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Discussion Starter · #1 ·
I would like some opinions on altitude killing horsepower.. I know that 460-480 rwhp in a stock S197 is about the (safe) limit. My question is if I am running 13psi on a twin screw to get 460 at the wheels because of the altitude is it still safe? The timing is 18 and the afr is 11.7.
 

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I have no idea, but 460 is low for 13psi, which shows altitude killed the HP. Some 10PSI guys are getting 470-480
 

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I would like some opinions on altitude killing horsepower.. I know that 460-480 rwhp in a stock S197 is about the (safe) limit. My question is if I am running 13psi on a twin screw to get 460 at the wheels because of the altitude is it still safe? The timing is 18 and the afr is 11.7.
I don't get it. Regardless of the altitude, 13 psi is 13 psi. All that I would think would happen is you would need to run a smaller pulley to get the same amount of boost vs someone at sea level. Obviously for an n/a application the altitude makes a huge difference. But with boost, as long as you have the same psi, shouldn't the hp be the same?

What twinscrew do you have? Saleen? Whipple?

Also, for what its worth, I didn't realize any gain in horsepower once I went over 12psi boost until I got a larger throttle body. I picked up alot going to the BBK, then later to the Black Diamond throttle body.
 

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Where are you located and what is your Altitude and DA's?

Yes, of course HP and boost are compromised by altitude. Obviously not as much as an NA car, but boosted cars especially blower cars are affected by altitude. I live in Denver, and our track is at 5300ft, and seeing DA's of 8000-9000ft above sea level is the usual. I have a whipple HO kit, and with the supplied 10lb pulley I see about 8lbs most of the time. I did the math with my trap speed, et's, and race weight, and I am making anywhere from 370-400hp. So there is no way you are always going to make the same hp as someone living at sea level, even if you are making the same amount of boost. Even top fuel cars come up here and have to make more boost, but are still down on hp...they don't run as fast up here as they do at sea level, even with power adjustments. Living at altitude is not drag racing friendly, and you have no idea what its like unless you've experienced it.
 

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You wont make as much power with the same boost at altitude because you are working the blower harder (spinning faster).
 

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Where are you located and what is your Altitude and DA's?

Yes, of course HP and boost are compromised by altitude. Obviously not as much as an NA car, but boosted cars especially blower cars are affected by altitude. I live in Denver, and our track is at 5300ft, and seeing DA's of 8000-9000ft above sea level is the usual. I have a whipple HO kit, and with the supplied 10lb pulley I see about 8lbs most of the time. I did the math with my trap speed, et's, and race weight, and I am making anywhere from 370-400hp. So there is no way you are always going to make the same hp as someone living at sea level, even if you are making the same amount of boost. Even top fuel cars come up here and have to make more boost, but are still down on hp...they don't run as fast up here as they do at sea level, even with power adjustments. Living at altitude is not drag racing friendly, and you have no idea what its like unless you've experienced it.
Wow I thought 3200 feet was bad. My dyno guy said going to sea level would be like my car having another 40-50hp. I gotta say it sure felt like it when I drove to the coast last spring. I can't imagine the difference from 5300 feet vs the coast, it would be like having a brand new more powerful car!
 

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Let me clarify---as I said in my first post with regards to N/A cars, altitude certainly does effect performance as there is nothing to compensate for the difference in atmospheric pressure.

But you just said the same pulley in Denver would only give you 8 psi, vs 10 psi elsewhere, so of course you would be down on power, because your not at the same boost level. But, go with a smaller pulley so your back up to 10lbs, lets say, and you should be close.

What CataclysmGT said makes more sense to me, as the blower would be spinning faster, therefore drawing more hp.

I run a whipple, and from running in DA's ranging from 600 feet at Famoso to over 3000 feet at CA Speedway at Fontana, the difference is not even 2/10ths. N/A cars, howver, experience a much larger difference though--I have seen when we used to race at LACR, the DA being close to 4000, and then run at terminal island and the et would improve a 1/2 second.
 

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Discussion Starter · #8 ·
Altitude is a hp killer. In reply to some of your thoughts think about it this way. Boost is just a measurement of restriction in the engine. If you leave your blower pulley the same and put on a great set of heads and free flowing exhaust you lose boost pressure but gain horespower because you are moving more air. I know the blower is working harder to get the engine the air but if I make 480 at 6000 feet do you think it is the same stress on the rods and pistons?
 

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A blower is a fixed rate compressor, once inmmersed in a pre-compressed environment it will compress in a proportional ratio. That is why NA, blower and some turbo cars are affected by BP. We are about 3000ft high, a 10psi car at our height can get to make about 1.xpsi more at sea level. The only config that would stay the same no matter the height will be a MAP referenced boost controlled turbo car.

Corrections (STD, SAE, etc.) help to compensate these results, our dyno actually any compensation used inflate considerably the real non corrected results because our height. There are a couple of excellent threads regarding this issues that are very worth reading:

http://www.modularfords.com/showthread.php?t=111643&highlight=analyzedhttp://www.modularfords.com/showthread.php?t=111643
http://www.modularfords.com/showthread.php?t=54099
 

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It'd be awesome to drive up a steep hill in your mustang, and cruise down it for a few miles, then noticing your car is a lot more powerful since your at a lower altitude.
 

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interesting thread. Volume of air is one thing and amount of oxygen per volume of air is another. meaning that 10 psi at 10,000' feet has less oxygen (what is needed for combustion, not nitrogen which accounts for 70%) than 10 psi at sea level. pilots may use an "oxymeter" regulator for this, and it is true that the blower will have to work harder to accomplish that amount of boost because air also gets "thinner" with altitude. Turbo-propeller aircraft do variable pitch to "bite" more at altitudes for trust.

atmosphere in a can? Nitrous...
 

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interesting thread. Volume of air is one thing and amount of oxygen per volume of air is another. meaning that 10 psi at 10,000' feet has less oxygen (what is needed for combustion, not nitrogen which accounts for 70%) than 10 psi at sea level. pilots may use an "oxymeter" regulator for this, and it is true that the blower will have to work harder to accomplish that amount of boost because air also gets "thinner" with altitude. Turbo-propeller aircraft do variable pitch to "bite" more at altitudes for trust.

atmosphere in a can? Nitrous...
I may be wrong but as I understand it the proportion of oxygen in the air not vary with altitude (usually at 21%) just the density of it and its proportional for all of its components (nitrogen, etc.).

A blower does not work any "harder" it justs compress air, you just submerge it into a "pre-compressed" environment and it will deliver more pressure in a quite proportional ratio. The only ones that work "harder" are the MAP referenced boost controlled turbos as BP lowers.
 

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^^
That's right. The composition of air doesn't change until you get to really high altitudes.

But since drive power of the compressor goes as mdot*cp*ΔT, for a given pressure ratio the drive power will actually drop at higher elevation since the mass-flow rate (mdot) goes down with air density. (This assumes a constant pressure ratio and inlet temp. ΔT is a function of both. cp is just the constant-pressure specific heat of air.)
 

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Both are incorrect; same car, same tune, same track, different seasons, different times by a couple of tenths 'cause change of air composition:

GasRatio compared to Dry Air (%)Molecular Mass
- M -
(kg/kmol)Chemical SymbolBoiling PointBy volumeBy weight(K)(oC)Oxygen20.9523.2032.00O290.2-182.95Nitrogen78.0975.4728.02N277.4-195.79Carbon Dioxide0.030.04644.01CO2194.7-78.5Hydrogen0.00005~ 02.02H220.3-252.87Argon0.9331.2839.94Ar84.2-186Neon0.00180.001220.18Ne27.2-246Helium0.00050.000074.00He4.2-269Krypton0.00010.000383.8Kr119.8-153.4Xenon9 10-60.00004131.29Xe165.1-108.1

Big difference in amount of oxygen at sea level vs. 10,000' as well as density (thinner), thats why they "pressurize" a cabin 'cause of oxygen when you fly commercial... not an expert but an FAA certified PPA since '94 and NAUI certified scuba diver plus open water since '78... same fixed impeller will not "move" the same amount of air in Florida (sea level) vs. Colorado at Salida (10,000 above msl)... same goes for mixture:bigwink:
 

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rice, the race was held in the same exact environmental conditions (temp, humidity and BP), did you use the exact same tires, you had the same exact level of experience from one year to another, the track had the same level/type of rubber both years, etc?

so the only change was O2 ratio in the air, ok.
 

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yeah, correct. It might even change in the same day, cold winter morning vs. hot humid afternoon... example: a plane might take 500' to take off at sea level vs. 800' at higher altitude just because of "air" and either you trim the fuel (when carbureted) or crash... lol
 

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Yes, there is a big difference in the density of air at sea level vs altitude. But the composition, (i.e., percentage of oxygen, etc.), is virtually a constant within the troposphere.
 

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Yes, there is a big difference in the density of air at sea level vs altitude. But the composition, (i.e., percentage of oxygen, etc.), is virtually a constant within the troposphere.
Wrong.
 

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riceburner - I'm not a pilot nor a scuba diver, but I have been doing the engineering thing for about 30 years, have BS and MS degrees in engineering, and am a card-carrying member of ASME and IEEE. I think you are confusing air density with composition. Composition doesn't change until you get to very high altitudes. Do a Google search. I won't argue it with you.
 
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