impact of elevation on power

[NEVRL8]

First off, my Cobra is a 2003 Black convertible with parchment interior and roof. #2681 of 5082 built 03/03/03 (perfect build date for an '03 Cobra) So far CAI is the only mod, but I was down in Califronia in January and brought back some souveniers. Magnaflow x-pipe and cat-back, 2.93 pulley, and a Predator. I understand MPH is more a function of horsepower, and 107 is the fastest I've run here in Calgary ele. 3400' (stock with CAI).
I know the rule of thumb for a n/a car is -4% hp per 1000' of elevation, but before I ran at the track I really thought that the supercharger would compensate for that.

Question 1. Does 8PSI of boost (manifold pressure) at sea level equal 8PSI of boost (manifold pressure) at 3400'?

Question 2. Given the new mods above, what should I run next time at the track? (best with CAI 13.3 @ 107 / 2.3 60')

Thanks,
Colin

ps I wanted to post a couple pics of the car but don't know how - forum rules say 'may not' ?!?

 

[NEVRL8]

any opinions?

 

[ToolemanTX]

oh

you lose pressure or something in higher elevation.. your car should be good for a 12.9 if you can get your launch down.. your 2.3 sucks.. you should be atlease 2.1 to 2.0..

 

[Darcsun]

Sea level is 14 psi (14.7 I beleive is more acutate). Every 2100' or so above sea level, you loose a full PSI of pressure. You will be loosing around 1.4 PSI from you're boost. (I live at 6200' and loose about 2.5 PSI).

There are a few advantages of altitude, however. Non boosted cars far far worse then boosted cars. The second is that, due to the lack of oxygen, the fuel systems run rich. This allows a pully or extra N2O with out the need of computer programing or fuel system upgrades. Of course, those are always nice insurance polocies.

 

[NEVRL8]

Originally posted by Darcsun
Sea level is 14 psi (14.7 I beleive is more acutate). Every 2100' or so above sea level, you loose a full PSI of pressure. You will be loosing around 1.4 PSI from you're boost. (I live at 6200' and loose about 2.5 PSI).

There are a few advantages of altitude, however. Non boosted cars far far worse then boosted cars. The second is that, due to the lack of oxygen, the fuel systems run rich. This allows a pully or extra N2O with out the need of computer programing or fuel system upgrades. Of course, those are always nice insurance polocies.

That's what I was thinking as far as the impact being more of an issue for the n/a cars, but I'm surprised at the mph of some stock Cobra's posted here and even MMFF's comparison test between Cobra Mach1 and GT. The Cobra was a 'vert right from the factory, and I think it ran 112mph.

Darcsun, have you run your car at the track stock and if so - how fast?

 

[Chris_H]

I'll jump in on this too since I live at 3,900' and race at 4,100'. There is a webpage from NHRA that shows elevation correction and how to figure it. Once I get home I will post the link to it. I have a Novi 1000 on my 87 LX and with the stock set up I could barely get 4psi. I threw on a smaller blower pulley and a powerpipe on and got a whopping .5 psi extra. I threw on a bigger crank pulley, and most guys with that combo at sea level were seeing 10 psi or better. I saw 7. So yes, the thin air definately affects the blower. Just like on a colder day/night the car runs a lot better, and you see more boost because of the thicker air, the elevation works the same way with thinner air. We run in the summer out here with it being over 100* outside and our elevation, we get some seriously slow times. I will post the link when I get home.

Chris

 

[NEVRL8]

Thanks Chris, that would really be helpful. My Avatar is my goal, and I want to make it there as close to stock as possible, then blow it away with the rest of my California souveniers(above). :thumbsup:

 

[Chris_H]

I know the feeling. I still have stock heads and cam in my LX. With crappy 60's (1.8) I told my wife I wouldn't get the heads and cam until I run a 12.99. I am sitting at 13.4 @ 102.4 right now. With the new crank pulley I bought, I am hoping to see 12's with it. Just need to get the 60's down.

Chris

 

[NEVRL8]

Originally posted by Chris_H
With crappy 60's (1.8)
Chris

If 1.8 is crappy, I have to shave off half a second just to get to crappy, that should put my et somewhere around 12.7 :burnout:

 

[SynMan5.0]

I believe there is a big misconception about boosted cars at higher elevations. From what I've personally seen on dynos, boosted cars seem to lose more at higher elevations than N/A cars. I live at over 6200 ft. elevation so I definitely feel a decrease in power. My ex-'03 Cobra (now 78mphgpr's car) ran a 13.1 @ 110 mph (2.3 60' :bash: ) at Bandimere (~5800 ft.) with a 2.93 pulley, chip, catback, and CAI. Terry (78mphgpr) told me he ran an 11.60 in Florida with the car with the only change being a set of 18" drag radials. Quite the difference. Anyway, on a dyno we had here in Cheyenne for a while (Dynojet), if we looked at actual HP & TQ between two cars with similiar SAE numbers (one boosted, one N/A), the N/A car lost a whole lot less than the boosted car. Why? Well, I believe it's because of the thinner air. As someone mentioned earlier, you lose boost at higher elevations. My '04 only makes about 6-7 psi on the stock gauge where sea level guys peg their boost gauges stock. Before I sold my '03, I took it on a trip to Pennsylvania. With the pulley and an Autometer boost gauge, I went from 11.5 psi here in WY to close to 15 in PA! Need more convincing? My '00 Lightning with a chip, full Bassani exhaust, and K&N ran a best of 14.9 at Bandimere. Same truck, same mods at WFC 5 in St. Louis ran a 13.0. Now nitrous cars on the other hand have an advantage at higher elevations (mostly because the nitrous basically creates a new atmosphere in the engine). My '89 coupe, when it was on the spray with no blower, went 11.6 @ 118 at Bandimere and in the same form went 11.1 @ 121 in St. Louis. Anyway, just thought I'd clear that up a little. I could be wrong about boosted cars losing more at elevation, but I'm just speaking from my personal experience.:beer:

 

[Chris_H]

I'm with you on that one Adam. I fully agree with the blower cars losing more hp than n/a or nitrous cars. After all, with nitrous, you are giving the car the oxygen it needs. All we are doing is compressing the lack of oxygen that we have.

http://www.smokemup.com/auto_math/nhra_correction_factors.php

That is the website I was talking about earlier. Take it as a grain of salt. I am guessing they are refering to n/a cars on that. but either way, my current 13.4 @ 104 would be close to a 12.72 @ 107.95. Now that would mean I have the same exact boost as I do now, which we know to be false. The lower the altitude, the higher the boost. With my new 8" crank pulley, I am hoping even for a 12.99 @ 104ish. If thats the case, with the same boost at sealevel, I would be running 12.33 @ 109.56. Just some food for thought.

BTW, can you guys at the higher altitudes get anything better than 91 octane at the pump? My understanding is that we don't need it because of the thin air, but that doesn't mean I wouldn't mind running some through it.

Chris

 

[NEVRL8]

Originally posted by svt46sc
My ex-'03 Cobra (now 78mphgpr's car) ran a 13.1 @ 110 mph (2.3 60' :bash: ) at Bandimere (~5800 ft.) with a 2.93 pulley, chip, catback, and CAI.

Wow, that's a little discouraging, but thanks for the input.

Colin

 

[sambandit]

You lose mega power at elevation, same or worse than a NA car. My 19-20 lb sea level setup gives me 14-15 here. Also, get this, my dyno is 508 SAE corrected. My actual uncorrected number is 428hp. That's enough to make you cry, isn't it? Do believe it when people say you don't lose much with a supercharged car.

 

[sambandit]

Originally posted by svt46sc
I believe there is a big misconception about boosted cars at higher elevations. From what I've personally seen on dynos, boosted cars seem to lose more at higher elevations than N/A cars. I live at over 6200 ft. elevation so I definitely feel a decrease in power. My ex-'03 Cobra (now 78mphgpr's car) ran a 13.1 @ 110 mph (2.3 60' :bash: ) at Bandimere (~5800 ft.) with a 2.93 pulley, chip, catback, and CAI. Terry (78mphgpr) told me he ran an 11.60 in Florida with the car with the only change being a set of 18" drag radials. Quite the difference. Anyway, on a dyno we had here in Cheyenne for a while (Dynojet), if we looked at actual HP & TQ between two cars with similiar SAE numbers (one boosted, one N/A), the N/A car lost a whole lot less than the boosted car. Why? Well, I believe it's because of the thinner air. As someone mentioned earlier, you lose boost at higher elevations. My '04 only makes about 6-7 psi on the stock gauge where sea level guys peg their boost gauges stock. Before I sold my '03, I took it on a trip to Pennsylvania. With the pulley and an Autometer boost gauge, I went from 11.5 psi here in WY to close to 15 in PA! Need more convincing? My '00 Lightning with a chip, full Bassani exhaust, and K&N ran a best of 14.9 at Bandimere. Same truck, same mods at WFC 5 in St. Louis ran a 13.0. Now nitrous cars on the other hand have an advantage at higher elevations (mostly because the nitrous basically creates a new atmosphere in the engine). My '89 coupe, when it was on the spray with no blower, went 11.6 @ 118 at Bandimere and in the same form went 11.1 @ 121 in St. Louis. Anyway, just thought I'd clear that up a little. I could be wrong about boosted cars losing more at elevation, but I'm just speaking from my personal experience.:beer:

All very true! I know another guy who's lightning runs 12 flat at sea level and ran 13.2 here. Probably lose a sec to a sec1/2 here due to elevation. Probably at least 1/2sec at 3500ft.

 

[Chris_H]

Yeah it definatelty sucks knowing my car is pretty much .7 faster in the 1/4 at sea level. Thats ok, because the faster I get it here, the faster I know it will run at sea level. Just have to get it to sea level! I feel so slow when I see everybody else's time, and a lot of people don't know that elevation makes a big difference. They just see your times and laugh. Oh well, guess thats the name of the game. When people here see the times, they freak out. Seems most camaros and mustangs run high 14's low 15's here as the norm, so its nice running low 13's :rockon:

Chris

 

[hogger]

Hey Colin, I just wanted to say welcome to the board. It's always nice to see another Canuck Cobra owner! If there's an answer to be found, this board usually has it.

By the way, sambandit's car even at elevation is fricken' fast!! :thumbsup: He whupped my stock vert! (oops, maybe that was his wife ;-) )

hogger

 

[Smrgol]

And what really sucks is that turbo cars don't suffer as much . The downside of any forced induction tied to the crank shaft is that it's boost loss is proportional to the loss of atmospheric pressure as altitude increases. That's why those little turbo'd procket rockets do so well up here in the nose bleeds.

 

[NEVRL8]

Originally posted by hogger
Hey Colin, I just wanted to say welcome to the board. It's always nice to see another Canuck Cobra owner! If there's an answer to be found, this board usually has it.;-) )
hogger

Thanks for the welcome! There's no doubt, this board is THE place to go for info on these cars. In fact the moderator at performance-shop.com (board based here in Calgary) directed me here when I was looking for info on exhaust systems.

Colin:beer:

 

[KWladyka]

There’s no question that high altitude affect the power output on any car. But with pressurized cars (supercharge, turbo, etc.), the correction factor for altitude depends on the boost you run.
Sea Level air pressure is 14.7 psi. If you go to a track in Boise, Idaho (my location - 2850 feet above sea level) the air pressure is around 13.25 psi. That's 90.1% of sea level pressure. If the temperature doesn't change and you have a normally aspirated car, your power output will now be 90.1% of what it used to be, so you can correct by multiplying your calculated HP by an extra 10.9% (1/.901, or 1.109).

However, let's say you were running 10 psi of boost in the first place. So at sea level, your car was really getting 24.7 psi (14.7 + 10). Now in Boise, your manifold pressure is going to be 23.25 psi (13.25 + 10). Note that the power isn't down as much, and in this case it's down to 94.1% of what it is at sea level. So the correction would equal an extra 6.2% (1/.941, or 1.062).

In addition to the conversion based on altitude, ultimately it is the “Density Altitude” that affects your power. This combines the Temp, Humidity, Pressure and adds that to the track altitude. All these factors contribute to the density of the air that enters the combustion process. For a density altitude correction, try http://www.modulardepot.com/density.php.

For the detailed oriented there are also other variables that impact the calculation such as the effects of higher pressure ratios in the compressor, lower density air across the intercooler and even the fact that there's less wind drag at higher altitudes but these are much more difficult to quantify and from what I understand have a much lower impact on overall performance.

I’ve run 11.97 at a 4891 feet DA and a 11.81 at a 3,442 ft DA in my 03 Cobra. Using a DA correction, the 11.97 is the faster time (11.21 to an 11.32) although the actual time is almost 2 tenths slower. To me knowing the impact environmental conditions have on my car lets me know when I’m actually doing better even though my times may be much slower. Density altitude conversions assist with understanding the puzzle and comprehending the effects of certain modifications run under various conditions. I think it is unfortunate that as a general rule conversions are accepted for Dyno tests but not so much for track results. To me, if a density attitude is not posted with any time, it really has no useful value.

 

[Smrgol]

However, let's say you were running 10 psi of boost in the first place. So at sea level, your car was really getting 24.7 psi (14.7 + 10). Now in Boise, your manifold pressure is going to be 23.25 psi (13.25 + 10). Note that the power isn't down as much, and in this case it's down to 94.1% of what it is at sea level. So the correction would equal an extra 6.2% (1/.941, or 1.062).

You have to be careful here. This analysis assumes that the boost pressure is constant between altitudes. For a turbo car this is true because the turbo is not tied to the crank and it's boost pressure is usually regulated mechanically. A 10 psi wastegate is a 10 psi wastegate at any altitude. The turbo is simply spinning faster at higher altitude to create the necessary boost to open the wastegate.

However, roots and centrifugal blowers are driven by the crank, therefore they cannot compensate for the loss of air density by changing rotation rates. Thus, a blower suffers a boost loss as altitude increases at the same rate as the motor does. Using numbers from the example above, a blower with peak boost of 10 psi at sea level will show a peak boost of 10 * 13.25/14.7 = 9 psi in Boise.

The interesting thing to note is that the SAE correction (and most others) assume a specific rate of absolute manifold pressure reduction with altitude increase. The roots blowers conform to this because they cannot compensate so the correction is still meaningful. However, this correction factor "overrates" turbo cars with mechanical boost regulation by about 5% to 10%.