below sea level.

Racing in a higher altitude or just a higher density altitude means you have less air to burn. Less air means you can't use as much fuel which means the engine makes less HP.

If you want to know the altitude of race tracks, simply look them up here
http://chasinracin.com/track-locator/index.html

Density altitude is a formula taking elevation, temperature, humidity and barometric pressure to give an equivalent altitude based on a sea level standard day.

You can use the NHRA correction factor to calculate what your car would run at different altitudes. Theoretically, if I raced at sea level, my car should get an 8 second time slip.

To see how bad altitude affects performance, just look at ProStock cars. At sea level, they run 6.5 seconds at 210 MPH. That's what they were running in Georgia this year with a DA of 2000'. Even when they were in Seattle, the DA was 2000'. Seattle has an altitude of 150' so it's considered a sea level track. When they went to Denver, the air was 8700' and they were running 6.9's at 197 MPH. This year was the first time ProStock cars were able to get into the 6's in Denver and they still haven't broken 200 MPH yet.

Saying you have a 12 second street car at a sea level track doesn't mean much. Saying you have a 12 second street car in someplace like Denver tells me you have a very fast car. Both similar cars run the same ET but the one that's doing it at high altitude is making a lot more power to do it.

It's easy to get misled by track altitudes. You can run a good time at Cayuga in bad air then go to a higher altitude track that has really good air and run even faster. If the higher altitude track has a lower DA than the other track did then you'll see an improvement in time.

You can also get mislead with performance changes. Go out one weekend and run a 13.5. Decide to change the intake and put headers on the car and go out the following week and still run 13.5 then bitch and complain that the car runs the same. Not realizing that the second week, the DA had increased which should have slowed the car down but the changes you made made it speed up. The increased power put you back to where you were but actually if you didn't do any changes, the car would have run slower.

1qk4mula

Thanks I got it now.. Next time I go the track I'm going to record the weather cond, humidity etc etc, to conpare apples with apples..

AlkyIROC

I use an inexpensive digital temperature/humidity meter I picked up from Canadian Tire. For barometric pressure, something inexpensive isn't very accurate. I use a digital handheld meter that was around $200. It's still not as accurate as I would like but it's close enough.

I record my barometric pressure in hPa and at station pressure. Station pressure is the actual air pressure. Knowing the altitude isn't required. The barometric pressure given in weather forecasts is in kPa and is corrected to altitude. For aviation use and weather maps, it gives a relative pressure no matter what the altitude is of the weather station. If you don't know the altitude of the location, a corrected pressure means nothing since it can be manually changed by changing the altitude in the meter. Since station pressure is the actual pressure of the air, it doesn't matter if you're in Florida or Denver. The gauge will show the actual pressure and it will be much lower in Denver.

As an example, today is a nice high pressure area. My station pressure is currently 905 hPa. The local weather station at the airport is giving a corrected pressure of 102.3 kPa or millibars (1023 hPa). My meter won't register a decimal place in the hPa reading so the pressure needs to change 1 full hPa before the reading changes. I want a meter that shows that decimal place accuracy.

A low pressure day for me last year was 884 hPa. When I went to Edmonton last year for a race, the track is about 1000' lower in elevation than Calgary. It really showed up in the station pressure when my meter said 940 hPa.

Standard sea level pressure is 29.92 inHg which is 101.325 millibars. Since today is a high pressure day, the corrected pressure is 102.3. Since it's higher than a standard sea level pressure, they can chart the higher pressure on a weather map properly. Using station pressure, my high altitude racing is at 26.72 inHG in good air (today) and 26.1 in poor air. If you had pressure that low at sea level, you would be in the eye of a hurricane.

The accuracy of your results depends on the accuracy of your instruments. If you walk up to 5 different racers holding 5 different weather stations, you'll get 5 different results. Always use your own instruments and keep your own readings. If you have to ask someone at the track what the current density altitude is, only use it as a reference, not as log book material.

By tracking the weather conditions, you can eventually see a pattern of how the car runs in different air conditions. If you have enough data collected, after a while you could show up at the track, check the air conditions and calculate an ET the car should run without having to make a pass. Good if you arrive late and can't make time trial runs.

Those hot sunny days may look like good racing days but the heat is causing the DA to increase. If it's a muggy day like a typical southern Ontario summer day, the high humidity causes the DA to increase. If the barometric pressure is low, the DA will increase. Being able to race in a cool, dry, high pressure day at a low altitude track to set a personal best ET is very difficult. That's why the hemi cars go to Mission in the spring and fall to set records.

Although I track the humidity, I actually use it to calculate the dew point. Dew point is a more accurate way to show how much moisture is in the air. Over the course of a race day, it will go up or down slightly while the relative humidy goes all over the place. Relative humidity is just that, relative humidity. It's a percentage of water in the air relative to the air temperature. Early in the morning when the air is cool, the RH is high. Later in the afternoon when the temperature is high, the RH is very low. Although the heat of the day will dry things out, reducing the amount of water in the air, RH doesn't really show how much it's change like watching the dew point will do.

Trying to understand relative humidity is like this. 90F air at 50% humidity has a lot more moisture in the air than 70F air at 50% humidity. Saying you raced in similar air because the humidity was the same isn't similar conditions. Even if the temperature was 5-10F difference, it's not similar air.

The next time you go to the track, go through the pits and see how many regular racers have some sort of weather station. It's an invaluable tool for racing. The idea of sticking your finger in your mouth then sticking it up in the air to guess an ET doesn't usually get you through too many rounds unless you're very lucky.

ZONES89RS

iTrader: (2)

That is some great info. My closest quarter mile track said it was:Alt.: 40

LOL, that is pretty much sea level at its best.

AlkyIROC

But just like when the NHRA was in Georgia and Seattle. Those are both sea level tracks but the density altitude was still around 2000 feet. Even here in Calgary, I've seen race days in the early or late part of the season have a density altitude drop below 3500' in the evening when the temperature cools off. Those sea level tracks can also have air that's below sea level in similar conditions. Racers love those conditions because the engines make so much more power and that's usually when class records are broken.

The NHRA uses correction factors for each track (usually above 2000' elevation) however they don't take into account the density altitude. If the density altitude goes below the elevation, the cars will run quicker than their factored index and they set a record.

five7kid

iTrader: (14)

NHRA provides altitude correction factors for tracks starting at 1600'. http://www.nhra.com/competition/altitude.aspx

Note there are charts for different classes. Our cars fit best in the Stock/Super Stock table. I've found it to be reliable.

Since the change in air pressure is not linear with the change in altitude, there is more effect going from 2000' to 3000' than there is going from sea level to 1000'. And, that is also why they don't bother with altitude corrections below 1600'.