altitude effecting times

how much does every 1000ft. of elevation effect your 1/4mi time.

Like Raptor said there are a lot of factors that can affect it. That is why you want to look at the density altititude (DA). This will factor in barometric pressure, humidity, temp, and humidity. With a increase in 1000 ft of DA you can gain several 1/10ths in your ET while loosing a few MPH.

If you want to look only at altitude, you will find that the first 1,000-ft doesn't make a lot of difference.... about 1%. If you use the formula that NHRA uses for Stock/Super Stock class "corrections", you find that you need to multiply your performance at 1,000-ft by:

ET: 0.99
MPH: 1.01

That means a 13.0@108MPH at 1,000-ft would be 12.87@109.1MPH with the NHRA corrections. Note also, that the formula used by NHRA for these corrections actually corrects to an altitude of approx. 230 to 280-ft above sea level.

The correction factors for other altitudes are:

2,000-ft:

ET: 0.9770; MPH 1.0241

3,000-ft:

ET: 0.9640; MPH: 1.0381

4,000-ft:

ET: 0.9510; MPH: 1.0521

5,000-ft:

ET: 0.9380; MPH: 1.0661

6,000-ft:

ET: 0.9250; MPH: 1.0801

Remember.... these are multipliers that you use to convert your actual performance at a higher elevation to the numbers you would have run closer to sea level.

On a typical summer day in Denver at Bandimere speedway (5800'), bolt on lt1's run high 14's to mid 15's. Ls1's run mid 14's to low 15's. My hotcammed, long tubed etc lt1 runs very low 14's.

I have the NHRA Altitude Correction Table on my website at http://members.cox.net/bobcosby/cobra/altitude.html

There is no correction or formula to account for wind, and unless it is really strong, the effects are usually not great, and are generally reflected in MPH only.

Temp, humidity, and pressure are the 3 environmental factors that affect performance. As an FYI, dew point is used to figure humidity, so they are not considered seperately. Also, pressure is used to determine altitude, so they can be considered together (easiest) or seperately.

I use dyno correction formulas to get a corrected ET. I also use them to predict an ET. They're all based on density altitude and how much power is available from the engine in relation to the current weather conditions.

If my engine produces dyno corrected 600hp I may only be able to get 500hp to the wheels because of the altitude I race at and poor weather conditions. My best pass only used 529hp. That's a 70 hp loss just because of where I race and the weather conditions. Theoretically if I raced at sea level, the engine would produce 600hp. During this time of year with low DA at sea level, I would probably make even more HP because the DA can be below sea level.

That's the nice thing about dyno HP. They're all corrected to sea level so a 12.0 second car racing in Denver is actually faster than a 12.0 car racing in Florida because the Denver car is making more HP to run 12's at altitude.

Originally posted by Raptor1
What meteorologists don't tell you (or don't know themselves) is that vapor pressure is also used to figure dew point. And although dew point is related to humidity (and vice versa) it is separate and does affect performance, especially in EFI engines.

Several months ago I went in to detail about this on this forum, but it was lost in cyber space somewhere. But you can find a lot of info on the subject by doing a google search for "dew point". However, I learned a long time ago by keeping logs on dew point, as well as temperature, relative humidity (another misconception) and barometric pressure, that a change in dew point alone will affect performance. But don't take my word for it; buy a wet bulb or similar instrument and try it ;)

Thanks for the explanation. Here's some more - all my own words off the top of my head....feel free to post the textbook versions if you like:

Humidity - more correctly in the terms we are speaking "relative humidity" - is the amount of water vapor in the air compared to the amount of water vapor the air is capable of holding, at a specific temperature (VERY important) and pressure.

Dew Point is the temperature to which air must be cooled in order to reach saturation - ie...100% relative humidty.

Vapor Pressure is atmospheric pressure, which takes water vapor into account.

With that in mind...

I learned a long time ago by keeping logs on dew point, as well as temperature, relative humidity (another misconception) and barometric pressure, that a change in dew point alone will affect performance.

So long as you keep temperature and pressure constant, it is impossible for dew point to change without the humidity also changing. A higher dew point is an indication that more water vapor is in the air. More water vapor in the air means that the humidity is higher. The opposite is also true.

And although dew point is related to humidity (and vice versa) it is separate and does affect performance, especially in EFI engines.

Though they have different definitions and are useful in different ways, Dew Point and Humidity are both relative indicators of the amount of water vapor in the atmosphere. Assuming barometric pressure and temperature are held constant, you cannot change the amount of water vapor in the air and NOT change both Dew Point and humidity. Changing one changes the other, everything else staying the same (primarily temperature). Of course that will also change the vapor pressure.

I would also suggest that EFI engines adjust for atmospheric conditions better than carburated engines, and thus are less affected by changes in the weather. I cannot, however, back that up with solid proof.

But don't take my word for it; buy a wet bulb or similar instrument and try it

Good idea, but buying a wet-bulb thermometer will only give you wet-bulb temperature. It will not give you Dew Point, Humidity (be it specific, relative, or absolute), or vapor pressure.

I agree whole-heartedly with keeping logs, and much prefer using dew point vice humidity in my logs. However, give me Temperture, humidity, and pressure, and I can calculate the dew point (or wet-bulb). Similarly, give me Temp, dew point, and pressure and I can find the humidity.

BTW, I am a meteorologist.

Relative humidity:

There is more moisture in the air in 50% air at 80*F than there is in 50% air at 60*F because of the saturation pressure.

When keeping logs of the weather while at the track it's nice to use humidity but dew point and vapor pressure are a better indication of the water content in the air.

You are quite correct with your humidity example, and I most certainly agree that it is better to keep track of dew point rather than humidity.

Quick question....how do you measure "vapor pressure"?

Originally posted by Raptor1
Whatever :rolleyes: Who am I to tell you anything?
You're welcome to tell me or anybody else whatever you wish. Whether or not I or anybody else will agree with you, or believe you, is a decision you cannot make.

I did notice that you did not disagree, nor agree, with the actual points I made. Instead, your main comment is that we were often wrong in our forecasts. Interesting.

And I've never been impressed with any of you.
Darn the bad luck.

After all, how often do you guys hit the forecast? :p
We're paid to be wrong, how often do we need to?

Perhaps you should do some more research?
Perhaps you shouldn't stray outside your area of expertise?

I won't get in to actual humidity vs. relative humidity, 'cause you already know, right?
As a matter of fact I do. You can "know" too - it is quite easy to look up the differences on any number of sites - weather.com comes to mind. What does that make you? Someone that has looked up definitions on weather.com. Cool.

Btw, there are several ways to measure vapor pressure.
Would you mind stating a few? Specifically, I'd like to know of ways that racers could get this vital info for their logs. After all, we are relating it to racing, right?

But again, who am I to try to get anyone to think outside the box? ;)
Thinking "outside the box" can be a good thing. However, the box should at least be something that you have a working knowledge of (not implying that you do or do not have such knowledge - only stating an opinion).

In the end, a racer should log Temp, dew point (or humidity, if you understand how it relates to air density, and how it is related to temperature), and barometric pressure. He/she needs only those instruments that gives him/her these variables in order to keep accurate, meaningful records.

I guess I'm just a little behind the times. I generally just look at Veronica when it's time to dial-in and ask her, "What do you think Honey?".

Anyway, try not to beat up on each other too much!

Best Regards,

Daren

Originally posted by Raptor1
You can do another lengthy response rebutting what I've stated if it makes you feel better
Can I? Thank you very much.

The National Severe Storms Laboratory is one of NOAA's internationally known research laboratories, leading the way in investigations of all aspects of weather. Headquartered in Norman OK, the people of NSSL are in partnership with the National Weather Service.
That's a neat quote you copied and pasted here. Rather meaningless in the context of our discussion, but neat, none-the-less.

Pssst....I visited the NSSL in Norman. Have you? Doesn't make me any better, smarter, faster, etc, but you quoting from the site sure doesn't do much to impress.

From The National Severe Storms Laboratory web site:

http://www.cimms.ou.edu/~cortinas/1014/l11_3.html

Specifically paragraph 3, second sentence where it states: "The relative humidity indicates how close the air is to saturation and is not a direct measure of the amount of water vapor in the air." (underline added)
Well duh. You know, I just reread what I wrote. Nowhere could I find where I stated anything contrary to the stuff you just cut-n-pasted.

Also the last paragraph where it states: "Another way of indicating how much [actual] water vapor is in the air is by measuring the dew-point temperature. The dew-point temperature is the temperature that the air must be cooled, at constant pressure and [actual] moisture content, in order to reach saturation."
Another duh. Did I not define this in one of my earlier responses? My wording was, and now I do quote - " Dew Point is the temperature to which air must be cooled in order to reach saturation - ie...100% relative humidty."

One can extrapolate from this that if the relative humidity is not a direct measure of the actual amount of water vapor in the air, but the dew point is a "way of indicating how much water vapor is in the air", relative humidity and dew point should not be confused as being interchangeable, or that one can be used to obtain a true value of the other, even though most meteorologists regularly practice this mistake. (There actually is one in my area who doesn't do this and he is the one I prefer to listen to).
LOL. Ok Einstein. Your "extrapolation" is a bit off. But don't take my word for it....how about using a National Weather Service site for evidence? Here you go: http://www.srh.noaa.gov/elp/wxcalc/wxcalc.shtml

Scroll down to "moisture conversions".

Now, once again in laymans terms, lets relate Dew Point to Humidity. Dew Point does indeed indicate how much water vapor in the air. Humidity...relative humidity...compares the amount of water vapor in the air to the amount of water vapor the air is capable of holding, at constant temperature and pressure. Now tell me - how are those not directly related? Holding temp and press constant, how can I change one without also changing the other?

You are welcome to believe whatever you wish. Those that wish to believe you can do the same - I hope they have luck finding a Vapor Pressure gauge.

So, is dew point more related to: 1) Absolute humidity, 2) Relative humidity or, 3) Specific humidity?

Yes to all three. Would you like a more indepth explanation?

Relative humidity (RH) is directly related to Dew point (it goes up when DP goes up, and down when DP goes down, everything else held constant), and indirectly related to Temperature (it goes down when the temp goes up, everything else held constant). Pressure has only a small effect on RH at the surface.

Absolute Humidity is the amount of water vapor in the air compared to a volume of air (the metric itself can be cubic foot, cubic meter, whatever). It is directly related to Dew Point.

Specific Humidity is the amount of water vapor in the air compared to dry air, usually in grams per gram. The only way to change the specific humidity of the air is to add or take away water vapor. As such, Specific Humidity is directly related to DP.

How's that? Of note, RH and DP are terms that should be meaningful to a racer that wants to keep track of his/her performance. Absolute and Specific humidity - like vapor pressure - have little meaning in the context of this discussion.

It is foolish to ASSume what someone's "area of expertise" may be, or what they may or may not have a "working knowledge" of.
Good point. So what do you do? Glasswork? Nuclear Physicist? Basket weaving? Brain surgeon? Please do tell.

BTW...its also kind of foolish to believe that you know what you're talking about when really all you do is copy and paste stuff from th internet.

But there comes a point where you can say too much and simply show that you're just another cookie-cutter weather person who doesn't truly understand what they're doing.
LOL. Cute. And it is you that decides that I "don't know what I'm doing"? I suggest investing in a mirror, a tape recorder, and a printer. That way you can have firsthand knowledge of what you just implied.

Personally, I have more important things to do here in my laboratory than argue about the weather with someone on a message board.
Oh ok, so you're too important for this little debate? This explains why you have made several replies, and will continue to do so?

Right.

It's been fun!

Absolutely. I get a kick out of proving wrong those that say things like "there are several ways to measure vapor pressure". Speaking of which, along with several other things, you ignored my request. Please tell me how you measure vapor pressure. Remember that question?

And finally, to restate to all that might be interested in the original topic...I recommend keeping good logs of Temperature, dew point (or humidity, if you understand how it is related to temperature), and pressure. The point that was made about vapor pressure is meaningless in terms of what we need to know as racers - which is essentially air density.