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Old 05-24-2000, 10:10 AM   #1
Ender
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Location: Lakewood, CO
Question Effects of altitude?

Just a few quick thoughts...

I recently moved from CA (sea level) to Denver (mile high). Here are some questions that have popped into my head... I new to this kind of stuff, so forgive me if these don't make sense.

1. I've heard that each 1,000ft decreases performance by 3%, so 5200'= -15% power?

2. The air is not as dense in altitude, but does the %02 content remain constant? If not, does the car lose more power due to less oxygen or is that what the 02 sensor is supposed to calculate?

3. I recently took a trip up to the mountains (I think I might have been the "highest" RS in the world briefly at 14,000'+) and noticed that the car used less gas. Is that a function of the less dense air? BTW, the car had NO power climbing those high hills!

4. If I go w/ forced induction and tune it to Denver air, do I have to worry when I drive the car to lower altitudes?



Thanks for any light you guys can shed on these Q's.

Stephen
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Old 05-24-2000, 01:03 PM   #2
Yotsuya
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Post

In colorado they also sell lower octane gas. Regular is 85 because of the altitude.

Did you drive up Pikes Peak or Mount Evans? Those are the only fourteeners I know you get to the top of in a Subaru.
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Old 05-24-2000, 01:21 PM   #3
El Kabong
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Hey Ender, welcome to Colorado. If you are interested, here's the site to get on the Colorado club mailing list. There's lots of events on the calendar there both track days and autocrosses.
http://www.egroups.com/group/SPCR
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Old 05-24-2000, 01:57 PM   #4
Ender
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Thanks El Kabong. I'm joining the mlist now!

Regarding octane in gas... why would you need lower octane for less dense air? I thought octane is anti-knock rating, which shouldn't be affected by air density?

Stephen
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Old 05-24-2000, 02:22 PM   #5
Strepto
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Ender, I posted on the general topics board that we have an autoX this weekend. If you have time to go down to C. Springs on Sun come meet a few of us!!!
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Old 05-24-2000, 02:49 PM   #6
DuneHopper
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Lightbulb

Someone please correct the parts I get wrong. From my very limited understanding and experience:

The Subie's have a knock sensor and something called something like an Automatic Ingition Retard. The gist being that when knocking is sensed, like when climbing a hill, the ignition is retarded to eliminate the knock. This also reduces power, which might be what you are experiencing.

My OBS couldn't climb hills worth a darn until I switched from 89 to 93 octane. Not a problem since. Anyhow, might be worth trying higher octane to see if it helps.

DuneHopper
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Old 05-24-2000, 09:53 PM   #7
Ender
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what about the effects of running sc/tc at differing altitudes?
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Old 05-24-2000, 10:31 PM   #8
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If you tune it at one altitude it'll either go lean or rich depending on which altitude you go to. Go higher and it'll run richer, go lower and it'll run leaner (and cause detonation, I might add).
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Old 05-24-2000, 11:39 PM   #9
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Going lower and causing detonation is why the Rimmer S/C is rummored to be 8psi for Colorado and 5.5psi for sea-level. Anyone know if that is true?

Also, I figure a stock RS in Denver/Colorado Springs will do about a 16.8 qtr mile at 80mph, due to the altitude (using NHRA adjustment tables). I'm sure mine is now back to full power with the mods I've done.

Larry
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Old 05-25-2000, 12:33 AM   #10
Scooter
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Stephen,

Its really the air density that matters. The density can be found by the Perfect Gas Law. The following link has a post by me that explains this a bit. http://www.i-club.com/ubb-files/Foru...ML/001551.html

Basically, higher altitude means lower pressure, lower pressure means lower density. However, this can be offset some by the typically cooler temperatures at higher elevations. Cooler air higher density.

Here's some basic altitude vs pressure info.. This does not account for pressure flucuations associated weather fronts and the like (i.e. high & low pressure areas)

Altitude-----Pressure
in ft-------lb/sq ft
-5000--------2528
0------------2117
5000---------1761
10000--------1456
15000--------1195
20000--------973
25000--------786
30000--------630
35000--------499
40000--------393


[This message has been edited by Scooter (edited May 24, 2000).]
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Old 05-25-2000, 12:35 AM   #11
rao
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Here is some intersting info http://www.gnttype.org/techarea/misc/altitude.html

It shows the conversion factors to convert 1/4 mile times an trap speeds to sea level and should give you an idea of the magnitude of change to expect.
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Old 05-25-2000, 08:37 AM   #12
Strepto
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From the results they got last night, at 6000 ft above sea level a stock car (2.2L at that) runs an 18 sec 1/4. Mark (w/slush box) running ~14psi only ran a 14.69. Damn this altitude!!!
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