E.T. correction for altitude??

[Ted]

Is there a general rule or equation to figure out how much elavation effects E.T. and mph???

 

[Dan Buick Man]

You can use this as a guide.

http://www.nhra.com/tech_specs/altitude.html

 

[Ted]

I checked that one out. NHRA only does a calculation for 1500 and higher.

WHat I want is how much e.t. per 100 feet? Or something of the like. Make sense?

We ran at Salem and I think they are 1300 feet, trying to figure what the same run would be at 300 feet.

Thanks...

 

[ledzeppac]

Its not gonna be "that" much of a difference.... we are boosted remember....

 

[TurboDave]

led's right. The difference would be virtually non existent. Forced induction applications just aren't going to see that much difference. Like the NHRA page said. Forced induction cars use "half" factor.

 

[XLR8]

Wish I could race at a track only 1300 feet high. My last time to the track although it was hot - 87* - we had good air for us - the barometer was 23.9! - the highest I remember seeing.

No offense, but come to a 5800' track, race and then tell me there is not much difference from sea level becase we have turbos. And if you still have the stock converter with anything but the stock turbo, good luck spooling it on the line.

 

[TurboDave]

Wish I could race at a track only 1300 feet high. My last time to the track although it was hot - 87* - we had good air for us - the barometer was 23.9! - the highest I remember seeing.

No offense, but come to a 5800' track, race and then tell me there is not much difference from sea level becase we have turbos. And if you still have the stock converter with anything but the stock turbo, good luck spooling it on the line.

I wasn't really implying that. I know the differences. I've raced at Vegas (2,000) ft, and run my fastest and quickest, and LACR (around 2,000 but DA is always closer to 5,000) not quite so fast there. What I was saying that lower altitutes around a thousand or so aren't going to be seen in a forced induction time ticket. But the beauty of forced induction is, we can mitigate the altitude differences much more than the N/A guys can.

 

[lazaris]

Ted,

I have an ET predictor software thats quite accurate. Its use by alot of super comp car classes by me. It uses a horsepower correction factor with its software. It calculates the Baro,temp and humidity to give you a Density altitude. If you can give me your tracks elivation, 1/8 mi ET and mph along with your 1/4 mi et and mph I can give you a difference on what your Density altitude would do to the car. And yes turbocharged or not the DA will effect your ET and mph.

 

[denn454]

so if you figre for us 1 second drop in every 10,000 feet (easy math not too percise)

so if my brain calculator is working right every 100 feet would be worth .01 seconds.

look at density altitude, thats what matters. i'm running short on time or i'd explain it more but do a google search on it. great stuff.

 

[lazaris]

so if you figre for us 1 second drop in every 10,000 feet (easy math not too percise)

so if my brain calculator is working right every 100 feet would be worth .01 seconds.

To a point. Thats where the HCF comes into play. You could have a DA of 2000' with 80* temps and a DA of 2000' with a temp 40* but the ET can be different because as the temp goes to a certain point the correction factor changes. It gets confusing but it not a straight graph for the DA footage.

 

[Ted]

Ok guys, thanks for the help. Let me get the time slips, to do some calculations...

 

[JoyOf6]

I use a basic Altronics weather station to obtain atmospheric data/d.a wherever I race. I see less than a hundredth of a second change per 100' of density altitude. It's closer to 200' per hundredth. I do have years of timeslips with density altitude information from several tracks but it's not compiled into a single table - I got the big picture I needed from routinely recording the data. To try to make meaningful comparisons among runs that occur under very small d.a. differences (as in bracket racing) IMO requires consistent 60' times and launch technique to offset the dominant launch variable (staging depth, rpm, boost, etc.). Then the rest of the run has to be consistent, too.

Obviously, it's easier to quantify the effect by running under extremes of d.a., make the comparison, and work backwards to get your per-X' value. When I leave the 6000'-plus summer air here to run elsewhere under conditions of d.a. thousands of feet lower, I look (ballpark/conservative) to pick up a tenth per 2000'. If the track hooks better than at home, all the better for optimizing the delta of course.

Art

 

[denn454]

density altitude is "pressure altitude corrected for nonstandard temperatures" your thinking pressure altitude which just figures in the barometric pressure. to get density altitude you first need pressure altitude, thats pretty easy to get.

standard pressure is 29.92, every inch is worth 1,000 feet. so if you have a barometric pressure of 30.05 pressure altitude would be 113 feet lower than actual elevation. so the affects of pressure altitude are pretty minimal.

finding density altitude is not so simple. standard temperature is 15 deg. C at sea level, thats not average temp, thats standard temp, i don't know who came up with that stuff... there is a normal lapse rate of 4.5 deg. for every 1,000 feet of elevation. so basically standard temp. should go down 4.5 deg. for ever 1.000 feet higher you get.

SO if your at 2,000 feet standard temperature would be 6 deg. C anything hotter than that will give you higher density altitude, and worse air.

i don't remember the exact calculation for finding exact density altitude, i have an aviation calculator (e6b) that does all the hard work for me. so if anyone is really intrested in this stuff go to a small airport and pick up one of them, they cost like $6. OR do a search and i'm sure you can find the equasion. or you can also go to summit and pay a few hundred bucks for a weather box that does it all for you.

humidity also has its effects, but i don't know of any ways to findout how it effects anything. i know high humidity is usually a bad thing, but if your making boost it might help with some detionation. i don't know about that.

 

[denn454]

density altitude is "pressure altitude corrected for nonstandard temperatures" your thinking pressure altitude which just figures in the barometric pressure. to get density altitude you first need pressure altitude, thats pretty easy to get.

standard pressure is 29.92, every inch is worth 1,000 feet. so if you have a barometric pressure of 30.05 pressure altitude would be 113 feet lower than actual elevation. so the affects of pressure altitude are pretty minimal.

finding density altitude is not so simple. standard temperature is 15 deg. C at sea level, thats not average temp, thats standard temp, i don't know who came up with that stuff... there is a normal lapse rate of 4.5 deg. for every 1,000 feet of elevation. so basically standard temp. should go down 4.5 deg. for ever 1.000 feet higher you get.

SO if your at 2,000 feet standard temperature would be 6 deg. C anything hotter than that will give you higher density altitude, and worse air.

i don't remember the exact calculation for finding exact density altitude, i have an aviation calculator (e6b) that does all the hard work for me. so if anyone is really intrested in this stuff go to a small airport and pick up one of them, they cost like $6. OR do a search and i'm sure you can find the equasion. or you can also go to summit and pay a few hundred bucks for a weather box that does it all for you.

humidity also has its effects, but i don't know of any ways to findout how it effects anything. i know high humidity is usually a bad thing, but if your making boost it might help with some detionation. i don't know about that.

one more thing, if anyone has their barometer and temp i'd be glad to convert that into DA for you. sorry about all the temps. being metric, thats just how they are.

 

[XLR8]

I wasn't really implying that. I know the differences. I've raced at Vegas (2,000) ft, and run my fastest and quickest, and LACR (around 2,000 but DA is always closer to 5,000) not quite so fast there. What I was saying that lower altitutes around a thousand or so aren't going to be seen in a forced induction time ticket. But the beauty of forced induction is, we can mitigate the altitude differences much more than the N/A guys can.

I agree with you there Dave.

And Dennis, all your calculations bring to mind the time I went through ground school. And if I can recall standard tempurature is 59*. (It may be 57* - it,s been over 20 years)

 

[denn454]

thats where i learned all that. i didn't know you were a pilot royal.

ayour right, 59 is the same as 15C.

i was wrong about the lapse rate, i had to dig out some of my old books but its actually 2-3 depending on moisture content. that works out to be about 5.4deg F cooler for every 1,000 feet. i guess i mixed the numbers up in my head.

i don't have much else to do right now so i'll go through a little bit of this.

sealevel standard temp 59 deg F
1,000 standard temp 53.6
2,000 standard temp 48.2
3,000 standard temp 42.8
4,000 standard temp 37.4
5,000 standard temp 32
6,000 standard temp 26.6

thats all assuming the air is dry, low humidity or larg dewpoint temp spread. so it pretty well sucks for us up at altitude, our density altitude is typically in the 7,000's.

i hope my bordom helps someone!

 

[XLR8]

Not a pilot - it costs too much. Went through ground school because I could for free and stopped there.